The regulation of seed germination by components of gibberellin and abscisic acid signal transduction

The hormones gibberellic acid (GA) and abscisic acid (ABA) exert profound influence over the germination of seeds, with the former promoting and the latter inhibiting this event. Genes encoding components of the GA and ABA signal transduction pathways have been proposed to regulate germination. The protein product of DELLA genes inhibits GA responses, but a decline in the transcript abundance of these genes is not required for the completion of germination in tomato, Arabidopsis, soybean or lettuce seeds. This is also the case in tomato for the positively-acting GA-signaling component SLEEPY (LeSLY) and positively-acting ABA component ABI3 (LeABI3), although expression of the negatively-acting GA component SPINDLY (LeSPY) declines prior to the completion of germination in tomato seeds.; A signal passing from the endosperm to the embryo during tomato seed germination is described, which promotes LeABI3 expression in the latter tissue. The nature of this signal is either GA or GA-derived, or is an unknown factor that acts upon GA synthesis or response within the embryo. This suggests that bi-directional signaling occurs during tomato seed germination, perhaps mediating the co-ordination of events between the release from coat dormancy and the commencement of radicle elongation.; A mutation in the DELLA gene isolated from tomato ( LeGAI) is responsible for the constitutive GA-response phenotype of the tomato procera (pro) mutant. The pro mutation does not affect the release from coat dormancy, but does increase embryo growth potential and the speed of seed germination. Mutant pro seeds exhibit the same sensitivity to the ABA-mediated inhibition of seed germination as wild-type seeds, suggesting that ABA signaling acts downstream of DELLA action. Supporting this hypothesis is the observation that the tomato DELLA protein fused to GFP (LeGAI-GFP) is degraded in the presence of both GA and ABA when transiently transformed into onion cells. This suggests ABA action to be downstream of DELLA protein degradation, since seeds imbibed in these two hormones fail to germinate. A model is presented that outlines the possible mode of interaction between GA and ABA signaling in the regulation of seed germination.