A Potential Role for Mitochondrial Produced Reactive Oxygen 10 Species in Salicylic Acid - Mediated Plant Acquired

27 To characterize the function of salicylic acid (SA) in acquired thermotolerance, the effects of heat 28 shock (HS) on wild-type and sid2 (for SA induction deficient 2) was investigated. After HS 29 treatment, the survival ratio of sid2 mutant was lower than that of wild-type. However, 30 pretreatment with hydrogen peroxide (H2O2) rescued the sid2 heat sensitivity. HsfA2 is a key 31 component of acquired thermotolerance in Arabidopsis. The expression of HsfA2 induced by SA 32 was highest among those of heat-inducible Hsfs (HsfA2, HsfA7a, HsfA3, HsfB1, and HsfB2) in 33 response to HS. Furthermore, the application of AsA, an H2O2 scavenger, significantly reduced 34 the expression level of HsfA2 induced by SA. Although SA enhanced the survival of sid2 mutant, 35 no significant effect on the hsfA2 mutant was observed, suggesting that HsfA2 is responsible for 36 SA-induced acquired thermotolerance as a downstream factor. Further, real-time PCR analysis 37 revealed that after HS treatment, SA also up-regulated mRNA transcription of HS protein (Hsp) 38 genes through AtHsfA2. Time course experiments showed an increase in the fluorescence 39 intensity of DCF in the mitochondria occurred earlier than in other regions of the protoplasts in 40 response to SA. The cytochrome reductase activity analysis in isolated mitochondria 41 demonstrated that SA-induced mitochondrial ROS possibly originated from complex III in the 42 respiration chain. Collectively, our data suggest that SA functions and acts upstream of AtHsfA2 43 in acquired thermotolerance, which requires a pathway with H2O2 production involved and is 44 dependent on increased expression of Hsp genes. 45 www.plant.org on February 9, 2016 Published by www.plantphysiol.org Downloaded from Copyright © 2015 American Society of Plant Biologists. All rights reserved. RE TR AC TE D www.plantph siol.org on August 25, 2017 Published by Downloaded from