Among the naturally occurring growth hormones, gib-berellins (GAs) (see Chapter 20) can have a strong influence on flowering (see Web Topic 24.8). Recent studies suggest that gibberellin promotes flowering in Arabidopsis by activating expression of the LFY gene (Blazquez and Weigel 2000). Exogenous gibberellin can evoke flowering when applied either to rosette LDPs like Arabidopsis, or to dual-day length plants such as Bryophyllum, when grown under short days (Lang 1965; Zeevaart 1985).
In addition, application of GAs can evoke flowering in a few SDPs in noninductive conditions, and in cold-requiring plants that have not been vernalized. As previously discussed, cone formation can also be promoted in juvenile plants of several gymnosperm families by addition of GAs. Thus, in some plants exogenous GAs can bypass the endogenous trigger of age in autonomous flowering, as well as the primary environmental signals of day length and temperature.
As discussed in Chapter 20, plants contain many GA-like compounds. Most of these compounds are either precursors to, or inactive metabolites of, the active forms of GA. In some situations different GAs have markedly different effects on flowering and stem elongation, such as in the long-day plant Lolium temulentum (darnel ryegrass) (see Web Topic 24.9).
These observations suggest that the regulation of flowering may be associated with specific GAs, but they do not prove that GA is the hypothetical flowering hormone. In fact, a certain level of GA is likely to be required for flowering in many species, but other pathways to flowering are necessary as well. For example, a mutation in GA biosynthesis renders the quantitative LDP Arabidopsis thaliana unable to flower in noninductive short days but has little effect on flowering in long days, demonstrating that endogenous GA is required for flowering in specific situations (Wilson et al. 1992).
Considerable attention has been given to the effects of day length on GA metabolism in the plant (see Chapter 20). For example, in the long-day plant spinach (Spinacia oleracea), the levels of gibberellins are relatively low in short days, and the plants maintain a rosette form. After the plants are transferred to long days, the levels of all the gib-berellins of the 13-hydroxylated pathway (GAra ^ GA44 ^ GAW ^ GA20 ^ GA1; see Chapter 20) increase. However, the fivefold increase in the physiologically active gib-
berellin, GA1, is what causes the marked stem elongation that accompanies flowering.
In addition to GAs, other growth hormones can either inhibit or promote flowering. One commercially important example is the striking promotion of flowering in pineapple (Ananas comosus) by ethylene and ethylene-releasing compounds—a response that appears to be restricted to members of the pineapple family (Bromeliaceae). Thus, as discussed next, the floral stimulus may be composed of many components, and these components may differ in different groups of plants.
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