A novel role of PR 2 in abscisic acid ( ABA ) mediated, pathogen‐induced callose deposition in Arabidopsis thaliana

Summary Pathogenesis‐related protein 2 ( PR 2) 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 PR 2 in callose deposition in relation to ABA treatment and challenge with L. maculans and Pseudomonas syringae . Characterization of PR 2‐overexpressing plants and the knockout line indicated that PR 2 negatively affects callose deposition. Recombinant PR 2 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 PR 2 knockout line compared with the wild‐type. ABA pretreatment before challenge with L. maculans concomitantly repressed PR 2 and enhanced callose accumulation. Likewise, overexpression of an ABA biosynthesis gene NCED 3 resulted in reduced PR 2 expression and increased callose deposition. We propose that ABA promotes callose deposition through the transcriptional repression of PR 2 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 PR 2 may function as a modulator of callose‐ and SA ‐dependent defense responses.