Common and Distinct Functions of Arabidopsis Class A1 and A2 Heat Shock Factors in Diverse Abiotic Stress Responses and Development

Abstract

There are 21 heat shock factor (HSF) homologs in Arabidopsis (Arabidopsis thaliana), of which members of class A1 (HSFA1a/HSFA1b/ HSFA1d/HSFA1e) play the major role in activating the transcription of heat-induced genes, including HSFA2. Once induced, HSFA2 becomes the dominant HSF and is able to form heterooligomeric complexes with HSFA1. However, whether HSFA2 could function independently as a transcription regulator in the absence of the HSFA1s was undetermined. To address this question, we introduced a Cauliflower mosaic virus 35S promoter:HSFA2 construct into hsfa1a/hsfa1b/hsfa1d/hsfa1e quadruple knockout (QK) and wild-type (Wt) backgrounds to yield transgenic lines A2QK and A2Wt, respectively. Constitutive expression of HSFA2 rescued the developmental defects of the QK mutant and promoted callus formation in A2QK, but not in A2Wt, after heat treatment. Transcriptome analysis showed that heat stress response genes are differentially regulated by the HSFA1s and HSFA2; the genes involved in metabolism and redox homeostasis are preferentially regulated by HSFA2, while HSFA1-preferring genes are enriched in transcription function. Ectopic expression of HSFA2 complemented the defects of QK in tolerance to different heat stress regimes, and to hydrogen peroxide, but not to salt and osmotic stresses. Furthermore, we showed that HSFA1a/HSFA1b/HSFA1d are involved in thermotolerance to mild heat stress at temperatures as low as 27°C. We also noticed subfunctionalization of the four Arabidopsis A1-type HSFs in diverse abiotic stress responses. Overall, this study reveals the overlapping and distinct functions of class A1 and A2 HSFs and may enable more precise use of HSFs in engineering stress tolerance in the future.

HsfA2 Controls the Activity of Developmentally and Stress-Regulated Heat Stress Protection Mechanisms in Tomato Male Reproductive Tissues

“…In general, our transcriptome analysis revealed a lower number of stress-induced genes compared to other studies (Finka et al, 2011;Liu and Charng, 2013;Rizhsky et al, 2004). This on one hand can be attributed to the high stringency criteria we used here for the identification of differentially expressed genes and, on the other hand, on the fact that stressed samples were harvested after 1.5 h of recovery.…”

“…Overexpression of Arabidopsis HsfA2 in the quadruple HsfA1a/b/d/e knockout mutant restored thermotolerance, indicating that Arabidopsis HsfA2 is functional in the absence of the master regulator (Liu and Charng, 2013). Moreover, a preferential regulation of genes involved in metabolism and redox homeostasis by HsfA2 was shown, while genes preferentially controlled by HsfA1s are involved in transcription (Liu and Charng, 2013). Therefore, beyond the expected redundancy, diverse specificity or preferential functions are shared among different members of the Hsf family.…”

“…HSR is characterized by the activation of Hsfs, which induce the expression of a cohort of selected genes to facilitate primarily the protection of protein homeostasis and adjustment of cellular metabolism (Mishra et al, 2002;Liu and Charng, 2013). Different cell types with diverse functions show different sensitivity to HS Figure 8.…”

“…In some cases, homologous or heterologous ectopic expression of HsfA2 co-orthologs resulted in growth and developmental alterations, including growth retardation and dwarfism as well as accelerated plant and callus growth in the absence of HsfA1s in Arabidopsis (Liu and Charng, 2013;Ogawa et al, 2007). In addition, HsfA2 and other HS-induced Hsps that are minimally expressed in vegetative tissues under physiological conditions are strongly expressed at early stages of anther and pollen development (Frank et al, 2009;Giorno et al, 2010).…”

“…HSR requires a complex network of distinct and interconnected pathways involved in maintenance of protein homeostasis, reduction of cellular damages, and readjustment of metabolic activities. Genes encoding for heat shock proteins (Hsps), such as molecular chaperones, are the best studied HS-induced genes, but a plethora of other genes are upregulated to contribute to HSR as shown by transcriptome analysis in several plant species (Finka et al, 2011;Jung et al, 2012;Liu and Charng, 2013;Sarkar et al, 2014;Fragkostefanakis et al, 2015b). The rapid and strong transcriptional activation of the majority of genes is controlled by the activity of the heat stress transcription factors (Hsfs).…”