Overexpression of bHLH95, a basic helix-loop-helix transcription factor family member, impacts trichome formation via regulating gibberellin biosynthesis in tomato

Trichomes are epidermal protuberances on aerial parts of plants known to play an important role in biotic and abiotic stresses. To date, our knowledge of the regulation of trichome formation in crop species is very limited. Through phenotyping of the Solanum pennelliixS. lycopersicum (cv. M82) introgression population, we identified the SIbHLH95 transcription factor as a negative regulator of trichome formation in tomato. In line with this negative role, SIbHLH95 displayed a very low expression in stems where trichomes are present at high density. Overexpression of SIbHLH95 resulted in a dramatically reduced trichome density in stems and a significant down-regulation of a set of trichome-related genes. In addition to the lower trichome density, overexpressing lines also showed pleiotropic alterations affecting both vegetative and reproductive development. While most of these phenotypes were reminiscent of gibberellin (GA)-deficient phenotypes, expression studies showed that two GA biosynthesis genes, SIGA20ox2 and SIKS5, are significantly down-regulated in SIbHLH95-OE plants. Moreover, in line with a decrease in active GA content, the glabrous and dwarf phenotypes were rescued by exogenous GA treatment. In addition, yeast one-hybrid and transactivation assays revealed that SIbHLH95 represses the expression of SIGA20ox2 and SIKS5 via direct binding to their promoters. Taken together, our study established a link between SIbHLH95, GA, and trichome formation, and uncovered the role of this gene in modulating GA biosynthesis in tomato.