Salicylic acid synthesized by benzoic acid 2-hydroxylase participates in the development of thermotolerance in pea plants

To investigate importance of salicylic acid (SA) and SA biosynthesis pathway in the development of plant thermotolerance, the research tested the effects of heat acclimation on pea plants (Pisum sativun L.) at 37 °C for 2 h and examined in vivo pretreatments of the plants in a medium containing either SA or SA biosynthesis inhibitors before heat treatment. The results showed that both heat acclimation and SA application could reduce leaf injury caused by subsequent heat stress and induce the synthesis of heat shock proteins (Hsp 70 and Hsp 17.6). Further, membrane lipid peroxidation caused by the heat stress was found to decrease, suggesting that plant's thermotolerance developed as a result of heat acclimation and SA application. A rapid transient increase of endogenous free SA and a subsequent enrichment in Hsp 70 were both elevated by heat acclimation. Changes of phenylalanine ammonia lyase (PAL) and benzoic acid 2-hydroxylase (BA2H) activities roughly paralleled those of free SA. In addition, pretreatment with the inhibitors of either PAL or BA2H effectively blocked SA accumulation and Hsp 70 enrichment in the plants subjected to heat acclimation, thus reduced plant tolerance to heat stress. This finding indicates that inhibition of SA biosynthesis during heat acclimation can annul the development of plant thermotolerance and that SA synthesized by BA2H participates in the acquisition of heat acclimation-induced thermotolerance.