HsfA1d promotes hypocotyl elongation under chilling via enhancing expression of ribosomal protein genes in Arabidopsis

How plants maintain growth under nonfreezing low temperatures (chilling) is not well understood. Here we use hypocotyl elongation under dark to investigate the molecular mechanisms for chilling growth in Arabidopsis thaliana. The function of HsfA1d (Heat shock transcription factor A1d) in chilling growth is investigated by physiological and molecular characterization of its mutants. Subcellular localization of HsfA1d under chilling is analyzed. Potential target genes of HsfA1d were identified by transcriptome analysis, chromatin immunoprecipitation, transcriptional activation assay and mutant characterization. HsfA1d is a positive regulator of hypocotyl elongation under chilling. It promotes expression of a large number of ribosome biogenesis genes to a moderate but significant extent under chilling. HsfA1d could bind to the promoter regions of two ribosome protein genes tested and promote their expression. The loss-of-function of one ribosome gene also reduced hypocotyl elongation under chilling. In addition, HsfA1d did not have increased nuclear accumulation under chilling and its basal nuclear accumulation is promoted by a salicylic acid receptor under chilling. This study thus unveils a new HsfA1d-mediated pathway that promotes the expression of cytosolic and plastid cytosolic and plastid ribosomal protein genes which may maintain overall protein translation for plant growth in chilling.

Tomato HsfA1a plays a critical role in plant drought tolerance by activating ATG 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. HsfA1a expression was induced by drought stress. Virus-induced HsfA1a gene silencing reduced while its overexpression increased plant drought tolerance based on both symptoms and membrane integrity. HsfA1a-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 HsfA1a-induced drought tolerance. On the other hand, HsfA1a-silenced plants increased while its overexpression decreased the levels of insoluble proteins which were highly ubiquitinated under drought stress. Furthermore, drought stress induced numerous ATGs expression and autophagosome formation in wild-type plants. The expression of ATG10 and ATG18f, and the formation of autophagosomes were compromised in HsfA1a-silenced plants but were enhanced in HsfA1a-overexpressing plants. Both electrophoretic mobility shift assay and chromatin immunoprecipitation coupled with qPCR analysis revealed that HsfA1a bound to ATG10 and ATG18f gene promoters. Silencing of ATG10 and ATG18f reduced HsfA1a-induced drought tolerance and autophagosome formation in plants overexpressing HsfA1a. These results demonstrate that HsfA1a induces drought tolerance by activating ATG genes and inducing autophagy, which may promote plant survival by degrading ubiquitinated protein aggregates under drought stress.