Investigation into the role of calmodulin-binding transcriptional activators in the heat stress response of Arabidopsis thaliana

Abstract

Global warming is imposing stress on natural ecosystems and commercial crops. In plants, heat stress negatively impacts key physiological processes such as photosynthesis, respiration, and reproduction. Plants respond to elevated temperatures through a process termed the heat stress response (HSR). The cellular mechanisms underlying the HSR are not yet fully understood but involve calcium (Ca2+) signal transduction and changes in gene expression. Plants have many Ca2+ sensor proteins that decode Ca2+ signals during growth and stress responses. Calmodulin (CaM) is the canonical Ca2+ sensor in eukaryotes, but plants also have evolved many calmodulin-like proteins (CMLs), some of which function in abiotic stress signalling. Among the downstream targets of CaM/CMLs are calmodulin-binding transcription factors (CAMTAs), a family of transcription factors that regulate stress-responsive genes in plants. CAMTAs have been implicated in response to cold, drought, pathogen defense, and other environmental stresses. However, their roles during heat stress have not been studied. Here, using a combination of genetics and physiological assays, I test the hypothesis that CAMTAs function as regulatory proteins during the Arabidopsis HSR. In heat shock phenotyping assays, camta mutants exhibited distinct phenotypes compared to wildtype (WT) plants. RT-qPCR analysis revealed that several heat responsive genes are misregulated in camta6 knockout mutants, indicating their potential role as regulators of the HSR in Arabidopsis. To explore CAMTA regulation, I tested interactions between CAMTAs and Ca2+ sensors using split-luciferase assays. These experiments revealed specific binding between CAMTA IQ domains and CaM/CML proteins. Understanding how CAMTAs influence the HSR will provide insight into plant stress regulatory mechanisms and should help inform engineering strategies to enhance crop tolerance in our warming climate.