Perception and First Defense Responses Against <i>Pseudomonas syringae</i> pv. <i>phaseolicola</i> in <i>Phaseolus vulgaris</i>: Identification of Wall-Associated Kinase Receptors

Common bean (<i>Phaseolus vulgaris</i>) is attacked by several pathogens such as the biotrophic gamma-proteobacterium <i>Pseudomonas syringae</i> pv. <i>phaseolicola</i>. To study the <i>P. syringae</i> pv. <i>phaseolicola</i>-bean interaction during the first stages of infection, leaf discs of a susceptible bean cultivar Riñón were infected with pathogenic <i>P. syringae</i> pv. <i>phaseolicola</i>. Using this experimental system, we tested six new putative wall-associated kinase (WAK) receptors, previously identified in silico. These six <i>P. vulgaris</i> WAKs (PvWAKs) showed high protein sequence homology to the well-described <i>Arabidopsis thaliana</i> WAK1 (AtWAK1) receptor and, by phylogenetic analysis, clustered together with AtWAKs. The expression of <i>PvWAK</i>1 increased at very early stages after the <i>P. syringae</i> pv. <i>phaseolicola</i> 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 and salicylic acid (SA), as well as the expression of six defense-related genes: <i>MEKK-1</i>, <i>MAPKK</i>, <i>WRKY33</i>, <i>RIN4</i>, <i>PR1</i>, and <i>NPR1</i>. The results showed that overexpression of <i>PR1</i> occurred 2 h after <i>P. syringae</i> pv. <i>phaseolicola</i> 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 caused by the initial increase in total H₂O₂ early after infection. In conclusion, Riñón can perceive the presence of <i>P. syringae</i> pv. <i>phaseolicola</i>, but this recognition results in only a modest and slow activation of host defenses, leading to high susceptibility to <i>P. syringae</i> pv. <i>phaseolicola</i>.