Abstract

Common bean (Phaseolus vulgaris L.) is attacked by several pathogens such as the biotrophic gamma-proteobacterium Pseudomonas syringae pv. phaseolicola (Pph). In order to study the Pph-bean interaction during the first stages of infection, leaf disks of a susceptible bean variety named Riñón were infected with a pathogenic Pph. Using this experimental system, six new putative Wall-Associated Kinase (WAKs) receptors, previously identified in silico, were tested. These six bean WAKs (PvWAKs) showed high protein sequence homology to the well-described Arabidopsis WAK1 (AtWAK1) receptor and, by phylogenetic analysis, clustered together with AtWAKs. The expression of PvWAK1 increased at very early stages after the Pph infection. Time course experiments were performed to evaluate the accumulation of apoplastic H2O2, Ca2+ influx, total H2O2, antioxidant enzymatic activities, lipid peroxidation, and the concentrations of abscisic acid (ABA) and salicylic acid (SA), as well as the expression of six defense-related genes - MEKK-1, MAPKK, WRKY33, RIN4, PR1 and NPR1. The results showed that overexpression of PR1 occurred 2 h after Pph infection without a concomitant increase in SA levels. Although apoplastic H2O2 increased after infection, the oxidative burst was neither intense nor rapid and an efficient antioxidant response did not occur, suggesting that the observed cellular damage was due to the initial increase in total H2O2 at early time points after infection. In conclusion, the Riñón variety can perceive the presence of Pph, but this recognition only results in a modest and slow activation of host defenses, leading to high susceptibility to Pph.