SlHY5 Integrates Temperature, Light and Hormone Signaling to Balance Plant Growth and Cold Tolerance.

During the transition from warm to cool seasons, plants experience decreased temperatures, shortened days and decreased red/far-red (R/FR) ratios of light. The mechanism by which plants integrate these environmental cues to maintain plant growth and adaptation remains poorly understood. Here, we report that low temperature induced the transcription of PHYTOCHROME A and accumulation of LONG HYPOCOTYL 5 (SlHY5, a bZIP transcription factor) in tomato (Solanum lycopersicum) plants, especially under short-day conditions with low R/FR light ratios. Reverse genetic approaches and physiological analyses revealed that silencing of SlHY5 increased cold susceptibility in tomato plants, whereas overexpression of SlHY5 enhanced cold tolerance. SlHY5 directly bound to and activated the transcription of genes encoding a gibberellin (GA)-inactivation enzyme, namely GIBBERELLIN 2-OXIDASE 4, and an abscisic acid (ABA) biosynthetic enzyme, namely 9-CIS-EPOXYCAROTENOID DIOXYGENASE 6 (SlNCED6). Thus, phytochrome A-dependent SlHY5 accumulation resulted in an increased ABA/GA ratio, which was accompanied by growth cessation and induction of cold response. Furthermore, silencing of SlNCED6 compromises short day- and low R/FR-induced tomato resistance to cold stress. These findings provide insight into the molecular genetic mechanisms by which plants integrate environmental stimuli with hormones to coordinate their growth with impending cold temperatures. Moreover, this work reveals a molecular mechanism that plants have evolved for growth and survival in response to seasonal changes.