Starch degradation, abscisic acid and vesicular trafficking areimportant elements in callose priming by indole-3-carboxylicacid in response toPlectosphaerella cucumerinainfection

Abstract

A fast callose accumulation has been shown to mediate defence priming in certain plant–pathogen interac-tions, but the events upstream of callose assembly following chemical priming are poorly understood,mainly because those steps comprise sugar transfer to the infection site.b-Amino butyric acid (BABA)-induced resistance in Arabidopsis againstPlectosphaerella cucumerinais known to be mediated by callosepriming. Indole-3-carboxylic acid (ICOOH, also known as I3CA) mediates BABA-induced resistance in Ara-bidopsis againstP. cucumerina. This indolic compound is found in a common fingerprint of primed metabo-lites following treatments with various priming stimuli. In the present study, we show that I3CA inducesresistance in Arabidopsis againstP. cucumerinaand primes enhancement of callose accumulation. I3CAtreatment increased abscisic acid (ABA) levels before infection withP. cucumerina. An intact ABA synthesispathway is needed to activate a starch amylase (BAM1) to trigger augmented callose deposition againstP. cucumerinaduring I3CA-IR. To verify the relevance of the BAM1 amylase in I3CA-IR, knockdown mutantsand overexpressors of theBAM1gene were tested. The mutantbam1was impaired to express I3CA-IR, butcomplemented35S::BAM1-YFPlines in the background ofbam1restored an intact I3CA-IR and callose prim-ing. Therefore, a more active starch metabolism is a committed step for I3CA-IR, inducing callose priming inadult plants. Additionally, I3CA treatments induced expression of the ubiquitin ligaseATL31and syntaxinSYP131, suggesting that vesicular trafficking is relevant for callose priming. As a final element in the callosepriming, an intactPowdery Mildew resistant4(PMR4) gene is also essential to fully express I3CA-IR.