Lipid- and calcium-signaling regulation of HsfA2c-mediated heat tolerance in tall fescue
Introduction
Heat stress is a major detrimental abiotic stress limiting the growth of cool-season plant species during summer months. As sessile organisms, plants have evolved signaling pathways to rapidly detect changes in ambient temperature, which activate transcription factors that induce changes in downstream genes and proteins and regulate various metabolic processes that enable plants to function and survive under heat stress (Mittler et al., 2012). Plant responses to heat stress are very complex and are comprised of many signaling pathways, where many signal molecules involved in each pathway play important roles in high temperature responses (Brestic et al., 2014).
The major signaling molecules that accumulate in response to heat stress include those in metabolic pathways, such as reactive oxygen species (ROS) and hormones, as well as membrane-sensing pathways, such as phospholipids and Ca2+ (Brestic et al., 2014). Hydrogen peroxide (H2O2) is one of the key elements in the ROS signaling pathway and was found to induce plant heat tolerance through exogenous application (Banzet et al., 1998, Desikan et al., 2001, Larkindale and Huang, 2005). Cytokinins (CK) are among the most well-known hormones that play positive roles in improving abiotic stress tolerances, including heat tolerance (Brestic et al., 2014). Increasing endogenous CK content by foliar application of compounds containing CK, such as seaweed extract-based CK and trans-zeatin riboside (t-ZR), alleviated heat-induced damages in cool-season turfgrass (Zhang and Ervin, 2008). Salicylic acid (SA) has been known to be a stress defense activator (Raskin et al., 1987), and foliar treatment of plants with SA significantly improved plant heat tolerance (Dat et al., 1998, He et al., 2005, Larkindale and Huang, 2005). Plasma membranes are sensitive to heat stress, and changes in membrane fluidity in response to increasing temperature serve pivotal roles in activating stress signaling (Saidi et al., 2009, Sangwan et al., 2002). In response to heat stress, an inward calcium flux, which is most likely activated by a temperature-induced increase in fluidity of the plasma membrane, has been considered to be the primary pathway in heat signal transduction (Mittler et al., 2012, Saidi et al., 2009). Application of Ca2+ can enhance plant intrinsic heat tolerance and maintain antioxidant activities by decreasing membrane lipid peroxidation (Gong et al., 1997, Jiang and Huang, 2001). Heat-induced changes in membrane fluidity also trigger lipid signaling, resulting in the accumulation of various lipid-signaling molecules, including phosphatidic acid (PA) (Mishkind et al., 2009, Munnik, 2001), which targets phosphoinositide-dependent kinase 1 (PDK1), abscisic acid insensitive 1 (ABI1), Phosphoenolpyruvate carboxylase (PEPC) and calcium-dependent protein kinase (CDPK) in response to various biotic and abiotic stress factors in plants (Testerink and Munnik, 2005). Despite the known positive physiological effects of H2O2, CK, SA, Ca2+ and PA on heat tolerance, the relative effectiveness of those molecules are not well documented, particularly for heat-sensitive cool-season plant species. Additionally, the molecular factors regulated by the major signaling molecules that may lead to improved heat tolerance are not well understood.
Heat signal transduction pathways are composed of cascades of kinases and some transcription factors (Mittler et al., 2012). As the terminal components of the signal transduction chain, heat stress transcription factors (HSFs) are key factors that receive heat stress signals from upstream pathways and trigger the transcription cascades of downstream heat responsive genes (Mittler et al., 2012). It was reported that calcium-activated calmodulin (Ca2+-CaM) was directly involved in the heat stress signal transduction pathway in wheat (Triticum aestivum L.) and up-regulated heat shock proteins (HSPs) under heat stress (Liu et al., 2003). HSPs of various molecular sizes (i.e. HSP18, HSP70, HSP90 and HSP101) are known to be positive regulators of heat tolerance in various plant species (Zhou and Abaraha, 2007). Moreover, pretreatment of whole cell extract of maize (Zea mays L.) seedlings with Ca2+ increased in vitro DNA binding of HSFs at room temperature, whereas the Ca2+ chelator ethylene glycol tetraacetic acid (EGTA) abolished HSF binding when activated by heating (Li et al., 2004). H2O2 is also known to be involved in the early phase of heat stress and is required for effective expression of HSPs and HSFs in Arabidopsis and rice (Oryza sativa L.) (Volkov et al., 2006, Wang et al., 2009). Other signal molecules involved in early perception and transduction of heat stress signals for activating HSF-mediated heat stress response are not well documented.
The plant HSF family is large with a multitude of complex structures, classifications, and functions; however, among all the members of this family, A2 group members of HSF (HsfA2s) are shown to be crucial factors in heat stress response (Scharf et al., 2012). In our previous study, we identified the positive roles of heat stress transcription factor A2c (HsfA2c) in conferring heat tolerance in tall fescue (Festuca arundinacea Schreb.) (Wang et al., 2016). We hypothesized that PA, Ca2+, H2O2, SA, and CK may improve heat tolerance in tall fescue involving the regulation of HsfA2c and related HSP genes. The objectives of this study were to examine the effectiveness of these selected signal molecules for improving heat tolerance of cool-season grass species and to determine the signal transduction pathways regulated by those compounds through activating HsfA2c and related HSPs.
Section snippets
Plant materials and growth conditions
Seeds of tall fescue (cv. ‘Kentucky 31′) were sown in plastic pots (10 cm diameter and 20 cm depth) filled with a mixture of soil and sand (2:1, v/v) and were established in a greenhouse controlled at 25/20 °C (day/night temperature) for a month. During the establishment period, plants were irrigated every two days, fertilized weekly with half strength of Hoagland’s nutrient solution (Hoagland and Arnon, 1950), and trimmed every three days to keep the canopy height at 8 cm. Plants were transferred
Physiological effects of exogenous treatments on improving heat tolerance
Tall fescue plants treated with PA, CaCl2, H2O2, SA and t-ZR had significantly higher turf quality rating (5.05, 5.10, 3.63, 3.88 and 3.35, respectively) than the water control (2.55) at 35 d of heat stress at 38/33 °C, with PA and CaCl2 being most effective on improving plant heat tolerance among the five treatments (Fig. 1A, B). In addition, plants treated with these five exogenous substances had significantly lower electrolyte leakage compared to the water control (Fig. 1C), indicating
Discussion
PA, CaCl2, H2O2, SA and t-ZR have been reported to play positive roles in plant tolerance to abiotic stresses, including heat stress (Larkindale and Huang, 2005, Liu et al., 2003, Mishkind et al., 2009, Munnik, 2001, Zhang and Ervin, 2008). This study found that exogenous application of all these five chemicals enhanced heat tolerance of tall fescue, while PA and CaCl2 had more pronounced effects compared to the other chemicals.
The primary sensing event of heat signaling has been shown to occur
Conflict of interest
The authors declare no competing financial interests.
Author contributions
Bingru Huang developed the research ideas, and provided support of research expenses. Xiuyun Wang performed all the experiments. Both authors designed the study, performed the data analysis and writing the manuscript.
Acknowledgements
We thank Dr. Weili Zhang and Patrick Burgess for assistance with the propagation and maintenance of plant materials. Thanks go to Cathryn Chapman and Stephanie Rossi for critical review of the manuscript. Thanks to the China Scholarship Council for stipend support and to Rutgers Center for Turfgrass Science for research support of Xiuyun Wang for collaborative research at Rutgers University.
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