Overexpression of SlbHLH95, 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. Up to date, our knowledge of the regulation of trichome formation in crop species is very limited. Through phenotyping the S. pennellii x S. lycopersicum (cv. M82) introgression population, we identified SlbHLH95 transcription factor as a negative regulator of trichome formation in tomato. In line with its negative role, SlbHLH95 displayed a very low expression in stems where trichomes are present at high density. Overexpression of SlbHLH95 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 GA deficient phenotypes, expression studies showed that two GA biosynthesis genes, SlGA20ox2 and SlKS5, are significantly downregulated in SlbHLH95-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 SlbHLH95 represses the expression of SlGA20ox2 and SlKS5 via direct binding to their promoters. The outcome of the study established a link between SlbHLH95, GA and trichome formation and uncovered the role of this gene in modulating GA biosynthesis in tomato.