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.

The R2R3MYB proteins comprise one of the largest families of transcription factors and play regulatory roles in developmental processes and defense responses in plants. However, there has been relatively little effort to systematically carry out comprehensive genomic and functional analyses of these genes in tomato (Solanum lycopersicum L.), a reference species for Solanaceae plants, and the model plant for fruit development. In this study, a total of 121 R2R3MYB genes were identified in the tomato genome released recently and further classified into 29 subgroups based on the phylogenetic analysis of the complete protein sequences. Phylogenetic comparison of the members of this superfamily among tomato, Arabidopsis, grape, rice, poplar, soybean, cucumber and apple revealed that the putative functions of some tomato R2R3MYB proteins were clustered into the Arabidopsis functional clades. The chromosome distribution pattern revealed that tomato R2R3MYB genes were enriched on several chromosomes and 52 % of the family members were tandemly duplicated genes. Tissue specificity or different expression levels of SlR2R3MYBs in different tissues suggested differential regulation of tissue development as well as metabolic regulation. The transcript abundance level analysis during abiotic conditions identified a group of R2R3MYB genes that responded to one or more treatments suggesting that the SlR2R3MYBs played major roles in the plant response to abiotic conditions and involved in signal transduction pathways. This study not only provides a solid foundation for further functional dissection of tomato R2R3MYB family genes, but may also be profitable for, in the future, the improvement of tomato stress tolerance and fruit quality.