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Nicotiana Biotypes and Its Bearing on the Regulation of Flower Formation (Photoperiodism in Plants/Plant Tissue Culture) MANGALATHU S

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Nicotiana Biotypes and Its Bearing on the Regulation of Flower Formation (Photoperiodism in Plants/Plant Tissue Culture) MANGALATHU S Proc. Natl. Acad. Sci. USA Vol. 90, pp. 4636-4640, May 1993 Plant Biology Flower-bud formation in explants of photoperiodic and day-neutral Nicotiana biotypes and its bearing on the regulation of flower formation (photoperiodism in plants/plant tissue culture) MANGALATHU S. RAJEEVAN* AND ANTON LANGt MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824-1312 Contributed by Anton Lang, December 31, 1992 ABSTRACT The capacity to form flower buds in thin-layer other photoperiodic biotypes of Nicotiana, with a qual. and a explants was studied in flowering plants of several species, quantitative (quant.) photoperiodic response, and also another cultivars, and lines of Nicohana differing in their response to DNP, Nicotiana rustica. The latter was used because it was photoperiod. This capacity was found in all biotypes examined reported (5) to be recalcitrant to flower formation in vitro, and could extend into sepals and corofla. It varied greatly, explants forming only few flower buds and only after forma- depending on genotype, source tissue and its developmental tion of two leaves. We examined the capacity of the different stage, and composition of the culture medium, particularly the plants to form flower buds in thin-layer explants and its levels ofglucose, auxin, and cytokinin. It was greatest in the two variations; we discuss some of the information research on in day-neutral plants examined, Samsun tobacco and Nicohana vitro flowering provides to our understanding ofthe regulation rustica, where it extended from the inflorescence region down of flower formation in general. the vegetative stem, in a basipetally decreasing gradient; it was least in the two qualitative photoperiodic plants studied, the MATERIALS long-day plantNicotiana silvestis and the short-day plant Mary- AND METHODS land Mammoth tobacco, the quantitative long-day plant Nico- Plants and Growing Conditions. The Nicotiana plants used tiana alata and the quantitative short-day plant Nicotiana oto- were (i) N. tabacum L. cv. Samsun, (ii) N. rustica L. cv. phora line 38-G-81, where it was limited to the pedicels (and, in Brasilia (DNPs), (iii) N. tabacum cv. Maryland Mammoth some cases, the sepals). Regardlessofthe photoperiodic response (qual. SDP), (iv) N. silvestris Speg. et Comes [qual. long-day of the source plants, the response was the same in explants plant (LDP)], (v) N. alata Link et Otto and (vi) N. otophora cultured under long and short days. The finding that capacity to Griseb. line 38C-G (quant. LDPs), and (vii) N. otophora line form flower buds in explants is present in all Nicotiana biotypes 38-G-81 and (viii) N. tomentosiformis Goodsp. (quant. SDPs). studied supports the idea that it is regulated by the same Maryland Mammoth and N. silvestris were from lines propa- mechanism(s), regardless ofthe plant's photoperiodic character. gated in A.L.'s laboratories for ca. 40 years. Seeds ofSamsun However, the source plants were all in the flowering stage, and were kindly provided by K. Tran Than Van (Le Phytotron, no flower-bud formation can be obtained in explants from Gif-sur-Yvette, France), those ofN. rustica cv. Brasilia were strictly vegetative Nicotana plants. Hence, flower formation in provided by H. H. Smith (Brookhaven National Laboratory, the explants is not identical with de novo flower formation in a Upton, NY), and those ofN. tomentosiformis and the two N. hitherto vegetative plant: it is rather the expression of a floral otophora lines were provided by the U.S. Department of state already established in the plant, although it can vary widely Agriculture Tobacco Research Laboratory (Oxford, NC). All in extent and spatial distribution. Culture conditions that permit plants have been checked for their photoperiodic response flower-bud formation in an explant are conditions that maintain under our standard growing conditions. the floral state and encourage its expression; conditions under The SDPs were started in a greenhouse on 20-hr LDs and which no flower buds are formed reduce this state and/or after 7-10 wk were transferred to 8-hr SDs, to induce flower prevent its expression. formation. The LDPs were grown for 10 wk in SDs and then in LDs. The greenhouse temperature was maintained at an A large part ofresearch on the physiology offlower formation average of 28°C. The plants were grown in a greenhouse soil in the last decades has been conducted using explants from mixture or, in later experiments, a commercial planting vegetative parts ofplants. The first majorwork ofthis kind was mixture; apart from water they received three times weekly that of Aghion-Prat (1), who used entire stem sections as a fertilizer solution. explants. In 1973, Tranh Than Van (2) introduced the use of Tissue Culture. Explants were prepared from pedicels of small segments consisting ofa few ofthe outermost cell layers fully opened and of unopened flowers, the latter being 1-1.5 of the stem, and these "thin-layer explants," having several cm in length without the pedicel, the basal region of inflo- advantages over stem sections, have been used in most rescence branches, various positions in the main stem, and in subsequent work on "in vitro" flower formation. Most ofthis some experiments from sepals and corollae of unopened work was done with day-neutral (DN) cultivars of tobacco flowers. Most explants (5-7 mm long, three to six cell layers) (Nicotiana tabacum L.); attempts at obtaining flower forma- were prepared when the first (terminal) flower of the source tion in explants from photoperiodic biotypes of Nicotiana plant was 3-4 days past anthesis, but sepal and corolla were much fewer and largely unsuccessful (for a briefreview, explants (3-4 mm long, excised under a dissecting micro- see ref. 3). This apparent difference between DN and photo- periodic plants has even led to some question as to whetherthe Abbreviations: BA, N6-benzylaminopurine; DN(P), day-neutral regulation offlower formation in these plants is fundamentally (plant); LD(P), long-day (plant); NAA, a-naphthaleneacetic acid; different. Some years ago we (4) showed that thin-layer qual., qualitative; quant., quantitative; SD(P), short-day (plant). explants of the tobacco cultivar Maryland *Present address: U.S. Department of Agriculture-Agricultural Re- Mammoth, a qual- search Service, Microbial Products Research Unit, P.O. Box 5677, itative short-day plant (qual. SDP), were capable of forming Athens, GA 30613. flower buds. We have expanded this work, including several tTo whom reprint requests should be addressed. 4636 Downloaded by guest on September 27, 2021 Plant Biology: Rajeevan and Lang Proc. Natl. Acad. Sci. USA 90 (1993) 4637 scope) were prepared using source plants before any flower Table 2. Distribution of flower-forming capacity in explants was open. Unopened flowers were sterilized in toto before of Nicotiana excising explants; of the other source organs, large sections Explant source were sterilized and explants were excised from these. Ster- Plant Cor Sep Ped IB SI SII SIII ilization was for 5 min in ca. 1.25% NaOCl, preceded by dipping in 95% ethanol and followed by rinsing in sterile N. tabacum cv. ++ +++ +++ ++ + distilled water. Explants were excised under aseptic condi- Samsun 87 100 100 87 45 0 tions, using scalpels with exchangeable blades. N. tabacum cv. - + ++ The culture media consisted of macro- and micronutrients Maryland Mammoth 55 100 0 0 0 0 according to Murashige and Skoog (6), 100 mg of myoinositol N. silvestris - + + per liter, 0.1 mg of thiamine hydrochloride per liter, and 55 75 0 0 0 various amounts of glucose, the auxin a-naphthaleneacetic N. otophora + + acid (NAA), and the cytokinin N6-benzylaminopurine (BA). 38C-G 0 75 25 0 0 0 They were solidified with 0.64% agar and sterilized by N. otophora + autoclaving (15 min at 121°C). All cultures were grown in 38-G-81 0 72 0 0 0 0 Petri plates in controlled-environment rooms at 25C and 16 N. tomentosiformis + ++ + hr of light daily from fluorescent and incandescent lamps for 0 75 85 25 0 0 the DNP and LDP and 8 hr for the SDP. (These light regimes N. rustica + +++ +++ + + + were chosen before it was recognized that flower-bud for- 62 100 95 100 47 32 mation in the explants was independent of photoperiod; see Cor, corolla; Sep, sepal; Ped, pedicel; IB, inflorescence branches; Table 3.) The culture period was 4 wk (after which time the SI, main stem above node 1 (nodes were counted basipetally, node flower buds undergo little, if any, further development); the 1 being the first node bearing a leaf >5 cm in length); SII, main stem cultures were examined under a dissecting microscope to between nodes 5 and 10 (in N. silvestris, nodes 4 and 8); SIII, main count the number of flower and vegetative buds formed and stem below node 15. + + + = Ten flower buds per explant and above; determine the developmental state reached by the former. + + = five to nine buds per explant; + = four and fewer buds per explant; denominator = percentage ofexplants with flower buds; not tested. Media as in Table 1. RESULTS The extent of flower-bud formation in the explants varied Flower-Bud Formation in Nicotiana Explants: Extent and considerably, regarding its extent in the plant, the fraction of Variation. Our main results are compiled in Tables 1 and 2. Flower-bud formation was obtained in explants from all explants forming flower buds, the number ofbuds per explant, species, cultivars, and lines of Nicotiana examined, in some and the development of the buds. It was greatest in the two cases not only from vegetative parts but also from sepals and DNPs studied, Samsun tobacco and N. rustica cv. Brasilia. All in the one case where it was tried from the corolla. In all cases explants from pedicels and sepals of Samsun formed flower the buds formed directly in the original tissue, with no buds whose number could reach 20 and above.
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