Floral Development in Legumes1

Home , Acacia myrtifolia, Amherstia, Ateleia, Bauhinia divaricata, Burkea (plant), Cadia (plant)

Update on Floral Development

Shirley C. Tucker* Department of Ecology, Environment, and Marine Biology, University of California, Santa Barbara, California 93106–9610

Species of flowering plants are most reliably iden- center of the flower is superior in position; i.e. its

tified by their flowers, the sexually reproductive or- base is attached at the same level as those of the Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 gans. A flower is similar to a vegetative short shoot sepals, petals, and stamens. The carpel differentiates (lacking appreciable internodes) that bears four kinds as a gynoecium with ovary, style, and stigma, and of laterally attached organs in successive whorls: se- eventually forms a pod-like fruit. Although most pals, petals, stamens, and carpels. Significant floral people think of papilionoid-type flowers (or flag distinctions among plant families include symmetry, flowers; Fig. 1A, F) as representative of legumes, whether organs are organized in whorls or helically, many legume taxa differ markedly from this type of number of parts per whorl, carpel position relative to flower (Fig. 1, B–E; Tucker, 1987a). the surrounding organs, fusion among organs within Superimposed upon this basic floral ground plan a whorl or between different whorls, and whether are prominent differences among the three legume both male and female organs are present in the same subfamilies. These include differences in flower posi- flower. tion in the inflorescence, floral symmetry, sepal and A flower, like a vegetative shoot, has a terminal petal aestivation, fusion, loss or increase of floral or- floral apical meristem that initiates organs laterally, gan number, heterogeneous organs within a whorl, usually in acropetal succession (although exceptions unisexual flowers, etc. Ontogenetic differences among are common in some taxa): sepals first, followed by the subfamilies are of particular interest. petals, stamens, and carpels. Because each flower With this background in mind, I will describe the lives for a very short time, the floral apical meristem details of floral development in each subfamily of is determinate, meaning that it ceases activity after a legumes. These details are necessary as a foundation certain number of organs have initiated. Vegetative for molecular studies on legume flower develop- apical meristems, in contrast, are usually indetermi- ment and organ identity relative to the ABC model nate, continuing to initiate new organs, such as (Meyerowitz et al., 1991; Irish, 1999; Jack, 2001). leaves, indefinitely. In this Update, I will focus on flowers of the plant SUBFAMILY CAESALPINIOIDEAE family Fabaceae. This family comprises three large subfamilies: Caesalpinioideae, Mimosoideae, and The subfamily Caesalpinioideae encompasses 170 Papilionoideae. Although the family is widely ac- genera and about 3,000 species. It has a basal position cepted as monophyletic (Chappill, 1995; Doyle, 1995; in phylogenetic schemes (Doyle, 1995; Doyle et al., Doyle et al., 2000), these subfamilies differ greatly in 2000; Bruneau et al., 2001) and is highly diverse in floral symmetry. Fabaceae is a large family (about floral form and ontogeny. It is currently divided into 700 genera and about 18,000 species), and is nearly four or five tribes: Cercideae, Caesalpinieae, Cass- ubiquitous over temperate and tropical parts of the ieae, and Detarieae, with Macrolobieae (derived from world (Polhill and Raven, 1981). Many agronomically within Detarieae) recently gaining acceptance. important plants are members of this family. The information base is huge, and the taxa are relatively Inflorescences easy to obtain. Most legume flowers share a pentamerous ground Caesalpinioid inflorescences are usually racemes or plan with five sepals, five petals, two whorls of five panicles, although solitary flowers and cymes occur stamens each, and a single carpel, or 21 organs in all. more often than in either of the other subfamilies. A Members of each of the four whorls alternate with raceme, as in Senna didymobotrya (Fres.) Irwin & Barn. those of the preceding whorl. The single carpel at the (Fig. 2A; Tucker, 1996), is the most common kind of inflorescence among legumes. Racemes, together

1 with panicles, spikes, and some umbels, have a ter- The research was supported in part by the National Science minal apical meristem that grows indeterminately Foundation (grant nos. BSR84–18922, BSR87–22514, DEB92–07671, and DEB94–20158 [DEB–9596281]) and by Louisiana State Univer- and initiates bracts (modified leaves) in acropetal sity (Baton Rouge; Boyd Professor funds). succession along the inflorescence axis. A single flo- * E-mail [email protected]; fax 805–893–4724. ral bud is initiated in the axil of each bract. A pair of Article, publication date, and citation information can be found bracteoles (reduced leafy organs) is usually produced at www.plantphysiol.org/cgi/doi/10.1104/pp.102.017459. below each flower. Developmental differences distin-

Plant Physiology, March 2003, Vol. 131, pp. 911–926, www.plantphysiol.org © 2003 American Society of Plant Biologists 911 Tucker Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021

Figure 1. A through F, Drawings of legume flowers. A, “Papilionaceous” flower of redbud (Cercis canadensis) with three forms of petals: standard or vexillum, wing, and keel. B, Paramacrolobium caeruleum, zygomorphic flower with large bracteoles, five tiny sepals, one large petal, the carpel, and three stamens. C, Saraca declinata, radially symmetrical flower with sepals, no petals, a carpel, and only four stamens. D, Labichea lanceolata, asymmetric flower with sepals, four reduced petals, carpel (not shown), and only two stamens. E, Strongly zygomorphic flower of Amherstia nobilis, with petalloid bracteoles, four sepals, three large petals, 10 stamens, and an elongate hypanthium. F, Papilionoid flower of Lupinus succulentus, with standard or vexillum, wings, and keel. Bl, Bracteole; C, calyx; G, gynoecium; H, hypanthium; K, keel petal; P, petal; V, standard or vexillum petal; S, sepal; St, stamen; Sy, style; W, wing petal. Scale bars ϭ 4 mm for A through C, E, and F; scale bar ϭ 2 mm for D.

912 Plant Physiol. Vol. 131, 2003 Floral Development in Legumes

Figure 2. A through L, Floral initiation in Cae- salpinioideae (SEM micrographs). Abaxial side is at base of figure in C through L. Scale bar ϭ 25 ␮m in G; scale bars ϭ 50 ␮m in H through K; scale bars ϭ 100 ␮m in B, E, F, and L; scale bar ϭ 500 ␮m in A; scale bars ϭ 1mminCand D. A and B, Inflorescences with most bracts removed. A, Successive raceme of Senna didymobotrya with the oldest, first formed flowers at base, and successively younger ones above. B, Cyme of Chamaecrista nictitans, with oldest flower at top, younger one below. C, Radially

symmetrical flower bud of Isoberlinia angolen- Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 sis. Sepals are removed, and petals are all similar. D, Zygomorphic symmetry in flower bud of Gilbertiodendron klainei, sepals removed, with one large petal, four smaller. E, Floral bud of Bauhinia malabarica (polar view) showing median petal on adaxial (upper) side, and alternating whorls of organs. F, Ascending cochleate petal aestivation in Cercis canadensis.G through L, Organ initiation series of redbud (C. canadensis). G, Bracteole initiation. H, First sepal initiated on abaxial side in median sagittal plane. I, All five sepals initiated in helical order and first two petals initiating on abaxial side (at arrowheads). J, All five petals initiated and first three stamens of outer whorl initiated (at arrowheads). K, All petals, outer stamens (A), and at least two inner stamens (at arrowheads) initiated. L, All organs initiated; sepals and petals removed. Two whorls of stamens alternate. The carpel cleft is forming. A, Outer-whorl stamen; a, inner-whorl stamen; Ap, inflorescence apical meristem; B, bract; Bl, bracteole; C, carpel; F, flower bud/floral apex; K, keel petal; P, petal; S, sepal/calyx tube; S1 - S5, order of sepal initiation; V, standard or vexillum petal; W, wing petal.

guish variations on the raceme, e.g. a spike has flow- cyme has bracteoles, the pattern of successive flower ers that lack pedicels and are crowded along the axis initiation in bracteole axils can be repeated indefi- without appreciable internodes, and a panicle has nitely. Cymes are found in Dialium guineense Willd. second order branches along the first order axis. (Tucker, 1998), Gleditsia triacanthos (Tucker, 1991), Racemes of caesalpinioids (Fig. 2A) show successive, and Poeppigia procera Presl. (Kantz, 1996) among cae- acropetal initiation and development among the salpinioids. Solitary flowers, each subtended by a flowers. In other words, each inflorescence includes foliage leaf, occur in the caesalpinioid Petalostylis flowers of many ages, with the oldest at the bottom labicheoides R. Br. (Tucker, 1998). and the younger ones above. A cymose inflorescence (Fig. 2B, Chamaecrista nicti- Symmetry tans L. Moensch) has determinate growth, with a terminal flower forming first, followed by younger Floral symmetry among Caesalpinioideae is highly flowers in the axils of two bracteoles that are located variable, reflecting the fact that the subfamily is basal below the terminal flower. Because every flower in a and polyphyletic, based on molecular phylogenies

Plant Physiol. Vol. 131, 2003 913 Tucker

(Doyle, 1995; Doyle et al., 2000). Most caesalpinioid mechanisms have evolved for increase or decrease in flowers are radially symmetrical through midstage of organ number among caesalpinioids. Is their ontog- development, when all organs have formed but have eny abbreviated as well, or is ontogeny normal but not yet differentiated. In radial symmetry (Fig. 2C), with subsequent suppression of organs? an object can be bisected along any radius to produce Reductions in the number of organs initiated are two halves that are mirror images. Radial symmetry seen in some examples. The only organs initiated in persists to anthesis in most taxa of tribe Caesalpin- L. lanceolata (Figs. 1D and 3E) are five sepals, four ieae, e.g. Gleditsia triacanthos (Tucker, 1991), and in petals, two stamens, and a carpel, whereas Saraca selected members of the other tribes, e.g. Ceratonia declinata initiates five sepals, a carpel, and four petal siliqua (Tucker, 1992) in tribe Cassieae, and many primordia that are converted to stamens (Figs. 1C tropical taxa of tribe Detarieae such as Saraca declinata and 3D). Far more commonly, legumes apparently

(Fig. 1C; Tucker, 2002b, 2000c). However, some cae- lack some floral organs after initiating all 21 organs; Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 salpinioids such as redbud (Fig. 1A) have flowers some are suppressed after initiation and fail to de- that become moderately to strongly zygomorphic at velop. The caesalpinioid genus Bauhinia includes anthesis (Tucker, 2002a) and show convergence with many species showing suppression of some of the subfamily Papilionoideae in their papilionoid type of petals and stamens. The number of functional sta- flower. Strongly zygomorphic but non-papilionoid mens per flower varies among species: one, two, flowers occur in many other tropical woody caesal- three, five, nine, or 10. B. divaricata (Fig. 3F), for pinioids such as detarioids Paramacrolobium caer- example, has two large petals (several are rudimen- uleum (Taub.) J. Le´on. (Fig. 1B) and Gilbertiodendron tary) and one stamen. The other nine stamens are klainei (Pierre and Pellegr.) Le´on. (Fig. 2D), in which initiated but remain small, sterile, and form a toothed the standard petal is very large, but the other petals collar around the ovary (Tucker, 1987a, 1988b). are absent or reduced in size (Tucker, 2000a, 2002d). Increase in the number of organs is not simply The flower of Amherstia nobilis Wall. (Fig. 1E), an- achieved in development of a highly synorganized other caesalpinioid in tribe Detarieae, is also strongly flower with a set number of organs in a set number of zygomorphic but nonpapilionoid, having just three whorls. (Synorganization is the spatial and functional petals. Some caesalpinioid flowers are asymmetric, integrated connection of organs to form a functional such as Labichea lanceolata Benth. (Fig. 1D) in tribe apparatus; definition after Endress [1994].) Some le- Cassieae. gumes have evolved novel kinds of meristems such as ring meristems (Fig. 3A), common primordia, or additional whorls, by which extra organs are added Organ Positions to the basic ground plan. A ring meristem, a circular ridge on which numerous stamens (up to 200) may be The positions of floral organs differ among the initiated, is a regular feature of ontogeny in several subfamilies. In Caesalpinioideae, the whorls of se- tropical tree genera of caesalpinioids including pals, petals, and the two whorls of stamens are each Monopetalanthus durandii (Fig. 3A; Tucker, 2000a). pentamerous and alternating. The sepal in the median sagittal plane (the vertical median) is on the abaxial or lower side of the flower (Fig. 2, A, H, and I). Because organs of each whorl alternate around the Organogenesis circumference of the flower, the median sagittal petal An ontogenetic series of organ initiation is shown (the standard petal) initiates on the adaxial or upper in Figure 2 (G–L) in the caesalpinioid redbud. Bracte- side (Fig. 2E). This positional arrangement of caesal- oles (Fig. 2G) are first initiated, then the first sepal pinioids is shared by papilionoids but differs in (Fig. 2H), then the rest of the five helically initiated mimosoids. sepals and the first two petals (Fig. 2I). Petal initiation begins on the abaxial side of the flower and Number of Organs continues unidirectionally toward the adaxial side. The next stage shows all five petals plus the first Most legume flowers consistently have 21 floral three outer stamens (Fig. 2J); then the inner stamens organs in alternating whorls: five sepals, five petals, initiate (Fig. 2K). Each of the two stamen whorls also 10 stamens in two whorls of five, and a single carpel. show unidirectional order of initiation. A midstage However, many caesalpinioid taxa have undergone view has all organs present (Fig. 2L) before enlarge- complete loss of some organs, so that there are ex- ment and differentiation begin (Tucker, 2002a). amples of missing sepals, petals, or stamens (Tucker, Caesalpinioids share some features of floral ontog- 1988c). Entire whorls may be missing, as in the inner eny with mimosoids (e.g. helical calyx in some taxa) stamen whorl of Labichea lanceolata (Figs. 1D and 3E; and some with papilionoids (e.g. having the median Tucker, 1998) and S. declinata (Figs. 1C and 3D; sagittal petal on the upper or adaxial side of the Tucker, 2000b), or only one petal and one sepal may flower, and unidirectional order in some taxa). be initiated as in Monopetalanthus durandii F. Halle´ & Among Caesalpinioid taxa, organogenesis varies Normand (Fig. 3A; Tucker, 2000a). Developmental greatly. One finds every possible combination of he-

914 Plant Physiol. Vol. 131, 2003 Floral Development in Legumes

Figure 3. A through K, Specializations among Caesalpinioideae (SEM micrographs). Loss and fusion of floral organs. Abaxial side is at base of figure in A through D and G. Scale bar ϭ 50 ␮m in A; scale bars ϭ 100 ␮m in B, C, and E; scale bars ϭ 250 ␮m in D, F, I, and K; scale bar ϭ 500 ␮m in I; scale bars ϭ 1 mm in G and H. A, Monopetalanthus durandii flower, showing only one sepal, one petal, two stamens and the carpel initiating, and a ring meristem (at arrowheads) around the carpel primordium. B and C, Brownea latifolia. B, Five sepals including two

adaxial sepals (at arrowheads) initiated. C, Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 Slightly older flower than B, showing fusion of two adaxial sepals (at arrowheads) by differential growth. D, Saraca declinata, sepals removed, showing four homeotic stamens (at arrowheads) in petal positions, and the carpel. E, Labichea lanceolata, sepals removed, with four petals, two stamens, and carpel. F, Bauhinia divaricata with three petals (one reduced), one functional stamen, and several suppressed stamen primordia (at arrowheads). G, Polar view of large flower bud of Amherstia nobilis, with three stamens and petals of three shapes. H, Side view of flower of Senna pendula, sepals and petals removed, showing four stamen morphs, labeled A1 through A4. Two labeled A1 will function in pollen transfer. I through K, Gleditsia triacanthos. I, Midstage flower with both stamen and carpel primordia. Sepals and petals removed. J, Male flower, sepals removed, with stamens and a central mound; no carpel is present. K, Female flower with carpel, sepals (two removed), and a petal; stamens have aborted. A, Outer-whorl stamen; a, inner-whorl stamen; A1 through A4, differing stamen morphs; C, carpel; F, flower bud/floral apex; G, gynoecium; P, petal; S, sepal/calyx tube.

lical and unidirectional organogenesis among whorls al., 2000), which have evolved developmental se- (Table I), from completely helical (in Gleditsia triacan- quences different from those of the other tribes. thos) to completely unidirectional (in 17 taxa, mostly Other combinations of organogenesis found rarely species of Caesalpinia (Kantz, 1996), and in several among caesalpinioid taxa include several with bidi- species of Bauhinia (Tucker, 1988b). Completely uni- rectional or erratic order in some whorls (Table I). In directional order is the same as that prevailing in the a few, the number of organs per whorl is so reduced papilionoid subfamily. This pattern is considered the that order cannot be determined. Only one sepal and most specialized one among legumes. It is interesting one petal are initiated in flowers of Aphanocalyx dju- to find the most derived type of organogenesis rep- maensis (de Willd.) J. Le´on. and Monopetalanthus du- resented in three of the caesalpinioid tribes: Caesal- randii (Fig. 3A; Tucker, 2000a). Only two stamens are pinieae, Cercideae, and Detarieae. The caesalpinioid initiated in Labichea lanceolata (Figs. 1D and 3E) and tribe Cassieae has especially diverse development, Dialium guineense (Tucker, 1998), and they are in with every combination of organogenesis except for anomalous positions that make order of initiation uniformly unidirectional (Table I), the only tribe that difficult to compare with other taxa. lacks any known taxa that are consistently unidirectional. This tribe includes some of the most unusual Unisexuality patterns involving bidirectional or erratic order, as well as reduced or missing whorls (Table I). Interest- Reduction in organ number results in unisexual ingly, Cassieae is polyphyletic and includes at least flowers among some Caesalpinioideae such as Gledit- three distinct groups of taxa (Doyle, 1995; Doyle et sia triacanthos and Bauhinia malabarica Roxb. All flow-

Plant Physiol. Vol. 131, 2003 915 Tucker

Table I. Order of floral organ initiation per whorl in tribes and 83 species of Caesalpinioideae Tribe: Cae, Caesalpinieae; Cer, Cercideae; Cas, Cassieae; and Det, Detarieae. Order of organ initiation: A, absent; B, bidirectional; E, erratic; H, helical; R, reduced no.; S, simultaneous; U, unidirectional; ?, unknown. Tribe, No. Sepals Petals Outer Stamens Inner Stamens Gleditsia (three spp.; Caes) H H H A Ceratonia siliqua (Cas) H A H A Burkea africana Hook. (Caes) H H H U Gymnocladus dioica C. Koch (Caes) H H ? A Dialium guineense Willd. (Cas) B R R A Labichea lanceolata Benth. (Cas) H U R A Petalostylis labicheoides R. Br. (Cas) B S B A Aphanocalyx djumaensis (de Willd.) (J. Léon. ͓Det͔)R U E E Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 Monopetalanthus durandii F. Hallé & Normand (Det) R R U ? Cae: 2 H H U U Cer: 1 H H U U Cas: 6 H H U U Det: 6 H H U U Cae: 3 H U U U Cer: 2 H U U U Cas: 5 H U U U Det: 17 H U U U Cae: 4 B or U S or U U U Det: 6 B or H B, H, or U B or U U Cae: 17 U U U U Cer: 1 U U U U Det: 1 U U U U

ers of G. triacanthos (Tucker, 1991; Fig. 3I) are initially keel margins overlap the adjacent margins of the bisexual, initiating both stamens and carpels, but wings, and wing margins overlap the adjacent mar- reproductive organs are suppressed selectively to gins of the standard or vexillum petal (Fig. 2F, produce either male or female flowers (Fig. 3, J and redbud). K). Three floral morphs (male, female, and bisexual) are found in B. malabarica (Tucker, 1988b) and Cera- tonia siliqua (Tucker, 1992). In many other caesalpin- Organ Differentiation and Specialization ioid legumes such as other species of Bauhinia (Tuck- er, 1988b) and Saraca (Tucker, 2000b), bisexual Differentiation of floral organs occurs late in their flowers are the rule, but occasional flowers are func- development. Developmentally, organs in the same tionally male; the gynoecium is suppressed and whorl are initially alike, and will develop alike in nonfunctional. radially symmetrical flowers (predominant in tribe How is unisexuality brought about developmen- Caesalpinieae; Kantz, 1996). Floral specializations are tally? All floral buds are bisexual at first in caesal- rather few in this tribe, according to Kantz (1996). pinioid legumes examined, developing both stamen Uniform whorls also can be found in some radially and carpel primordia. Late in development either the symmetrical taxa of tribe Detarieae [e.g. Fig. 2C, Iso- stamens or the carpel become suppressed in flowers, berlinia angolensis (Benth.) Hoyle & Brenan]. so that they are functionally unisexual. Pollen fails to In many other caesalpinioids, however, same-whorl form in the anthers of female flowers; the gynoecium organs differentiate as dissimilar structures, especially in male flowers may contain ovules but the gyno- in flowers with zygomorphic symmetry. How do ecium remains much smaller than usual and the style these dissimilarities arise in development? Three petal and stigma do not develop normally. morphs are seen in the flower of Amherstia nobilis (Fig. 3G; Tucker, 2000c), and only three functional stamens. Although uniform stamens per flower predominate, Petal Aestivation exceptions with heterogeneous stamens can be found. As the petal primordia enlarge and broaden, their Two dissimilar stamens occur in the flower of Labichea margins overlap. The pattern of petal aestivation (or lanceolata (Figs. 1D and 3E), a flower that is asymmet- overlapping) distinguishes the three legume subfam- rical. Bauhinia divaricata (Fig. 3F) has one large func- ilies. The petals of caesalpinioid flowers become im- tional stamen and nine suppressed stamen rudiments. bricate in a pattern called ascending cochleate; the Stamen diversification is even more noticeable in

916 Plant Physiol. Vol. 131, 2003 Floral Development in Legumes

Senna pendula (Humb. & Bonpl. ex Willd.) Irwin & Only one (Cercideae) of the tribes of Caesalpin- Barneby, in which the 10 stamens include four differ- ioideae appears to be monophyletic (Bruneau et al., ent morphs (Fig. 3H, with stamen types labeled as 2001). The flowers are generally showy, with zygo- A1–A4; Tucker, 1996). S. pendula flowers are zygomor- morphic symmetry. Several species of two of its gen- phic and dorsiventrally heteromorphic, having two era (Bauhinia and Cercis) have been studied develop- large pollinating stamens (A1) with curved anthers mentally (Tucker, 1984b, 1988b, 2002a), but the other and terminal pores; four intermediate “fodder” sta- three are poorly known. mens (A2) with straight anthers; a median adaxial Members of tribe Caesalpinieae (Polhill and Raven, stamen (A3) with a longer filament, and three short 1981; Kantz, 1996) are characterized by simple floral staminodia (A4) with coiled or arched nonfunctional organization, primarily radial symmetry and lack of anthers. Flowers of Senna spp. are adapted for “buzz” specializations. These unspecialized floral features pollination by large bees. An ontogenetic study of this may be considered plesiomorphic (primitive shared) Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 species shows that the stamen primordia are alike in character states. Floral ontogeny is relatively uniform each whorl up until late stages, when the two lateral among Caesalpinieae (Table I; Tucker, 1984b, 1991; abaxial stamens of the inner whorl enlarge ahead of Kantz, 1996). Specializations are rather few and mi- the others of both whorls, and each stamen type dif- nor in this tribe, according to Kantz (1996). Recent ferentiates uniquely. Specialized features of floral or- molecular-based phylogenetic analyses (Doyle, 1995; gans, such as these stamens, commonly are expressed very late in ontogeny. Doyle et al., 2000; Bruneau et al., 2001) suggest that tribe Caesalpinieae is not monophyletic. Aggrega- tions of taxa having unspecialized flowers, such as Caesalpinieae, lack the shared, specialized character Organ Fusion states that indicate close relationship by convention Fusions occur among caesalpinioid floral organs. in cladistic analysis. Fusions are of two kinds: edge-to-edge fusion and Tribe Cassieae is not monophyletic, including at fusion that results from intercalary growth. The first least three disparate lineages, according to molecular- type of fusion may be either temporary or perma- based phylogenetic analyses (Doyle, 1995; Doyle et al., nent, whereas the second is usually permanent. 2000). The great developmental diversity in floral Edge-to-edge fusion is exemplified by sepal and petal form, with every combination of organogenesis except margins that may fuse temporarily in bud and then for uniformly unidirectional (Table I), supports this split open as the flower expands and opens. Carpel lack of close relationship among members of tribe margins also fuse by appression of the edges, cells Cassieae. Specializations among caesalpinioids in- interlocking, and subsequent cell division. The carpel clude heterogeneous androecial whorls and adapta- margins are permanently fused in this way during tions for “buzz” pollination in the Cassia group (Tuck- seed development, although anatomical mechanisms er, 1996), and unisexuality and complete loss of petals of the developing fruit may later lead to separation in Ceratonia siliqua (Tucker, 1992). along the suture. Tribe Detarieae sensu lato (here including Mac- Fusion also occurs by intercalary growth. Many rolobieae) is both the largest (84 genera, 50% of the caesalpinioid flowers appear to have only four se- total in the subfamily Caesalpinioideae) and the pals, because the two adaxial sepals fuse to appear as least well-known tribe of caesalpinioids. Floral one (Fig. 3, B and C, Brownea latifolia Jacq.; Tucker, structure is diverse, with numerous specializations 2000c). The sepal primordia initiate separately, but including suppression of some organs after initia- intercalary growth of the receptacle below the bases tion (Tucker, 2000c, 2001, 2002c, 2002d), nearly com- of two adjacent sepal primordia creates the appear- plete loss of sepals and/or petals (Tucker, 2000a), ance of a single organ. Intercalary growth also occurs and homeotic conversion of petal primordia into in the receptacle below the ring of 10 stamens in some stamens in Saraca declinata (Figs. 1C and 3D; Tucker, caesalpinioids, raising the stamens on a tubular sheath (Tucker, 2002c, 2002d). 2000b). The molecular-based phylogenetic analysis by Bruneau et al. (2001) shows two major clades, Macrolobieae and Detarieae sensu stricto. The Ma- crolobieae clade appears monophyletic and derived Comparisons of Caesalpinioid Tribes from within the diverse tribe Detarieae sensu The subfamily Caesalpinioideae is basal in phylo- stricto. genetic schemes (Doyle, 1995; Doyle et al., 2000; Bru- In the last 10 years, concerted efforts have been neau et al., 2001) and is highly diverse in floral form made to investigate many aspects of the Detarieae and ontogeny. The number of tribes continues to be tribe. Molecular evidence suggests the non-mono- controversial, with current opinions including four phyly of Detarieae (Bruneau et al., 2001), although or five: Cercideae, Caesalpinieae, Cassieae, and De- some groups within it are monophyletic. Develop- tarieae; Macrolobieae is segregated from the latter in mental floral evidence is complex and is not altogether some views (Bruneau et al., 2001). congruent with molecular results.

Plant Physiol. Vol. 131, 2003 917 Tucker

SUBFAMILY MIMOSOIDEAE at the same stage of development in an individual inflorescence. Synchronous development in mi- Mimosoideae includes 65 genera and about 3,000 mosoid inflorescences contrasts with the successive species. There is general agreement that the Mi- development found in inflorescences of the other two mosoideae subfamily is a monophyletic group aris- subfamilies. ing from among the caesalpinioids, based upon both morphological (Chappill, 1995) and molecular evidence (Doyle et al., 2000; Luckow et al., 2000). Al- Symmetry though its flowers are radially symmetrical and appear unspecialized, the group as a whole is derived. Flowers are radially symmetrical in nearly all taxa The subfamily includes four tribes: Mimoseae, Aca- in the mimosoid subfamily (Fig. 4F). Radially sym- cieae, Ingeae, and Parkieae. metrical mimosoid flowers usually have four or five organs in each whorl, and all members of a whorl are Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 alike. Inflorescences

Mimosoid flowers are usually aggregated in a Organ Positions raceme (Fig. 4A, Mimora strigillosa T. & G.) or a paniculate inflorescence (Tucker, 1987a). The taxa Flowers of Mimosoideae differ as a group from examined share an unusual developmental feature: other legumes in organ position (Tucker, 1987a, synchronous development of the flowers in any one 1988a; Ramirez-Dome´nech, 1989; Derstine and inflorescence. As in racemes of the other subfamilies, Tucker, 1991). In pentamerous taxa of Mimosoideae, these undergo acropetal, successive order of flower the sepal in the median sagittal plane is on the adax- initiation, but each floral bud pauses after its initia- ial or upper side (Fig. 4, B and C; Neptunia pubescens tion until all are initiated in that inflorescence. Then, Benth.). In Caesalpinioideae and Papilionoideae, the all flowers undergo synchronized initiation of sepals, sagittal plane sepal is on the abaxial or lower side then petals, and so on. As a result, all flowers will be (Fig. 2, H and I). This difference in the architecture of

Figure 4. A through I, Floral initiation and specializations in Mimosoideae (SEM micrographs). Abaxial side is at base of figure in B through G. Scale bars ϭ 50 ␮m in B through H; scale bar ϭ 250 ␮m in A; scale bar ϭ 1 mm in I. A, Raceme type of inflorescence of Mimosa strigillosa with most bracts removed. All floral meristems are synchronous, at the same stage. B through G, Neptunia pubescens. B through F, Ontogenetic series showing simultaneous whorls of organs. B, Five sepals have initiated simultaneously. Note that the adaxial (uppermost) sepal and the sub- tending bract are on the median sagittal plane. C, Five petals have initiated simultaneously. The abaxial (lowermost) petal is on the median sagittal plane. D, The outer whorl of stamens is initiating simultaneously (three at arrowheads). E, The inner whorl of stamens is initiating simultaneously (three at arrowheads). F, Midstage after all floral organs are present and the carpel cleft is begin- ning adaxially, forerunner of the ovary locule. G, Valvate corolla, showing edge-to-edge temporary fusion of petals that will split apart at anthesis. H, Tubular fused calyx of Shrankia microphylla resulting from intercalary growth. This type of fusion is permanent. I, Floral bud of Calliandra houstoniana Standley (sepals and petals removed), showing specialized multiple stamens that are fused at their bases by intercalary growth. A, Outer-whorl stamen; a, inner-whorl stamen; B, bract; C, carpel; F, flower bud/floral apex; P, petal; S, sepal/calyx tube.

918 Plant Physiol. Vol. 131, 2003 Floral Development in Legumes the flower between Mimosoideae and the other two bat-pollinated tree genus Parkia (Polhill and Raven, subfamilies is a major developmental difference, and 1981, and refs. therein). one in which there are unlikely to be intermediate states or exceptions. It is difficult to see an adaptive value for either of these states in themselves, but an Petal Aestivation advantage may lie in associated or derivative char- Mimosoid petal aestivation is predominantly val- acters. The lack of a petal in the median adaxial vate (meeting edge-to-edge), without overlap (Fig. position in mimosoids may suppress in some way the 4G, N. pubescens). This pattern contrasts with imbri- tendency toward zygomorphy that prevails among cate (overlapping) petal aestivation in the other two papilionoids and many caesalpinioids. subfamilies. Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021

Number of Organs Organ Differentiation and Specialization Mimosoid flowers are either tetramerous or pen- Organs are all alike in each whorl in mimosoid tamerous. Flowers of tribes Mimoseae and Parkieae flowers, unlike many caesalpinioids. Within-whorl have eight or 10 stamens in two whorls. In tribes organs all develop similarly. Organ specializations Acacieae and Ingeae (Fig. 4I, Calliandra houstoniana among mimosoids include unisexual flowers in Standley), stamens are numerous. The genus Acacia Parkia spp. and some Mimoseae, glandular-tipped in Acacieae includes numerous multistaminate spe- anthers in Parkieae and some Mimoseae, a fused cies, i.e. Acacia myrtifolia Willd., with as many as 500 staminal tube in Ingeae, and polyad pollen grains in stamens per flower. Stamens proliferate on “common Acacieae and Ingeae. primordia” in Acacia baileyana F. Muell. (Derstine and Tucker, 1991). Common primordia are apical meristem sectors below each of the first five stamen pri- Organ Fusion mordia, on which additional stamens are initiated, giving a total of 30 or more per flower. Most mi- Fusions among organs, either temporary or perma- mosoids have a single carpel per flower, although nent, are prevalent among mimosoids. Temporary some taxa of Ingeae have multicarpellate flowers. edge-to-edge fusions most commonly involve sepals Reduction in organ number is uncommon among or petals (Fig. 4G). Permanent fusions occur occa- mimosoids, except for some taxa with only one whorl sionally in the calyx through intercalary growth (Fig. of four or five stamens, rather than two whorls. Uni- 4H, Shrankia microphylla [Dryander] MacBride), and sexual flowers also have reduced numbers of organs commonly among stamen filaments (Fig. 4I, C. hous- at anthesis, although generally the reduction results toniana) in tribe Ingeae as the result of intercalary from failure of either stamens or carpel to enlarge. growth. Stamens are initiated individually, but intercalary growth occurs in the receptacle below their bases so that all the stamens are raised on a ring and Organogenesis appear fused (Fig. 4I). The flowers of mimosoid taxa nearly all show simultaneous initiation of each whorl (Fig. 4, B–F, Nep- Comparison among Tribes tunia pubescens). The only exception is that sepals may be either simultaneous or helical, depending on Traditionally, the four tribes of Mimosoideae differ the taxon. Members of each whorl alternate with on morphological bases. Stamen number distin- those of the previous whorl. Among Mimosoideae, guishes Mimoseae and Parkieae (eight or 10 stamens developmental innovations such as ring meristems per flower; Fig. 4, E and F) from the other two tribes, and common primordia produce numerous addi- Acacieae and Ingeae (both with numerous stamens). tional organs beyond the 21 found in flowers of most The latter two are distinguished by free stamens in Caesalpinioideae and Papilionoideae. Large numbers Acacieae versus fused stamens in Ingeae. The Mi- of stamens per flower are initiated on ring meristems moseae have valvate sepals, whereas the Parkieae or common primordia in tribes Acacieae and Ingeae have imbricate sepals. (Fig. 4I, Calliandra houstoniana). Recent molecular-based analyses by Luckow et al.(2000, 2002) suggest that none of the four mimosoid tribes is monophyletic. Mimoseae is basal Unisexuality and paraphyletic, with taxa of the other tribes derived from within it. Parkia, the major genus in tribe Unisexual flowers are quite common among mi- Parkieae, is nested within tribe Mimoseae. Acacieae mosoids, particularly andromonoecy (Kalin Arroyo, and Ingeae are each polyphyletic; Acacia subgenus 1981). Three sexual floral morphs (male, female, and Acacia forms one monophyletic clade, whereas the neuter) occur in each inflorescence in Neptunia pubes- remainder of the huge genus Acacia, together with cens (Tucker, 1988a) and in species of the tropical, Ingeae, forms a second clade.

Plant Physiol. Vol. 131, 2003 919 Tucker

SUBFAMILY PAPILIONOIDEAE Dougl. (Fig. 1F) and Erythrina herbacea (Fig. 6, B and C), are typical examples of zygomorphic symmetry Papilionoideae is the largest of the three subfami- (Tucker, 1987a). Each flower has three petal forms: a lies, with 30 tribes, 455 genera, and about 12,000 single standard petal (or vexillum), two wing petals, species. It is a specialized monophyletic group that is and two keel petals (Fig. 5F, Genista tinctoria). The derived from within the caesalpinioid subfamily, petals are alike, however, throughout much of their based on morphological (Chappill, 1995) and molec- development (Fig. 5D), and only differentiate late in ular evidence (Doyle, 1995; Doyle et al., 2000). Most ontogeny (Figs. 1F and 5F). Other expressions of zy- taxa have the familiar “papilionoid” flower form gomorphy also are manifested at late stages, e.g. up- (Fig. 1F), although there are exceptions in tribes So- turning of the style and stamens, the horizontal posi- phoreae and Swartzieae. tioning of the entire flower, differential elongation of

sepal lobes, and formation of one or two “windows” Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 or fenestrae into the nectar-containing filament tube Inflorescences (Fig. 6G, Erythrina caffra Thunb.). Zygomorphy expressed late in floral development is very common, Inflorescences of Papilionoideae are usually especially among papilionoid taxa. racemes or panicles. In a raceme of Lupinus affinis Two papilionoid tribes, Sophoreae and Swartzieae, Agardh (Fig. 5A; Tucker, 1984a), the apical meristem include exceptions to the rest of the subfamily in initiates bracts in acropetal succession, and a floral floral form and uniform ontogeny. These exceptions bud develops in the axil of each bract. The number of include (in some but not all taxa) radial symmetry flowers per raceme depends on how long the apical (Pennington et al., 2000; Tucker, 2002b; Mansano et meristem is active. The flowers in racemes are initi- al., 2002), non-papilionaceous corollas, complete loss ated in succession and develop in succession, as in of some petal primordia, atypical petal aestivation, Caesalpinioideae. Hence, an individual inflorescence polystemony (resulting from an innovative develop- contains flowers of many different ages, the oldest at mental feature, the ring meristem), and lack of sta- the bottom, and younger ones above. men fusion. Representing taxa with radial symmetry, Two other kinds of inflorescences occur among Cadia purpurea Forsk. (Fig. 5E; Tucker, 2002b) in tribe Papilionoideae: pseudoracemes and cymes (rarely). Sophoreae has a uniform calyx, corolla, and andro- Pseudoraceme inflorescences (Tucker, 1987b) have ecium. Representing zygomorphic but nonpapilion- evolved in five tribes of Papilionoideae: Abreae, Des- oid symmetry is Swartzia (Fig. 5, G and H) in tribe modieae, Millettieae, Psoraleeae, and Phaseoleae. All Swartzieae, which has a completely fused calyx, a of these tribes are currently grouped in the “phase- single petal, and multiple stamens of two size classes. oloid” clade based on molecular evidence (for sum- The order of initiation of organs in Swartzieae also mary, see Doyle et al., 2000). Pseudoracemes (Fig. 5B) diverges from that of all other papilionoids. differ from racemes in that two to several flowers are initiated in each bract axil rather than just one as in a raceme. The cluster of flowers at each node is called Organ Positions a fascicle. The order of initiation among flowers at a node (Fig. 5B, Psoralea macrostachys DC) shows the Papilionoid flowers are pentamerous, with mem- fascicle to be a short shoot topped by a second order bers of each whorl alternating with those of adjacent inflorescence apical meristem. This meristem initiates whorls. The basic ground plan has the median sagit- flowers in a bilaterally symmetrical order: a single tal sepal on the lower or abaxial side, and the median abaxial flower, then two lateral flowers, another me- sagittal petal on the upper or adaxial side. This ar- dian abaxial, then two more laterals. The number of rangement is similar to that of Caesalpinioideae, but flowers per fascicle depends on the duration of the contrasts with that of Mimosoideae, in which the axillary inflorescence apex of the short shoot, which median sagittal sepal is on the upper or adaxial side. ceases activity after initiating the few flowers in the fascicle. No flowers are initiated adaxially (toward the first order axis) on the short shoot (Tucker, 1987b; Number of Organs Tucker and Stirton, 1991). The short shoot in a pseudoraceme can be distinguished from a cyme in that Most taxa of Papilionoideae have a full comple- every flower is bract subtended in a pseudoraceme. ment of organs: pentamerous alternating whorls of sepals, petals, two stamen whorls, and the single carpel. Where organs appear to be fewer, ontogeny Symmetry shows that all are initiated but that some organs are subsequently suppressed during development. Most Papilionoideae (in 28 of 30 tribes) have spe- Loss and increase in floral organ number are rare cialized zygomorphic flowers with papilionaceous among papilionoids, but both occur in one tribe, features that result from surprisingly uniform ontog- Swartzieae, a group of tropical trees. In Swartzia, the enies. Papilionoid flowers, such as Lupinus succulentus corolla is reduced to one petal (Fig. 5, G, S. sericea

920 Plant Physiol. Vol. 131, 2003 Floral Development in Legumes Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021

Figure 5. A through H, Floral initiation and specializations in Papilionoideae (SEM micrographs) Abaxial side is at base of figure in C, D, and F. Scale bars ϭ 100 ␮m in C, D, and G; scale bars ϭ 200 ␮m in B and F; scale bars ϭ 500 ␮minA,E, and H. A and B, Inflorescences with most bracts removed. A, Raceme of Lupinus affinis with flower buds developing successively and acropetally. Each flower is subtended by a bract. B, Pseudoraceme inflorescence of Psoralea macrostachys with three flowers in each bract axil. C, Floral bud of garden pea (Pisum sativum) showing overlap in time of initiation among whorls of sepals, petals, stamens, and carpel. All organ types have initiated on the abaxial side but only sepals on the adaxial side. A common primordium (at arrowheads) has initiated one stamen primordium and would have initiated two more primordia. D, Polar view of floral bud of Genista tinctoria at midstage with all organs initiated. Three of the inner stamen primordia are at arrowheads. The median sagittal sepal is on the abaxial (lower) side, and the median petal is on the adaxial (uppermost) side. E, Side view of flower bud of Cadia purpurea showing all petals of same size, none overlapping at this stage. F, Near-polar view of large bud of Genista tinctoria, sepals removed, to show descending cochleate aestivation of petals. G, Side view of flower bud of Swartzia sericea, showing single petal and ring meristem (at arrowheads), on which numerous stamen primordia have initiated. H, Older flower bud of Swartzia aureosericea, sepals removed. Flower has a single petal, three large stamens, about 100 small stamens (some at arrowheads), and a gynoecium. A, Outer-whorl stamen; a, inner-whorl stamen; Ap, inflorescence apical meristem; B, bract; C, carpel; F, flower bud/floral apex; G, gynoecium; K, keel petal; P, petal; S, sepal/calyx tube; V, standard or vexillum petal; W, wing petal.

Vogeli-Zuber; and H, S. aureosericea Cowan) or absent Organogenesis altogether. Numerous stamens (20–200) are initiated on a ring meristem, a circular ridge that is an inno- In most Papilionoideae, all floral organs are initi- vation in ontogeny in Swartzia (e.g. Fig. 5G; Tucker, ated in a unidirectional successive order in each 1988c). whorl starting on the abaxial side (next to the sub-

Plant Physiol. Vol. 131, 2003 921 Tucker

Figure 6. A through G, Specializations among Papilionoideae (SEM micrographs) including fusion, loss, and/or suppression of organs. Scale bar ϭ 200 ␮m in A; scale bars ϭ 500 ␮minC and E; scale bars ϭ 1mminB,D,F,andG.A through C, Floral buds of Erythrina herbacea showing petal diversification. A, Midstage (side view, sepals removed) when petal primordia are starting to differ in size. The vexillum or standard petal primordium (at arrowhead) is larger than the other petal primordia. B and C, Late-stage flower buds with three petal morphs differentiat-

ing as standard or vexillum, wing, and keel. Keels Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 have fused along their adjacent margins. D, Fused keel petals of Indigofera heteranthera with spurs. Hairs show the line of fusion. E through G, Three types of fusion in stamen filament tubes. E, Monadelphous androecium, with all filaments fused, in Lupinus affinis. F, Diadelphous androecium in Robinia hispida, with nine stamens fused and the 10th (at arrowhead) free. G, Pseudomonadelphous androecium in Erythrina caffra, showing all stamens fused, with the adaxial stamen having fused last and bordered by two fenestrae (at arrowheads). A, Outer-whorl stamen; A1 through A4, differing stamen morphs; C, carpel; K, keel petal; P, petal; S, sepal/calyx tube; V, standard or vexillum petal; W, wing petal.

tending bract). The ontogenetic series of papilionoids Later in development, stamen or carpel development resembles that shown in Figure 2, G through L, for a is suppressed, resulting in female or male flowers. caesalpinioid, except that all whorls have unidirec- Male sterility occurs in some cultivated species of tional order in papilionoids. Timing of one whorl Vigna, Lathyrus, and Lupinus (Kalin Arroyo, 1981). may overlap with that of the next among some papilionoids. For example, stamens commonly start to Petal Aestivation initiate before the last petals have been initiated. An extreme example is in garden pea (Fig. 5C; Tucker, Papilionoid petal aestivation is expressed as imbri- 1989; Ferra´ndiz et al., 1999), in which petal and sta- cate in a pattern called descending cochleate, in which men whorls as well as the carpel overlap in their time the standard petal margins overlap the adjacent wing of initiation. This flower also is unusual in having margins, which overlap the adjacent keel margins “common primordia,” ephemeral meristems that (Fig. 5F, Genista tinctoria). This pattern contrasts with each initiate two or three individual primordia. that of caesalpinioids (ascending cochleate, Fig. 2F) The pattern of organogenesis differs in papilionoid and mimosoids (valvate, Fig. 4G; no overlap). The tribe Swartzieae, which has a ring meristem on which pattern of overlap among petals is an easy way to numerous stamen primordia are initiated (Fig. 5, G distinguish the legume subfamilies. These distinctions and H, species of Swartzia; Tucker, 1988c). This genus in petal aestivation among subfamilies are stable and also is exceptional in having either a single petal are presumably highly canalized. How do these dif- initiated, or no petals. Most species have a single ferences arise in development? carpel but a few have several carpels, a rare condition A few taxa in Papilionoideae have a random pat- among legumes. tern of petal aestivation, e.g. Cadia purpurea (Fig. 5E) in tribe Sophoreae, and three taxa of the Lecointea Unisexuality group (Mansano et al., 2002), with currently uncer- tain affinity (Pennington et al., 2000), but closely Unisexual flowers are rare among papilionoids, but related to Sophoreae. The basis for this aberrant petal Ateleia herbert-smithii Pitt. in tribe Swartzieae (Tucker, aestivation appears to be that all petal margins in C. 1990, and refs. therein) is dioecious, with male and purpurea grow straight outward like the vexillar or female flowers on different trees. It is wind- standard petal in most papilionoids. In papilionoid pollinated, a rare feature among legumes. All flowers corollas with descending cochleate aestivation, over- of A. herbert-smithii initiate both stamens and carpels. lapping petal margins grow essentially straight out-

922 Plant Physiol. Vol. 131, 2003 Floral Development in Legumes ward, whereas those that will be overlapped curve amples include an enclosing calyx in bud, fused keel inward, due to slightly greater cell enlargement ab- petals, fused carpel margins that form the ovary lo- axially than adaxially (for details, see Tucker, 2002b). cule, and the fused stamen tube. How do these fu- In C. purpurea, chance determines which petal over- sions occur during development, and what adaptive laps another. Taxa with random aestivation such as advantages do they provide? C. purpurea provide an opportunity to determine the First, both edge-to-edge fusions and intercalary basis for genetic control of the prevailing descending growth fusions occur, as described for the other two cochleate petal aestivation among Papilionoideae subfamilies. Sepal, petal, and carpel margins fuse by and the ascending cochleate aestivation among appression of the edges, interlocking between epider- Caesalpinioideae. mal cells, and (often) subsequent cell division (Tuck- er, 1987a). Edge-to-edge fusions between keel petals are common among papilionoids (Fig. 6, B and C, E. Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 Organ Differentiation and Specialization herbacea; and D, I. heterantha), and these fusions gen- Specializations among papilionoid flowers include erally persist during the short life of the petals. Keel different types of inflorescences, some asymmetry, enclosure of the stamens and gynoecium is one of the elaborations of petals and stamens, heterogeneous mechanisms that restricts the type of pollinator and whorls, and fusions among petals and among sta- its activity to species-faithful insects. Carpellary mar- mens. Same-whorl organs often differentiate as dis- ginal fusion usually involves cell division and en- similar morphs, especially in papilionoid flowers largement that obscure the line of contact and is with zygomorphic symmetry. How do these dissim- permanent until fruit ripening. ilarities arise in development, and at what stage do Permanent fusion due to intercalary growth is re- they first appear? In the papilionoid Erythrina herba- sponsible for the fused stamen sheaths of papilionoids cea, petal primordia are uniform in size up through (Tucker, 1987a). Ten stamen primordia are initiated in midstage, when size differences first become evident two successive, alternating whorls of five (Fig. 5D, (Fig. 6A). The standard or vexillum petal (at arrow- Genista tinctoria). They become reoriented into a single head in Fig. 6A) is slightly taller than the wing and whorl as the receptacle enlarges, and their filaments keel petals at midstage. In late stages, the petals have then become fused into a tube (Fig. 6E), with the taken on different shapes, symmetries, and degree of anthers and upper parts of filaments free. This fusion fusion. The standard or vexillum petal enlarges occurs by elongation in the receptacle below the bases greatly and envelops the rest of the flower, but re- of the stamens, so that they are raised up on a cylin- mains bilaterally symmetrical (Fig. 6, B and C). The drical or tubular base. The stamen primordia them- two wing and two keel petals remain small and have selves do not undergo fusion. Papilionoid filament each become asymmetrical. The two keel petals have tubes are of three kinds: monadelphous, diadelphous, attenuate tips, and their adjacent margins become and pseudomonadelphous. In the monadelphous type fused (Fig. 6, B and C). At anthesis in E. herbacea, only (Lupinus affinis Agardh, Fig. 6E), the cylinder includes the standard petal, 3 to 5 cm long, is visible; it wraps all 10 stamens and is closed; in the diadelphous type around the other petals that are 0.5 to 1.3 cm long. (Robinia hispida, Fig. 6F), only nine stamens are fused These changes in petal size and form are expressions and one is free. The free stamen is always the median of zygomorphy and are manifested late in develop- adaxial one, and its separate base permits insect access ment of the flower. to the nectary between the carpel and stamen bases. Petals of other papilionoids may show additional A diadelphous androecium may undergo a further changes in form: auricles, transverse ridges, folds, elaboration during ontogeny to form the third type, a pegs, and corresponding pits that interlock petals pseudomonadelphous cylinder (Erythrina caffra, Fig. together, even though they may not be physically 6G; Tucker, 1987a). Edge-to-edge fusions occur be- fused. Papilionoid petals may have elaborate tween the free stamen filament and the sides of the pouches, knobs, ridges, and other elaborations. An adjacent filament tube, so that it becomes a continu- example is shown in the inflated fused keel petals of ous tube. Fusion is not complete, however, at the Indigofera heteranthera Wall. (Fig. 6D) that also have base of the filaments. In fact, two “holes” or fenestrae spurs. In many papilionoids, the knobs and pits be- enlarge at these points, which facilitate entry of a tween adjacent petals interlock to form a tube-like bee’s proboscis into the area of the nectary. Thus, this corolla that encloses the stamens, gynoecium, and androecium is called pseudomonadelphous because nectary, thereby restricting the type of pollinator and it mimics a monadelphous androecium, but results controlling its path of entry to pollen and nectar. from a different ontogeny. The stamen tube charac- terizes papilionaceous floral form and restricts or Organ Fusion controls pollinator behavior. The stamen tube has evolved via at least three different ontogenetic path- Fusions are especially common among floral or- ways among papilionoid legumes. gans of papilionoids. Although all floral organs are Selective fusion, by intercalary growth in the recep- initiated singly, many show fusion at anthesis. Ex- tacle and/or by edge-to-edge fusion, thus determines

Plant Physiol. Vol. 131, 2003 923 Tucker the type of androecium in papilionoids. Some papil- papilionoid and caesalpinioid legumes that have over- ionoids such as those in tribes Sophoreae and Swartz- lapping whorls. The ABC model also does not explain ieae generally lack fusion among stamens, and are the concurrent initiation of the carpel at the same time adapted to different kinds of pollinators. as petals or stamens, which is usual in legumes. I know of no legume in which the carpel is initiated last, after all stamens are present. In several respects, le- Comparison among Papilionoid Tribes gume flowers fail to conform to the ABC model. Much more molecular systematic work has been An intriguing array of mutant genes that control done on Papilionoideae than on either of the other two various aspects of normal floral ontogeny in legumes subfamilies. Only a brief summary of current opinion have been discovered. Genes control the transition about relationships can be offered here. Several major from inflorescence to flower, as well as affecting floral monophyletic groups or clades appear consistent organ expression (Singer et al., 1999; Hirsch et al., Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 among Papilionoideae, based on molecular evidence 2002; Taylor et al., 2002). Organ initiation in pea in- (for summary, see Doyle, 1995; Doyle et al., 2000). A volves common primordia, an extra step inserted dur- “genistoid” clade includes tribes Genisteae, Thermop- ing ontogeny. Each common primordium initiates two sideae, Crotalarieae, Podalyrieae, and Sophoreae pr. or three individual organ primordia (Tucker, 1989). p. An “aeschynomenoid” group includes tribes Ae- Ferra´ndiz et al. (1999) examined floral identity genes schynomeneae, Adesmieae, Desmodieae pr. p., and as well as mutations of A, B, and C classes in pea. They Dalbergieae pr. p. The “galegoid” clade (Hologale- concluded that A, B, and C factors specify organ iden- gina) is the largest papilionoid clade and is comprised tity in common primordia as well as in organ primor- of most of the temperate herbaceous tribes (Car- dia, although these factors do not control floral deter- michaelieae, Cicereae, Coronilleae, Galegeae, Hedys- minacy or organ number unlike some of the gene areae, Loteae, Trifolieae, and Vicieae). A “phaseoloid” homologs in Arabidopsis and Antirrhinum spp. Com- clade includes tribes Desmodieae pr. p., Indigofereae, mon primordia are quite rare among legumes and Millettieae, Phaseoleae, and Psoraleeae. Two Austra- probably represent an evolutionary specialization. lian tribes, Mirbelieae and Bossieae, form a clade. Sev- Most other papilionoid legumes (e.g. Lupinus affinis, eral other small tribes remain unresolved as to their Tucker, 1984a, 1984b; Melilotus alba, Hirsch et al., 2002) relationships. Neither Swartzieae nor Sophoreae, the share an unusually uniform pattern of organogenesis two anomalous tribes with non-papilionoid flowers, is and initiate organs directly without a common- monophyletic. Their status is currently in flux, as dis- primordial stage. This contrast in pattern of organ cussed by Pennington et al. (2000). initiation offers an opportunity to compare control of floral organ initiation with and without the stage involving common primordia. WHAT CAN WE LEARN FROM LEGUME Although research on the ABC model has concen- FLORAL ONTOGENY? trated on two angiosperms (Arabidopsis and Anti- Does Legume Flower Development Follow the rrhinum majus) belonging to the eudicots (the most ABC Model? highly derived group of dicotyledons), 17 basal angiosperms have recently been investigated for the B The successive and overlapping order of organ class genes that control petal and stamen identity initiation in some legume flowers is intriguing devel- (Kramer and Irish, 2000). Many of these basal or opmentally because of its conflict with prevailing lower dicots have only one perianth whorl (tepals), interpretations of hypotheses concerning timing of which have long been thought to be derived from determination of organ identity. The ABC model hy- modified leaves, in contrast to petals of higher dicots, pothesis of floral organ identity (Meyerowitz et al., which are considered to be evolved from stamens. 1991; Irish, 1999; Jack, 2001) applies to flowers such The basal angiosperms and lower dicots have as Arabidopsis, in which all organs of a whorl initiate PISTILLATA and a form of the APETALA gene simultaneously, the order of initiation is sepals, pet- (PaleoAP3) expressed in the tepals, although the tim- als, stamens, and carpels, and where whorls do not ing and degree of expression varied considerably overlap in time of initiation. It proposes that certain among the basal taxa. sets of genes or gene combinations act in the floral apex in succession, determining the type of organ being initiated. “A” alone produces sepals, “A” ϩ ϩ What Does Comparison of Floral “B” produces petals, “B” “C” produces stamens, Ontogenies of Numerous Related Taxa Tell and “C” alone produces carpels. This hypothesis ap- Us about Evolution in Legumes? plies only to organ identity determination and the order in which whorls occur in the flower and does Unspecialized floral structure, used as a basis for not explain timing within whorls or location of or- systematic relatedness in several groups of Fabaceae, gans within the whorl. It does not satisfactorily ex- appears deceptive in several cases. For instance, sim- plain a system in which more than one type of organ ple floral organization and lack of floral specializa- is being initiated at the same time, such as that in tions characterize members of tribe Caesalpinieae of

924 Plant Physiol. Vol. 131, 2003 Floral Development in Legumes subfamily Caesalpinioideae (Polhill and Raven, 1981; homogeneous versus heterogeneous whorls, and uni- Kantz, 1996). Similarly, the tribe Sophoreae includes sexuality in some taxa. the taxa of subfamily Papilionoideae that have the fewest floral specializations. However, recent molecular-based phylogenetic analyses (Doyle, 1995; Hierarchical Theory Doyle et al., 2000; Bruneau et al., 2001) indicate that The framework of this research is a systematic and neither Caesalpinieae nor Sophoreae are monophy- phylogenetic one rather than experimental. The em- letic groups, but instead each includes several di- phasis is on exploring concepts of floral diversifica- verse, distantly related groups. Unspecialized flow- tion and how such changes are produced during ers in legumes tend to share similar ontogenies development, expressed within subfamilies, tribes, (Kantz, 1996), although these taxa usually have a few genera, and species. Floral development in over 300 specialized character states that serve to distinguish representative legume taxa has been studied and Downloaded from https://academic.oup.com/plphys/article/131/3/911/6111090 by guest on 01 October 2021 them. Comparisons in such taxa demonstrate that compared using scanning electron microscopy (SEM) they share simple straightforward pathways of to date (124 Papilionoideae, 136 Caesalpinioideae, development. and 49 Mimosoideae). A hypothesis has been pro- Most legume flowers, both unspecialized and spe- posed (Tucker, 1984a, 1997) that a correlation exists cialized, share a similar ontogenetic pathway, at least between the timing of character expression and the for the early stages during organ initiation and mid- hierarchical level at which it is significant. For exam- stages as development begins. For example, flowers ple, order of initiation and positions of organs are of Papilionoideae share early and midstages of floral determined early in ontogeny (during organogene- ontogeny, whether they will become zygomorphic or sis). Both are significant at the level of subfamily. In remain radially symmetrical. Specialized flowers, in contrast, characters such as petal fusions, petal contrast, have extra developmental steps added in shapes, and absolute or relative size differences are late stages of ontogeny that produce the specializa- determined late in ontogeny, and these distinguish tions. Some examples include differing petal shapes taxa at the level of species. of the “papilionoid” or “flag” flower, fusion of stamen filaments into a tube, and poricidal anther Trends in Legume Evolution dehiscence. Shared assemblages of specialized characters gen- Developmental differences among suprageneric erally are viewed as evidence of close evolutionary taxa (subfamilies and tribes) have been emphasized relationships. Nevertheless, in one genus, Cassia here to show evolutionary trends in Fabaceae. Two of sensu lato in the caesalpinioid tribe Cassieae, shared the three traditional subfamilies, Mimosoideae and specializations are deceptive. Howard S. Irwin (Long Papilionoideae, are clearly monophyletic and have Island University, Greenvale, NY) and Rupert C. been derived from the third subfamily, Caesalpin- Barnaby (New York Botanical Garden, Bronx, NY) ioideae, which is basal and paraphyletic (Doyle, 1995; (Polhill and Raven, 1981, and refs. therein) split Cas- Doyle et al., 2000; Bruneau et al., 2001). sia into three genera: Cassia sensu stricto, Chamaec- The systematic diversity among legumes is re- rista, and Senna. These are superficially similar and flected in diverse floral ontogenies. Although devel- share many specialized characters such as yellow opmental differences among legumes have been em- flower color, pentamerous corolla, zygomorphy, dor- phasized here, they share a basic ground plan and a siventral heterostemony, and poricidal stamen dehis- basic pattern of floral ontogeny. Parallel current re- cence. Comparison of floral ontogenies (Tucker, search on molecular systematics of legumes greatly 1996) showed marked developmental differences in enhances the developmental investigations by pro- their inflorescence architecture, phyllotaxy, bracteole viding a phylogenetic framework that helps to make formation, order of organ initiation, amount of over- sense of developmental trends. lap in time between whorl initiations, the basis for asymmetry, and the basis for poricidal dehiscence. ACKNOWLEDGMENTS These differences in development strongly suggest that the superficial similarities in the flowers have The author thanks Jo Anna Bass (University of California, Santa Barbara) and former students Andrew Douglas (University of Mississippi, Universi- resulted from evolutionary convergence. ty), Katherine Kantz (Grand Valley State University, Allendale, MI), Eliza- Comparative floral ontogeny has demonstrated its beth Harris (Ohio State University, Columbus), and Jose´ Ramirez- usefulness in showing shared similarities in a basic Dome´nech (Xavier University, New Orleans) for their technical assistance ground plan throughout the monophyletic Fabaceae. with scanning electron microscopy and photography; Jan Beckert (Santa Barbara Botanical Garden, CA) for drawings; and the following for collec- It also has shown the developmental basis for signif- tions: Frans J. Breteler and Jan J. Wieringa (University of Wageningen, The icant differences in floral form. These modifications Netherlands), Gurilym Lewis (Royal Botanic Gardens, Kew, UK), and Bente of the basic ground plan result in different inflores- Klitgaard (Museum of Natural History, London). cence types, differences in symmetry, petal aestiva- Received November 11, 2002; returned for revision December 4, 2002; ac- tion, loss and increase in number of parts per flower, cepted December 26, 2002.

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