Genomic organization, phylogenetic comparison and differential expression of the SBP-box family of transcription factors in tomato

SBP-box genes represent transcription factors ubiquitously found in the plant kingdom and recognized as important regulators of many different aspects of plant development. In this study, 15 SBP-box gene family members were identified in tomato and analyzed with respect to their genomic organization and other structural features. Phylogenetic reconstruction based on the DNA-binding SBP-domain, allowed the classification of the SlySBP proteins into eight groups representing clear orthologous relationships to family members of other flowering plants and the moss Physcomitrella. In order to have a better understanding of their possible function in the development of a fleshy-fruit species like tomato, the mRNA expression levels of all SlySBP genes were quantified in vegetative and reproductive organs of plants, at different stages of growth. As transcripts of ten SlySBP genes were found to carry putative miR156- and miR157-response elements, the expression levels of the corresponding microRNAs were determined as well, revealing different patterns of expression. In addition, eight putative miR156 and four miR157 encoding loci could be identified in the tomato genome, four of them forming a polycistronic cluster. Whereas miR156 and miR157 levels were highest in seedlings, leaves and anthers of young flowers, most miR156-targeted SlySBP genes were found to be expressed in young inflorescences and during fruit development and ripening, suggesting a particularly important role during tomato reproductive growth. The data presented provide a basis for future clarification of the various functions that SBP-box gene family members play in tomato growth and development.