Tomato HsfA1a plays a critical role in plant drought tolerance by activating <i>ATG</i> genes and inducing autophagy

Autophagy plays critical roles in plant responses to stress. In contrast to the wealth of information concerning the core process of plant autophagosome assembly, our understanding of the regulation of autophagy is limited. In this study, we demonstrated that transcription factor HsfA1a played a critical role in tomato tolerance to drought stress, in part through its positive role in induction of autophagy under drought stress. <i>HsfA1a</i> expression was induced by drought stress. Virus-induced <i>HsfA1a</i> gene silencing reduced while its overexpression increased plant drought tolerance based on both symptoms and membrane integrity. <i>HsfA1a</i>-silenced plants were more sensitive to endogenous ABA-mediated stomatal closure, while its overexpression lines were resistant under drought stress, indicating that phytohormone ABA did not play a major role in <i>HsfA1a</i>-induced drought tolerance. On the other hand, <i>HsfA1a</i>-silenced plants increased while its overexpression decreased the levels of insoluble proteins which were highly ubiquitinated under drought stress. Furthermore, drought stress induced numerous <i>ATGs</i> expression and autophagosome formation in wild-type plants. The expression of <i>ATG10</i> and <i>ATG18f</i>, and the formation of autophagosomes were compromised in <i>HsfA1a</i>-silenced plants but were enhanced in <i>HsfA1a</i>-overexpressing plants. Both electrophoretic mobility shift assay and chromatin immunoprecipitation coupled with qPCR analysis revealed that HsfA1a bound to <i>ATG10</i> and <i>ATG18f</i> gene promoters. Silencing of <i>ATG10</i> and <i>ATG18f</i> reduced HsfA1a-induced drought tolerance and autophagosome formation in plants overexpressing <i>HsfA1a</i>. These results demonstrate that HsfA1a induces drought tolerance by activating <i>ATG</i> genes and inducing autophagy, which may promote plant survival by degrading ubiquitinated protein aggregates under drought stress.自噬（autophagy）在植物应对胁迫的过程中发挥关键作用。尽管目前已有大量关于植物自噬体（autophagosome）组装核心过程的研究数据，但我们对自噬调控机制的认知仍十分有限。本研究证实，转录因子HsfA1a对番茄的干旱胁迫耐受性具有关键调控作用，其部分机制是在干旱胁迫下通过正向诱导自噬过程实现的。干旱胁迫可诱导HsfA1a的基因表达。基于植株表型与膜完整性的检测结果显示，病毒介导的HsfA1a基因沉默会降低番茄的干旱耐受性，而过表达HsfA1a则可提升其耐旱性。沉默HsfA1a的植株对内源脱落酸（abscisic acid, ABA）介导的气孔关闭更为敏感，而过表达HsfA1a的株系在干旱胁迫下则表现出抗性，这表明植物激素ABA并非HsfA1a介导的干旱耐受过程中的核心调控因子。另一方面，干旱胁迫下，沉默HsfA1a的植株体内高度泛素化的不溶性蛋白水平会升高，而过表达HsfA1a的植株则会降低该类蛋白的积累量。此外，在野生型植株中，干旱胁迫可诱导大量自噬相关基因（autophagy-related gene, ATG）的表达以及自噬体的形成。在沉默HsfA1a的植株中，ATG10与ATG18f的表达水平以及自噬体的形成过程均受到抑制，而在过表达HsfA1a的植株中这些过程则得到增强。电泳迁移率变动分析（electrophoretic mobility shift assay, EMSA）与染色质免疫共沉淀（chromatin immunoprecipitation, ChIP）联合实时定量PCR（quantitative real-time PCR, qPCR）的检测结果均证实，HsfA1a可结合至ATG10与ATG18f的基因启动子区域。沉默ATG10与ATG18f会削弱过表达HsfA1a的植株中HsfA1a介导的干旱耐受性以及自噬体的形成能力。上述结果表明，HsfA1a可通过激活自噬相关基因的表达并诱导自噬过程，从而提升植株的干旱耐受性；该过程可能通过在干旱胁迫下降解泛素化蛋白聚集体，进而促进植株存活。