Tomato drought-responsive transcription factor TINY1 suppresses embryonic growth

Dehydration-Responsive Element-Binding (DREB) transcription factors play an important role in plant responses to drought. DREB subfamily A4, contains a sub-group named TINY. Previous studies in Arabidopsis suggest that TINYs suppress plant growth, promote the expression of drought-responsive genes and mediate abscisic acid (ABA)-induced stomatal closure. In this study, we investigated the function of the tomato TINY1, using CRISPR-derived tiny1 mutants. Under drought conditions, tiny1 mutants lost turgor and wilted more rapidly than control M82 plants. However, this sensitivity was attributed to their larger leaf area, rather than intrinsic differences in drought response. Measurements of stomatal conductance, leaf temperature, and osmotic adjustment revealed no significant differences between tiny1 and M82. Furthermore, whole plant daily transpiration under drought conditions of M82 and tiny1 with similar leaf area, showed no differences between the genotypes. Although TINY1 expression was strongly induced by drought in tomato leaves, RNA-seq analysis revealed only minor differences in the transcriptional drought response between tiny1 and M82. Nevertheless, several candidate TINY1-regulated genes were identified. Interestingly, the growth-promoting effect of tiny1 was confined to early developmental stages; enhanced embryo growth and larger seeds, hypocotyl elongation, and accelerated emergence of the first true leaves - traits that later contributed to increased leaf area. At later stages, the mutation had no observable impact on growth rate or plant morphology under either well-watered or drought conditions. Our data suggest that TINY1 suppresses embryonic growth by repressing gibberellin (GA) biosynthesis through downregulation of GA 20-oxidase 4 (GA20ox4) expression.