Exogenous CaCl₂ Enhances Heat Tolerance In Tobacco

In this study we investigated the role of exogenous CaCl2 application in improving heat tolerance in tobacco plants. The main results are as follows:(1) Heat stress resulted in a decrease in net photosynthetic rate and stomatal conductance, while the plants pretreated with CaCl2 maintained higher net photosynthetic rate and stomatal conductance, which was associated with less reactive oxygen species (ROS) accumulation.(2) The analysis of chlorophyll fluorescence demonstrated that CaCl2 application improved heat tolerance of photosystem II (PSII), suggesting that CaCl2 pretreatment increased tolerance to high temperature-enhanced photoinhibition, which was associated with improved resistance of oxygen-evolving complex (OEC) and the reaction center of PSII to heat stress.(3) Heat stress induced a significant accumulation of H2O2, superoxide (O2-) in tobacco leaves, but this accumulation was much less in plants pretreated with CaCl2, and CaCl2 application reduced this accumulation under normal condition. Exogenous Ca2+ decreased the accumulation of ROS through increasing or maintaining relatively higher antioxidant enzymes activities and antioxidants content.(4) The Ca2+ content in tobacco leaves increased under high temperature, whereas the Ca2+-ATPase and Mg2+-ATPase activities decreased. This might lead to the increase of Ca2+ content in gurd cell, which induced the closure of stoma. CaCl2 pretreatment maintained or highed the Ca2+-ATPase and Mg2+-ATPase activities, which improved the Ca2+ transportion, maintained the cytoplastic Ca2+ homeostasis.(5) Heat stress induced the expression of HSP70, and exogenous Ca2+ significantly enhanced the HSP70 expression. The higher HSP70 expression maybe associated with the enhanced heat tolerance in tobacco plants.(6) CaCl2 pretreatment futher improved the photosynthesis in transgenic plants with the ability to accumulate glycine betaine under high temperature. This might because that exogenous Ca2+ application reduced the ROS accumulation cooperated with the accumulation of glycine betaine, futher enhanced heat tolerance of PSII, alleviated the damage to the photosynthesitic machine. This was associated with the higher antioxidant enzyme activities and antioxidants content.(7) Though Ca2+ content in transgenic plants was higher than the wide-type ones, higher Ca2+-ATPase and Mg2+-ATPase activities mained the cytoplastic Ca2+ homeostasis. The transgenic plants pretreated with CaCl2 showed higher Ca2+ content, Ca2+-ATPase and Mg2+-ATPase activities.(8) Heat stress also induced the expression of HSP70, and the HSP70 content in transgenic tobacco plants were much higher than that of wild-type ones. Ca2+ application futher enhanced the HSP70 content.In conclusion, the mechanisms of the enhanced heat tolerance by CaCl2 application maybe associated with the followings:(1) CaCl2 pretreatment decreased the damage induced by ROS to photosynthetic apparatus through maintaining relatively higher antioxidant enzyme activities and antioxidants content.(2) CaCl2 application might activate the Ca2+-CaM, improved the HSP70 expression, which was accosiated with enhanced heat tolerance.(3) GB in vivo might cooperate with exogenous Ca2+, further improved the heat tolerance in transgenic tobacco plants with BADH gene.