A GAMYB-like gene in tomato and its expression during seed germination.

To understand the function of the gibberellin (GA) transduction pathway during germination, the transcription factor gene GAMYB, which responds to the GA signal, has been studied in tomato (Solanum lycopersicum) seeds. This gene, called LeGAMYBL1 is present as a single copy, and is expressed in both the embryo and endosperm during seed germination in gib-1 mutant (non-GA producing) and wild-type (cv. Glamour) seeds. It is also expressed in young vegetative tissues. There is an 83% similarity in the amino acid sequence of the binding domain of the protein that is encoded by this tomato GAMYB-like gene when compared to that encoded by the GAMYB genes from barley, rice and Arabidopsis. In both mutant and wild-type intact tomato seeds, LeGAMYBL1 expression increases during germination, is upregulated by gibberellic acid (GA(3)), and declines thereafter. LeGAMYBL1 transcripts are also present in non-germinating gib-1 mutant seeds imbibed in water, and they are upregulated by GA(3) during promotion of germination. However, dissected gib-1 embryos complete germination when imbibed in either water or GA(3), with almost no difference in the amount of mRNA transcribed by the LeGAMYBL1 gene during this event. This is indicative that GA(3) is not required for the expression of the LeGAMYBL1 gene, which is likely necessary, but not sufficient, for germination to be completed, especially in the intact seed. The germination-inhibiting hormone abscisic acid does not influence expression of this gene. Expression of the LeGAMYB1 gene also occurs in the endosperm, but there is no correlation between its expression and GA-promoted expression of the cell-wall-degrading enzyme endo-beta-mannanase.