CALMODULIN-BINDING TRANSCRIPTION ACTIVATOR 6 (CAMTA6) functions as a negative regulator of heat-stress response in Arabidopsis

Heat stress negatively impacts plant growth and development and reduces yield in agricultural crops. Plants respond to elevated temperatures through the heat stress response (HSR) in which Ca2+ signals trigger changes in cellular activity and gene expression that help limit cellular damage. Calmodulin-binding transcription factors (CAMTAs) are a family of transcriptional regulators that participate in various stress-responsive signalling pathways. However, roles for CAMTAs during HSR are unclear. Here, using a combination of genetics and physiological assays, we tested the hypothesis that CAMTAs function as regulatory proteins during the Arabidopsis HSR. Among the six-member CAMTA family, camta6 loss-of-function lines exhibited enhanced tolerance to HS relative to wild-type plants at the germination and seedling stages but not at later developmental stages. Qualitative promoter-reporter assays and quantitative gene transcription analyses indicate that CAMTA6 expression is repressed by HS in wild-type plants. Under control and HS conditions, we investigated the expression of various genes with promoters containing putative CAMTA-binding CGCG-box cis-elements and known to function in HSR. Notably, we found that in response to HS, several key HSR genes such as HEAT-SHOCK FACTOR-A1e (HSFA1e), HSFA2, HSFB1, HSFB2b, and DEHYDRATION-RESPONSIVE ELEMENT-BINDING 2A (DREB2A) are significantly up-regulated in camta6 knockout mutants in comparison to wild-type plants. In addition, we found that expression of ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOGUE (HYH) were suppressed in camta6 mutants. Collectively, our study indicates that CAMTA6 functions as an important negative regulator of the HSR in Arabidopsis and highlights the expanding roles of CAMTAs in abiotic stress signalling pathways.