Mutation in Arabidopsis β-cyanoalanine synthase overcomes NADPH oxidase action in response to pathogens

Plant responses to pathogens comprise a complex process, implying a plethora of signals and reactions. Among them, endogenous production of hydrogen cyanide (HCN) has been shown to induce resistance in Arabidopsis to the hemibiotrophic bacterium Pseudomonas syringae pv. tomato (Pst) DC3000. β-cyanoalanine synthase (CAS-C1) is responsible for the detoxification of HCN in Arabidopsis mitochondria. Here, we show that green fluorescent protein-tagged CAS-C1 is transiently reduced in leaves infected with an avirulent strain of Pst during early interactions and increased in leaves infected with a virulent strain of Pst, supporting previous transcriptional data. Genetic crosses show that mutation in CAS-C1 in Arabidopsis resembles the action of the NADPH oxidase RbohD independently of reactive oxygen species production and that the accumulation of salicylic acid is required for HCN-stimulated resistance to Pst. Finally, we show that the cas-c1 mutation acts on the salicylic acid-dependent response to pathogens by mechanisms other than protein ubiquitination or the increase of monomerization and entry to the nucleus of NPR1, the central regulator of the salicylic acid-mediated response. Considering these results, we propose new mechanisms for modulation of the immune response by HCN.