Nitric oxide produced during the hypersensitive response modulates the plant signaling network and inhibits the pathogen’s virulence machinery

Nitric oxide (NO) is a well-recognized key signaling mediator of the plant defense response to pathogens. Like in animals, NO acts directly or through mediators of its activity such as reactive NO derivatives (RNS) and second messengers like cGMP. We aim at deciphering the “signaling network” mediated in plant by NO and RNS upon pathogen attack, and by using the HKGreen-2 dye we demonstrated that peroxynitrite, the RNS originated by the reaction between NO and O 2 - , accumulates in plants during the hypersensitive response (HR). Peroxynitrite is not a “death messenger” in plants, and its accumulation in plants correlates with an increase in tyrosine nitrated proteins, and we demonstrated that in vitro peroxynitrite targets specifically some MAPK kinases, inhibiting their activity and thus precisely modulating the most complex plant signal transduction network by tyrosine nitration. Furthermore, we recently found that NO can participate in plant defenses also by inhibiting pathogen effector activity. The effector HopAI1, a phosphothreonine lyase produced by many Pseudomonas syringae strains which suppresses plant immunity via MAPK inhibition, is a target of NO and its activity is dramatically inhibited by S-nitrosylation. Whereas HopAI1 expressed in Arabidopsis thaliana does not affect the HR induced by the avirulent P. syringae AvrRpt2, the expression of a mutated (Cys free) form of HopAI1 strongly reduces the hypersensitive cell death as well as plant resistance. This suggests that NO modulates the plant signaling network through nitration of specific MAPK and inhibits pathogen’s effectors that would attenuate the plant defence response.