Nitric oxide: an effective weapon of the plant or the pathogen?

Abstract

An explosion of research in plant nitric oxide (NO) biology during the last two decades has revealed that NO is a key signal involved in plant development, abiotic stress responses and plant immunity. During the course of evolutionary changes, microorganisms parasitizing plants have developed highly effective offensive strategies, in which NO also seems to be implicated. NO production has been demonstrated in several plant pathogens, including fungi, but the origin of NO seems to be as puzzling as in plants. So far, published studies have been spread over multiple species of pathogenic microorganisms in various developmental stages; however, the data clearly indicate that pathogen-derived NO is an important regulatory molecule involved not only in developmental processes, but also in pathogen virulence and its survival in the host. This review also focuses on the search for potential mechanisms by which pathogens convert NO messages into a physiological response or detoxify both endo- and exogenous NO. Finally, taking into account the data available from model bacteria and yeast, a basic draft for the mode of NO action in phytopathogenic microorganisms is proposed.

Figure 1

The mode of nitric oxide (NO) action in the pathogen unit (A) and the role of pathogen‐derived NO during susceptible plant–pathogen interactions (B). NO generated by pathogenic microorganisms via the l‐arginine and/or nitrite pathway (red lines) is directly or indirectly [via the guanosine 3′,5′‐monophosphate (cGMP) pathway] involved in various developmental processes, and may also be responsible for the activation of toxins and virulence factors. NO accumulation by the invading pathogen may induce S‐nitrosylation of host membrane receptors and/or channels to effectively facilitate cellular entry. When the invader colonizes plant tissues, it destroys intracellular compartments of the host, which contributes to uncontrolled NO overproduction. Simultaneously, the host plant activates various defence responses, including its own programme of boosted NO production. Such huge amounts of NO in the susceptible genotype provoke nitrosative stress, leading to uncontrolled tyrosine nitration, facilitating tissue damage and colonization by the pathogen. Blue lines and areas, pathogen‐derived NO; white areas outlined in green, plant‐derived NO.