Sex Reprod (2001) 14:3Ð7 © Springer-Verlag 2001

REVIEW

M. Herrero signals for directional tube growth

Received: 15 December 2000 / Accepted: 13 June 2001

Abstract In angiosperms, the female has a work has focussed on growth along the stig- secluded life; it is protected by several concentric layers ma and style, the ovary has been neglected; it has often that envelop each other. The sac is surrounded been assumed that once the pollen tubes arrive at the by the nucellus, which in turn is wrapped by the integu- base of the style occurs. However, informa- ments forming the , which is nested in the ovary. tion is converging toward the concept (Herrero 2000) These wrappings are not hermetic, but contain little that the process is far from straightforward, and that in “gates” the pollen tube must traverse on its way towards the ovary, also, an interesting male-female interaction the embryo sac. Information is emerging that shows that occurs. the ovary and ovule provide signals orienting and direct- A close look at the ovary shows that the female game- imng the pollen tube on the right course. There are three tophyte has a secluded life, protected by a number of main bodies of evidence supporting this hypothesis. One concentric layers that consecutively envelop each other. relates to developmental changes in the female tissues Thus, the embryo sac is surrounded by the nucellus, and how they affect pollen tube growth. The second re- which in turn is wrapped by the integuments forming the fers to defective ovule mutants, which induce defective ovule, which is nested in the ovary. These structures are pollen tube guidance. And the third relates to the possi- highly conserved in flowering , and in spite of ear- ble molecules involved in this signalling. Here, informa- ly descriptions little attention has been devoted to their tion gathered along these three main lines of evidence is possible role. It is assumed that they have a protective reviewed. All converge to the conclusion that different function, first of the embryo sac and later of the develop- checkpoints exist all along the pollen tube pathway. ing embryo. However, these wrappings are not hermetic; These checkpoints provide active signalling that guides they provide little “gates” the pollen tube must traverse the pollen tube to its destination, the embryo sac. on its way toward the female . The pollen tube pathway in the ovary has been described in a number of Keywords Ovary á Ovule á Embryo sac á Pollen tube species, but there is little evidence showing how this trail growth is blazed. Information is emerging that indicates the ova- ry and the ovule provide signals that orient and direct the pollen tube to set the right course. There are three main Introduction bodies of evidence supporting this point. One relates to developmental changes in the female tissues and how The pistil is a clear stage for male-female interaction they affect pollen tube growth. The second refers to de- (Knox 1984). It plays a significant role in controlling fective ovule mutants, which induce defective pollen pollen tube growth (Linskens 1986; Mascarenhas 1993), tube guidance. Finally, the third relates to the possible fostering self-incompatibility mechanisms (Heslop- molecules involved in this signalling. Here, information Harrison 1983; Dickinson 1995) and favouring pollen gathered about these three main lines of evidence is re- competition and selection (Mulcahy 1979). Also, the en- viewed. tire pistil has been viewed as having a protective and prophylactic role (Heslop-Harrison 2000). While most Developmental changes in the ovary M. Herrero and ovule play a part in pollen tube guidance Unidad de Fruticultura, SIA-DGA, Campus de Aula Dei, Apartado 727, 50080 Zaragoza, Spain e-mail: [email protected] The idea that particular developmental stages have to be Fax: +34-976-716335 attained before the pistil is competent to receive the male 4 gametophyte appears to be universal in the reproductive number of different species (for reviews see Tilton 1980 process. Once the pollen grain germinates at the and Sage et al. 1994). Little is known about the possible and the pollen tube reaches the base of the style, the pol- functions of this secretion, although it is tacitly assumed len tube tip alights on the placental surface and crawls that, somehow, it is needed for pollen tube growth. on its surface until it eventually turns towards an ovule, While these changes occur in the sporophytic tissues, penetrates the ovule and achieves fertilisation. In some the megagametophyte itself also plays a role in pollen species and circumstances, pollen tube growth proceeds tube growth. Little is known about changes in the em- smoothly. In these cases, although changes can be ob- bryo sac and how they relate to pollen tube penetration, served in the pistil tissues, it is difficult to establish the but the main candidate as signal provider for pollen tube influence that these changes have on pollen tube growth. penetration is the synergid. While the precise role syner- But, in other instances, accelerations and decelerations gids play is still to be discovered, it is apparent that they of pollen tube growth occur along this path (Herrero and are involved in pollen tube penetration to the embryo Arbeloa 1989). Furthermore, in some instances pollen sac, since the pollen tube consistently enters through a tubes navigate in a chaotic way, lose direction and do not synergid (Jensen 1965). In addition, experiments with in achieve fertilisation (Herrero 2000). These apparently vitro fertilisation have shown that pollen tubes do not abnormal situations provide a useful tool for detecting arrive at embryo sacs containing disrupted synergids what is happening in the pistil and how these changes re- (Higashiyama et al. 1998). Likewise, the presence of a late to pollen tube behaviour. complete egg apparatus appears to be a requisite for pol- Information is accumulating that shows there is not a len tube attraction (Shimizu and Okada 2000). As the single “checkpoint”, but that several structures all along synergid degenerates, two actin coronas develop, one at the pollen tube pathway are involved in pollen tube regu- the chalazal end of the degenerating synergid and anoth- lation. So far, evidence is dispersed and fragmented, il- er at the interface between the and central cell. lustrating different cases in different species. The placen- The formation of these coronas is concomitant with pol- ta often has little bumps that face the physically len tube arrival and appears to be required for effective directing the pollen tube to the ovule entrance (the exos- migration of the sperm cells towards the egg cell and po- tome). In some species these bumps are called obtura- lar nucleus to achieve double fertilisation. (Huang and tors. The term was described early (Juel 1918) and Russell 1994; Russell 1996). reported in a number of unrelated species (Tilton and Thus, developmental changes occur all along the pol- Horner 1980). In these species the pollen tubes consis- len tube pathway in the ovary. All of this dispersed infor- tently traverse along the obturators to reach the ovules. mation appears to share a common idea, and this is that In peach (Prunus persica), the obturator, far from being events occurring along with pistil developments have a a passive structure, plays a clear part in regulating pollen bearing on pollen tube development. tube access to the ovule. When the pollen tubes arrive at the obturator, growth is arrested and does not resume un- til some 5 days later, when the obturator enters a secreto- Mutants with distorted pollen tube guidance ry phase (Arbeloa and Herrero 1987). Interestingly, in ki- wi (Actinidia deliciosa), a species where smooth and While the developmental approach shows that time plays rapid pollen tube growth occurs, obturators also lie along a major role in pollentube growth and that for successful the placenta, one facing each ovule. However, in this fertilisation male-female synchronism is required, defec- species a secretion is present in the obturators right from tive mutants provide a genetic dissection of the process- anthesis and the pollen tubes swiftly grow along on them es involved. Analysis of defective mutants which have to reach the ovules (Gonzalez et al. 1996). In Zea mays, reduced fertility due to failures in pollen tube guidance is a similar mechanism has been described. Papillar hairs an incipient field. However, it is surely going to clarify cover the ovary entrance. As the pollen tubes pass by, how the reproductive process is controlled, and also these hairs lose turgidity and prevent other pollen tubes which genes play clear roles in species isolation. The from entering the ovary (Heslop-Harrison et al. 1985). first evidence showing that a normal megagametophyte These mechanisms appear to act as drawbridges regulat- was required for directional pollen tube growth came ing pollen tube access to the ovule at a particular time. through the use of defective mutants that failed to devel- Once the pollen tubes traverse the placenta and face op an embryo sac and also failed to attract the pollen the ovule, further control appears to be exerted; ovule tube (Hulskamp et al. 1995). However, since these mu- penetration appears to be a chemotropic response tants were sporophytic, it could not be ruled out that the (Mascarenhas 1978). In peach, it has been described that loss of pollen tube guidance was due to a defect in the in some ovules the pollen tubes wander around the ovule sporophytic (Hulskamp et al. 1995; Drews et al. exostome, while in other ovules penetration occurs. Pol- 1998). To overcome this difficulty Ray et al. (1997) tried len tube penetration is concomitant with the production a different approach by using a reciprocal chromosomal of a secretion in the cells that line the exostome and fail- translocation to generate wherein approximately ure in the production of this secretion prevents pollen 50% of the ovules did not contain a functional female tube penetration (Herrero 2000). A secretion of unclear gametophyte, while at the same time no alterations were origin has also been reported in the micropylar canal in a expected in sporophytic tissue. In these flowers pollen 5 tubes were guided to normal, but not to abnormal, fe- fold the amount of calcium contained in the upper style male . The same strategy and similar re- (Mascarenhas and Machlis 1962b, 1964). Further work sults were obtained with a different approach, the use of has come to support a clear role of calcium in pollen meiotic mutants in which most ovules failed to go tube growth. Calcium is involved in pollen tube growth through meiosis (Couteau et al. 1999). Finally, by in vitro (Malho 1998) and it also plays a clear part in screening siliques which contained 50% of the normal pollen tube growth in vivo (Frankin-Tong 1999). The number of , several mutants have been recovered pollen tube pathway in the ovary is also layered with (Shimizu and Okada 2000). In two of these mutants, in loosely bound calcium (Tian and Russell 1997) and this which female gametophyte development was delayed, calcium, while maintained in unpollinated , is de- pollen tube guidance was affected. While pollen tubes pleted as the pollen tubes pass by (Tian and Russell were directed towards ovules, they lost their way just be- 2000). Finally, high levels of calcium have been record- fore entering the micropyle and elongated in random di- ed in synergids (Chaubal and Reger 1990), and it has rections. In addition, these ovules often attracted two been suggested that this calcium might be involved in pollen tubes (Shimizu and Okada 2000). A similar situa- pollen tube attraction (Reger et al. 1992). While a com- tion was recorded in maize, where mutants with two or plete picture of the role of calcium in the reproductive more degenerated synergids containing actin coronas at process is still to emerge, all of this information supports their chalazal ends received multiple pollen tubes and early reports on the chemotropic effect of calcium produced multiple (Huang and Sheridan 1998). (Mascarenhas and Machlis, 1962b, 1964). From all of this evidence, it appears clear that the mega- However, calcium appears not to be the single protag- gametophyte plays an active part in providing signals for onist (Heslop-Harrison 1986) and other molecules as correct pollen tube penetration, but open the pos- well appear to be involved in the signalling events taking sibility (Ray et al. 1997) that the gametophyte may ac- place within the ovary. The search for signalling mole- complish this indirectly through its influence on some cules that play a part in pollen-pistil interaction is an ac- sporophytic cells. tive field in which considerable progress is being made. The does indeed appear to play a role in A number of reviews have recently focussed on this sub- pollen tube direction, whether directly or indirectly. A ject (Cheung 1996; Franklin-Tong 1999; Wilhelmi and sporophytic genetic control of fertilisation has been Preuss 1999; Lord 2000) and the emerging picture is far shown in Arabidopsis (Wilhelmi and Preuss 1996). from simple. This is not surprising, considering the com- Ovules with apparently normal gametophytes failed to plexity of male-female interaction and the fact that these attract pollen tubes in particular genetic combinations. molecules have to fulfil an array of functions from rec- This was due to a failure in pollen tube adhesion to the ognition, to adhesion, to nutrition and attraction. Work placenta. It has been argued, though (Smyth 1997), that has been performed mainly in the stigma and style, but it the genes involved here may not be controlling guidance is to be expected that some of these molecules have to be per se, but are a precondition that is required for it to op- conserved in the ovary. erate. A role played by the sporophyte is also supported Recognition molecules have been researched mainly by the study of ovule developmental mutants (Gasser et in pollen-pistil incompatibility systems (Dickinson al. 1998), and with the use of transformed plants with 1995), where kinases appear to play a pivotal role antisense constructs for ACC oxidase (de Martinis and (Nasrallah et al.1994). On the other hand, the pollen tube Mariani 1999). These plants had underdeveloped ovules must adhere to pistil cells, and adhesive molecules have that failed to attract pollen tubes, illustrating the fact that recently been identified (Lord 2000). But the pollen tube particular stages of development have to be attained for also needs to derive nourishment, and pollen tube growth the ovule to attract pollen tubes. in the style proceeds in a heterotrophic way at the ex- pense of the stylar reserves (Herrero and Dickinson 1979). A number of pistil-specific glycoproteins have Signalling molecules been identified. Some of them play a nutritive role since they can be deglycosylated by growing pollen tubes (Wu Mascarenhas (1978) pointed out that, since pollen tube et al. 1995) and are incorporated into actively growing growth along the style is dependent on the style, the pollen tubes (For a review see Cheung 1996). missing component could act as the chemotropic factor, The specific involvement of these molecules in sig- but that from placenta to ovule a proper chemotropic fac- nalling events within the ovary remains to be elucidated. tor is required. A number of molecules appear as poten- But it is plausible that some of them may acquire a spe- tial candidates for chemotropic factors and the search cial relevance when they are missing, as in defective mu- and debate on chemotropism is still an active field (Lush tants. The same situation applies in systems, as the one 1999). Mascarenhas and Machlis (1962a) designed a bio- described in peach (Herrero 2000), in which particular assay to test pollen tube chemotropism. Pollen tubes secretions along the pollen tube pathway are required for were attracted toward ovules but, surprisingly, also to- the pollen tube to proceed toward the embryo sac. ward other tissues. These authors showed that the myste- rious molecule that caused a chemotropic response was calcium and that the ovules and placenta contained four- 6 Perspectives which fulfils a particular function at a certain time. Time plays a primary role and differences in time may lead to The different anatomical structures involved in pollen completely different results. The question remains about tube pathway within the ovary are highly conserved, and how these different checkpoints coordinate to work in the same appears to hold for what we know of the mech- sequence and to appear on the scene, each one at the anisms fostered in these structures. This uniform appear- right time, concomitant with pollen tube arrival. Do sig- ance contrasts with the fact that the system provides a nals at a particular checkpoint trigger the set up of the good stage for finely tuned male-female recognition pro- next one, or do they all respond to a general coordina- cesses, which could be at the base of the speciation pro- tion? Likewise, the question remains about how signals cess. The “tip of the iceberg” for the molecules involved at both the gametophytic and the sporophytic level inter- has started to show. While some of these molecules have act. This situation is further complicated if we take into been characterised mainly at the stigma and style, the account that fertilisation occurs with the two at ovary presents a unique opportunity to test the relevance the same cell cycle phase (Friedman 1999). And the of these molecules for pollen tube growth. This is be- question remains about how cell-cycle genes may inter- cause the various structures within the ovary provide an act with what is happening in the outside world to coor- on-and-off system, in which the pollen tube depends on dinate arrival and fertilisation at the right time. particular pistilar secretions to continue growing. While information to date is fragmented and dis- While work with mutants that have defective pollen persed, all converges to the idea that active signalling oc- tube guidance is still incipient, the involvement of differ- curs in the ovary that guides the pollen tube to its desti- ent groups in this field over the last few years foretells nation, the embryo sac. The merging of different ap- rapid development of this field in the future. The recent proaches from different fields and perspectives will elu- description of mutants with delayed development of the cidate the last, hidden and private phase of male-female embryo sac (Shimizu and Okada 2000) may form a interaction in plants. bridge between genetic dissection and developmental studies. It is clear that to support pollen tube growth Acknowledgements I want to thank Professor Jopseph Mascarin- the pistil must attain a particular stage of development has for his advice, encouragement and support. Thank to J. Sanzol, A. Hedhly, J. Rodrigo and J.I. Hormaza for valuable comments on (Kandasamy et al. 1994). However, upon opening the manuscript. Finacial support was provided by INIA (project this development is not complete, flower development is grant SC 98Ð049). a continuous process that proceeds throughout the flow- er’s life span. Pistil maturation occurs in a basipetal way, starting at the stigma and proceeding down to the ovary References (Herrero and Arbeloa 1989). The different parts of the pistil appear to come into view at particular times to fu- Arbeloa A, Herrero M (1987) The significance of the obturator lfil particular tasks (Herrero and Hormaza 1996), and in the control of pollen tube entry into the ovary in peach male-female synchronism appears to be a prerequisite (Prunus persica). Ann Bot 60:681Ð685 Chaubal R, Reger BJ (1990) Relatively high calcium is localised for a successful fertilisation. in synergid cells of wheat ovaries. Plant Reprod 3:98Ð102 It remains to be seen what the biological significance Cheung AY (1996) Pollen-pistil interactions during pollen-tube of this finely tuned signalling is and whether these signals growth. Trends Plant Sci 1:45Ð51 may also exert a genetic control. A number of both inter- Couteau F, Belzile F, Horlow C, Grandjean O, Vezon D, Doutriaux MP (1999) Random chromosome segregation without meiotic specific (Williams et al. 1982) and intraspecific (Seavey arrest in both male and female meiocytes of a dmc1 mutant of and Bawa 1986; Sage et al. 1994) barriers have been de- Arabidopsis. 11:1623Ð1634 scribed in the ovary. Early work in Oenothera, reported Dickinson HG (1995) Dry stigmas, water and self-incompatibility selective fertilisation (Schwemmle 1968). Since then, in- in Brassica. Sex Plant Reprod 8:1Ð10 Drews GN, Lee D, Christensen CA (1998) Genetic analysis of fe- formation has accumulated that calls into question wheth- male gametophyte development. Plant Cell 10:5Ð17 er mate choice exists in plants (Willson and Burley 1983; Franklin-Tong VE (1999) Signaling and the modulation of pollen Marshall and Folsom 1993). Mate selection is an attrac- tube growth. Plant Cell 11:727Ð738. tive possibility that has important evolutionary conse- Friedman WE (1999) Expression of cell cycle in sperm of quences. While the peculiar cytogenetics of Oenothera Arabidopsis: implications for understanding patterns of game- togenesis and fertilization in plants and other eukaryotes. De- make it relatively easy to demonstrate this phenomenon velopment 126:1065Ð1075 in this species, it may prove difficult to evaluate in other Gasser CS, Broadhvest J, Hauser BA (1998) Genetic analysis of species. The main body of evidence is based on distorted ovule development. Annu Rev Plant Physiol 49:1Ð24 segregations in the offspring produced in particular cross- González MV, Coque M, Herrero M (1996) Pollen-pistil interac- tion in kiwifruit (Actinidia deliciosa; Actinidiaceae). Am J Bot es, but so far we lack the physiological support for under- 83:148Ð154 standing mate selection. However, as Mulcahy et al. Herrero M (2000) Changes in the ovary related to pollen tube (1992) put forward knowing that it may exist at all sug- guidance. Ann Bot [Suppl A] 85:79Ð85 gests that the search seems worth undertaking. Herrero M, Arbeloa A (1989) Influence of the pistil on pollen tube kinetics in peach (Prunus persica). Am J Bot 76:1441Ð1447 From all of this research it appears clear that different Herrero M, Dickinson HG (1979) Pollen-pistil incompatibility in checkpoints exist all along the pollen tube pathway. Petunia hybrida: changes in the pistil following compatible These checkpoints lie in particular structures, each of and incompatible intraspecific crosses. J Cell Sci 36:1Ð18 7 Herrero M, Hormaza JI (1996) Pistil strategies controlling pollen Mascarenhas JP, Machlis L (1962a) Chemotropic response of An- tube growth. Sex Plant Reprod 9:343Ð347 tirrhinum majus pollen to calcium. Nature 196:292Ð293 Heslop-Harrison J (1983) Self-incompatibility: phenomenology Mascarenhas JP, Machlis L (1962b) The pollen tube chemotropic and physiology. Proc Roy Soc London Ser B 218:371Ð395 factor from Antirrhinum majus:bioassay, extraction, and par- Heslop-Harrison J (1986) Pollen tube chemotropism:fact or delusion. tial purification. Am J Bot 49:482Ð489 In: Cresti M, Dalai R (eds) Biology of reproduction and cell mo- Mascarenhas JP, Machlis L (1964) Chemotropic response of the tility in plants and animals. University of Siena, Siena Italy pollen Antirrhinum to calcium. Plan Physiol 39:70Ð77 Heslop-Harrison J, Heslop-Harrison Y, Reger BJ (1985) The pol- Mulcahy DL (1979) The rise of angiosperms: a genecological fac- len-stigma interaction in the grasses. VII. Pollen tube guidance tor. Science 206:20Ð23 and the regulation of tube number in Zea mays L. Acta Bot Mulcahy DL, Mulcahy GB, Searcy KB (1992) Evolutionary ge- Neerl 34:193Ð211 netics of pollen competition. In: Wyatt R (ed) Ecology and Heslop-Harrison Y (2000). Control gates and micro-ecology: The evolution of , Chapman and Hall, New York pollen-stigma interaction in perspective. Ann Bot [Suppl A] Nasrallah JB, Stein JC, Kandasamy MK, Nasrallah ME. (1994) 85:5Ð13 Signalling the arrest of pollen tube development in self-incom- Higashiyama T, Kuroiwa H, Kawano S, Kuroiwa T (1998). Guid- patible plants. Science 266:1505Ð1508. ance in vitro of the pollen tube to the naked embryo sac of Ray S, Park SS, Ray A (1997) Pollen tube guidance by the female Torenia fournieri. Plant Cell 10:2019Ð2031 gametophyte. Development 124:2489Ð2498 Huang B-Q, Russell SD (1994) Fertilization in Nicotiana taba- Reger BJ, Chaubal R, Pressey R (1992) Chemotropic rsponse by cum:cytoskeletal modifications in the embryo sac during syn- pearl millet pollen tubes. Sex Plant Reprod 5:47Ð56 ergid degeneration. Planta 194:200Ð214 Russell SD (1996) Attraction and transport of male gametes for Huang B-Q, Sheridan WE (1998) Actin coronas in normal and in- fertilization. Sex Plant Reprod 9:337Ð342 determinate gametophyte 1 embryo sacs of maize. Sex Plant Sage TL, Bertin RI, Williams EG (1994) Ovary and other late-act- Reprod 11:257Ð264 ing self-incompatibility systems. In: Williams EG, Clarke AE, Hülskamp M, Schneitz K, Pruitt RE (1995) Genetic evidence for Knox RB (eds) Genetic control of self-incompatibility and a long-range activity that directs pollen tube guidance in reproductive development in flowering plants. Kluwer, Arabidopsis. Plant Cell 7:57Ð64 Dordrecht, Netherlands Jensen WA (1965) The ultrastructure and histochemistry of the Seavey SR, Bawa KS (1986) Late-acting self-incompatibility in synergids of cotton. Am J Bot 52:238Ð256 angiosperms. Bot Rev 52:195Ð218 Juel HO (1918) Beiträge zur Blütenanatomie und zur systematik Schwemmle J (1968) Selective fertilization in Oenothera. Adv der Rosaceen. K Sven Vetenskapsakad Hand 58:5 Genet 14:225Ð324 Kandasamy MK, Nasrallah JB, Nasrallah ME (1994) Pollen-pistil Shimizu KK, Okada K (2000) Attractive and repulsive interac- interactions and developmental regulation of pollen tube tions between female and male gametophytes in Arabidopsis growth in Arabidopsis. Development 120:3405Ð3418 pollen tube guidance. Development 127:4511Ð4518 Knox RB (1984). Pollen-pistil interactions. In: Linskens HF, Smyth DR (1997) Attractive ovules. Curr Biol 7:R64-R66 Heslop-Harrison J (eds) Cellular Interactions (Encyclopedia of Tian HQ, Russell SD (1997) Calcium distribution in fertilized and , new series 17) Springer, Berlin, Heidelberg, unfertilized ovules and embryo sacs of Nicotiana tabacum L. New York Planta 202:93Ð105 Linskens HF (1986) Recognition during the progamic phase. In: Tian HQ, Russell SD (2000) Calcium changes in ovules and em- Cresti M, Dallai R (eds) Biology of reproduction and cell mo- bryo sacs of Plumbago zeylanica L. Sex Plant Reprod 13:11Ð tility in plants and animals. University of Siena, Siena, Italy 20 Lord E (2000) Adhesion and cell movement during : Tilton VR (1980) The nucellar and micropyle of cherchez la femme. Trends Plant Sci 5:368Ð373 Ornithogalum caudatum () with a review of these Lush WM (1999) Whither chemotropism and pollen tube guid- structures in other taxa. Can J Bot 58:1872Ð1884 ance? Trends Plant Sci 4:413Ð418 Tilton VR, Horner HT Jr (1980) Stigma, style and obturator of Malho R (1998) Pollen tube guidance Ð the long and winding Ornithogalum caudatum (Liliaceae) and their function in the road. Sex Plant Reprod 11:242Ð244 reproductive process. Am J Bot 67:1113Ð1131 Marshall DL, Folsom MW (1991) Mate choice in plants:an ana- Wilhelmi LK, Preuss D (1996) Self-sterility in Arabidopsis due to tomical to population perspective. Annu Rev Ecol Syst defective pollen tube guidance. Science 274:1535Ð1537 22:37Ð63 Wilhelmi LK, Preuss D (1999) The mating game:pollination and Martinis de D, Mariani C (1999) Silencing gene expression of the fertilization in flowering plants. Curr Op Plant Biol 2:18Ð22 ethylene-forming enzyme results in a reversible inhibition of Williams EG, Knox RB, Rouse T (1982) Pollination subsystems ovule development in transgenic tobacco plants. Plant Cell 11: distinguished by pollen tube arrest after interspecific crosses 1061Ð1072 in . J Cell Sci 53:255Ð273 Mascarenhas JP (1978) Sexual and chemotropism in Willson MF, Burley N (1983) Mate choice in plants:tactics, mech- plants. In: Hazelbaner GL (ed) Taxis and behavior. Elementary anisms and consequences. Monogr Pop Biol no 19, Princeton sensory systems in biology. Receptors and recognition, series University Press, Princeton, NJ B5. Chapman and Hall, London Wu H-M, Wang H, Cheung AY (1995) A pollen tube growth stim- Mascarenhas, JP (1993) Molecular mechanisms of pollen tube ulatory glycoprotein is deglycosylated by pollen tubes and dis- growth and differentiation. Plant Cell 5:1303Ð1314 plays a glycosylation gradient in the flower. Cell 82:395Ð403