Gibberellin Is Required for Flowering in Arabidopsis thaliana under Short Days 1

Abstract

Mutants of Arabidopsis thaliana deficient in gibberellin synthesis (gal-3 and gal-6), and a gibberellin-insensitive mutant (gai) were compared to the wild-type (WT) Landsberg erecta line for flowering time and leaf number when grown in either short days (SD) or continuous light (CL). The ga 1-3 mutant, which is severely defective in ent-kaurene synthesis because it lacks most of the GAI gene, never flowered in SD unless treated with exogenous gibberellin. After a prolonged period of vegetative growth, this mutant eventually underwent senescence without having produced flower buds. The gai mutant and the "leaky' gal-6 mutant did flower in SD, but took somewhat longer than WT. All the mutants flowered readily in CL, although the gal-3 mutant showed some delay. Unlike WT and gal-3, the gai mutant failed to respond to gibberellin treatment by accelerating flowering in SD. A cold treatment promoted flowering in the WT and gai, but failed to induce flowering in gal-3. From these results, it appears that gibberellin normally plays a role in initiating flowering of Arabidopsis. Exogenous GA2 has been shown to promote the switch from vegetative growth to flowering in a variety of plants. Most species in which applied GA can induce flowering are long-day or cold-requiring plants, and many of these normally grow as rosettes under noninductive conditions (17). Exogenous GA fails to stimulate flowering in many other angiosperms (11). It is still unclear what role, if any, endogenous GA plays in floral induction. In a very few cases, such as Samolus parviflorus, an inhibitor of GA biosynthesis has been shown to prevent flowering in a GA-reversible manner (17). In other species, however, the application of GA synthesis inhibitors failed to block flowering (8). For these plants, it remains uncertain whether GA is normally involved in the induction of flowering. Mutants that are specifically impaired in GA production have been obtained in a number of species. The GA-deficient mutants of rice (Oryza sativa), maize, Arabidopsis, pea (Pisum sativum), and tomato (Lycopersicon esculentum) all flower readily under normal growth conditions (11), although these flowers may show various structural defects, depending on 'This work was supported by a grant from the U.S. Department of Energy (DE-FG02-90ER20021). 2Abbreviations: GA, gibberellin (in this context, any biologically active structure); SD, short days; CL, continuous light; WT, wild type. the species (5, 10). A GA-deficient mutant of Brassica rapa takes somewhat longer to flower than normal (16), as does a GA-deficient mutant of Thlaspi arvense (9). A mutant of red clover never flowers without exogenous GA (2), but it is not clear whether this variant will prove to be defective primarily in GA metabolism. It is possible that all the GA biosynthesis mutants that have been examined to date are 'leaky' to some degree, and produce small amounts of active GAs, sufficient to induce flowering. To determine whether GA is necessary for flowering, it is essential to study a mutant that contains very little or no active GA. Therefore, we examined an extremely GAdeficient mutant of the quantitative long-day plant Arabidopsis thaliana, in which a major portion of the GAl gene is deleted (14). This gene is thought to encode a product necessary for carrying out the first committed step in GA biosynthesis, the formation of ent-kaurene (18). The availability of this apparently nonleaky mutant provided the opportunity for a definitive test of the role of GA in flowering of Arabidopsis. To gain a better understanding of GA action in flowering, we have also characterized the effect on flowering of the gai mutation, which impairs GA responsiveness. The results presented here indicate that under short photoperiods, GA is required for flowering in Arabidopsis. MATERIALS AND METHODS

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@inproceedings{WilsonGibberellinIR, title={Gibberellin Is Required for Flowering in Arabidopsis thaliana under Short Days 1}, author={Ruth Wilson and John Heckman and Robert A . Somerville} }