A novel role of PR2 in abscisic acid (ABA) mediated, pathogen-induced callose deposition in Arabidopsis thaliana.

Pathogenesis-related protein 2 (PR2) is known to play a major role in plant defense and general stress responses. Resistance against the fungal pathogen Leptosphaeria maculans in Arabidopsis requires abscisic acid (ABA), which promotes the deposition of callose, a β-1,3-glucan polymer. Here, we examined the role of PR2 in callose deposition in relation to ABA treatment and challenge with L. maculans and Pseudomonas syringae. Characterization of PR2-overexpressing plants and the knockout line indicated that PR2 negatively affects callose deposition. Recombinant PR2 purified from Pichia pastoris showed callose-degrading activity, and a considerable reduction in the callose-degrading activity was observed in the leaf extract of the PR2 knockout line compared with the wild-type. ABA pretreatment before challenge with L. maculans concomitantly repressed PR2 and enhanced callose accumulation. Likewise, overexpression of an ABA biosynthesis gene NCED3 resulted in reduced PR2 expression and increased callose deposition. We propose that ABA promotes callose deposition through the transcriptional repression of PR2 in Arabidopsis challenged by L. maculans and P. syringae. Callose by itself is likely to act antagonistically on salicylic acid (SA) defense signaling, suggesting that PR2 may function as a modulator of callose- and SA-dependent defense responses.