Novel interrelationship between salicylic acid, abscisic acid, and PIP2-specific phospholipase C in heat acclimation-induced thermotolerance in pea leaves

Increasing evidence suggests that heat acclimation and exogenous salicylic acid (SA) and abscisic acid (ABA) may lead to the enhancement of thermotolerance in plants. In this study, the roles that free SA, conjugated SA, ABA, and phosphatidylinositol-4,5-bisphosphate (PIP2)-specific phospholipase C (PLC) play in thermotolerance development induced by heat acclimation (38 degrees C) were investigated. To evaluate their potential functions, three inhibitors of synthesis or activity were infiltrated into pea leaves prior to heat acclimation treatment. The results showed that the burst of free SA in response to heat acclimation could be attributed to the conversion of SA 2-O-D-glucose, the main conjugated form of SA, to free SA. Inhibition of ABA biosynthesis also resulted in a defect in the free SA peak during heat acclimation. In acquired thermotolerance assessment, the greatest weakness of antioxidant enzyme activity and the most severe heat injury (malondialdehyde content and degree of wilting) were found in pea leaves pre-treated with neomycin, a well-known inhibitor of PIP2-PLC activity. PsPLC gene expression was activated by exogenous ABA, SA treatments, and heat acclimation after pre-treatments with a SA biosynthesis inhibitor. From these results, PIP2-PLC appears to play a key role in free SA- and ABA-associated reinforcement of thermotolerance resulting from heat acclimation.