Functional roles of PIL5 in Arabidopsis seed germination

Abstract

The first decision made by an angiosperm seed, whether to germinate or not, is based on integration of various environmental signals such as light, moisture and temperature. The phytochromes act as red and far-red light photoreceptors to mediate various light responses including seed germination. However, the molecular mechanism by which phytochromes regulate seed germination has not been elucidated. Here, I showed that PIL5 protein, a basic helix-loop-helix transcription factor, is a key negative regulator of phytochrome-mediated seed germination by phenotypic analyses using PIL5 loss of function and gain of function mutant. PIL5 also regulated other phytochrome mediated light responses including inhibition of hypocotyls elongation and hypocotyls negative gravitropism. Like, PIF3, PIL5 also preferentially interacted with Pfr form of phytochrome. Expression analysis revealed that PIL5 repressed the expression of gibberellin (GA) biosynthetic genes ($GA3ox1$ and $GA3ox2$) and activated the expression of a GA catabolic gene ($GA2ox$). Consistent with these gene expression patterns, the amount of bioactive GA was higher in the pil5 mutant and lower in the PIL5 overexpression line. I also showed that red and far-red light signals trigger PIL5 protein degradation through the 26S proteasome, thus releasing the inhibition of bioactive GA biosynthesis by PIL5. In addition to GA metabolic genes, PIL5 also regulates the expression of GA repressor (DELLA) genes (GAI and RGA) and ABA metabolic genes and ABA metabolic genes (ABA1, NCED6 and CYP707A2). Chromatin immunoprecipitation analyses indicated that PIL5 directly binds to GAI and RGA promoters through G-box element, but not to GA and ABA metabolic gene promoters. Taken together, these results show that light signals perceived by phytochromes cause a reduction in the PIL5 protein level, which in turn regulates the transcription of two DELLA genes directly and that of GA and ABA metabolic genes indirectly. To find more P...