MeSA and MeJA increase steady-state transcript levels of alternative oxidase and resistance against chilling injury in sweet peppers ( Capsicum annuum L.)

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Methyl salicylate (MeSA) and methyl jasmonate (MeJA) vapors increased resistance against chilling injury in freshly harvested green bell pepper ( Capsicum annuum L. cv Century). The period within 2 days of cold storage was considered the most critical because chilling injury symptom (surface pitting) was not apparent. The expression patterns of alternative oxidase (AOX) and seven other genes involved in defense against oxidative stress before and during the early chilling period suggested that pre-treatment of pepper fruit with MeSA or MeJA vapors increased preferentially the transcript levels of AOX. Overnight treatment with MeSA or MeJA vapors increased transcript levels of AOX (1.5 kb) even at room temperature of 25 °C, whereas no change was observed with untreated control. In addition to the expected 1.5 kb AOX transcript, RNA gel blot analysis revealed an extra 3.5 kb transcript that was induced only at 0 °C. At 0 °C, both AOX transcripts (1.5 and 3.5 kb) reached maximal levels firstly in MeSA treated fruit, secondly in MeJA treated fruit and lastly in controls. Compared with freshly harvested peppers, overnight treatment of wholesaler peppers with MeJA does not increase AOX transcript level and no differences in chilling injury symptom were observed between treated and control fruit. AOX transcript level in peppers from farm and wholesaler were maintained at a high level as long as the fruits were kept at 0 °C. Transcript levels of AOX (1.5 kb) were increased by storage at low temperature but the steady-state mRNA accumulation rate was faster at 0 than at 5 °C. Alternative respiratory pathway was proposed to mediate chilling injury. Here, we show that the increase in AOX transcript levels by MeJA or MeSA before cold treatment was correlated with reduced incidence of chilling injury.