Preformed defense responses in a powdery mildew-resistant Hungarian cherry pepper cultivar

Abstract

A Hungarian cherry pepper (Capsicum annuum var. cerasiformé) cultivar ('Szentesi') displays resistance to pepper powdery mildew caused by Leveillula taurica. Resistance also develops in susceptible sweet pepper (C. annuum) when grafted on resistant cherry pepper cv. Szentesi rootstocks. Powdery mildew (PM) resistance is correlated with high levels of the defense regulator reactive oxygen species superoxide (O2 ') even in healthy plants. In order to further elucidate the mechanisms of preformed defense responses in cherry pepper cv. Szentesi we have monitored levels of salicylic acid (SA), a key molecule of plant defense signaling and expression of so-called pathogenesis/defense related (PR) genes in healthy pepper plants. Assays of free and bound (glycosylated) SA by high performance liquid chromatography (HPLC) revealed that in leaves of PM-resistant pepper levels of free SA are ca. twice as high compared to that of PM-sensitive plants. No difference occurred in levels of bound (glycosylated) SA. Expression of the CaPR-1 gene was several times higher in leaves of PM-resistant pepper than in sensitive plants as assayed by real time reverse transcription quantitative polymerase chain reaction (real time RT-qPCR). On the other hand, high expression levels of the CaPR-2 (glucanase) gene did not entirely correlate with PM-resistance, being detectable only in PM-resistant cv. Szentesi plants but neither in PM-resistant sweet pepper cv. Totál grafted on cv. Szentesi rootstocks nor in susceptible controls (cv. Totál). It seems that graft-transmissible PMresistance of the cherry pepper cv. Szentesi is correlated not only with high levels of superoxide but also with elevated levels of free salicylic acid and enhanced expression of the defense-related CaPR-1 gene.