The pathogenic coronatine toxin hijacks host redox signalling to suppress plant immunity

Reciprocal antagonism between the hormones salicylic acid (SA) and jasmonic acid (JA) is particularly important during infection by the bacterial pathogen Pseudomonas syringae. P. syringae secretion of the virulence-promoting toxin, coronatine, is thought to suppress SA-induced immune responses by manipulating host JA signalling. Here, we report an unexpected JA-independent role of coronatine in promoting pathogen virulence. While JA induced resistance to P. syringae, coronatine promoted pathogen virulence by suppressing cellular accumulation of glutathione, a vital antioxidant required for immunity. Coronatine-mediated suppression of glutathione levels prevented activation of NPR1, a redox-sensitive master regulator of SA-responsive immune genes. Moreover, the accrual of nitric oxide (NO) restored virulence of coronatine-deficient P. syringae, but was counteracted by expression of the host S-nitrosothiol reductase, Thioredoxin h5. Thus, our findings indicate P. syringae utilises coronatine to suppresses host immunity by precisely manipulating glutathione- and NO-mediated redox signalling networks.