myb-related transcription factors contain highly conserved DNA-binding domains. Using a mixture of degenerate oligonucleotides derived from the highly conserved region as probe, 14 myb-related clones were isolated from a cDNA library constructed using tomato hypocoyl mRNA. The expression of these clones was studied by northern blot analysis using poly(A)+ RNA from 7 tissue types (hypocotyl, leaf, root, green and red fruit, immature and mature flower). This study has revealed a wide range of expression patterns which include multiple and single transcripts, some of which show marked tissue specificity. Two clones showing different expression patterns have been fully sequenced. The DNA-binding domains of these two tomato myb clones are compared with myb genes from other plant species and organisms. Of the three clones analysed so far by Southern hybridization, two are single-copy genes and one has multiple genomic copies.

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.