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Understanding how microorganisms manipulate plant innate immunity and colonize host cells is a major goal of plant pathology. Here, we report that the fungal nitrooxidative stress response suppresses host defenses to facilitate the growth and development of the important rice pathogen Magnaporthe oryzae in leaf cells. Nitronate monooxygenases encoded by NMO genes catalyze the oxidative denitrification of nitroalkanes. We show that the M. oryzae NMO2 gene is required for mitigating damaging lipid nitration under nitrooxidative stress conditions and, consequently, for using nitrate and nitrite as nitrogen sources. On plants, the Δnmo2 mutant strain penetrated host cuticles like wild type, but invasive hyphal growth in rice cells was restricted and elicited plant immune responses that included the formation of cellular deposits and a host reactive oxygen species burst. Development of the M. oryzae effector-secreting biotrophic interfacial complex (BIC) was misregulated in the Δnmo2 mutant. Inhibiting or quenching host reactive oxygen species suppressed rice innate immune responses and allowed the Δnmo2 mutant to grow and develop normally in infected cells. NMO2 is thus essential for mitigating nitrooxidative cellular damage and, in rice cells, maintaining redox balance to avoid triggering plant defenses that impact M. oryzae growth and BIC development.Global rice yields are significantly and negatively impacted each year by blast disease caused by the hemibiotrophic fungus Magnaporthe oryzae 1-3 (synonym of Pyricularia oryzae). Defining the full spectrum of molecular pro- Marroquin-Guzman et al. in Nature Microbiology 2 (2017) 2 cesses used by M. oryzae to manipulate rice innate immunity and allow fungal colonization of host cells might reveal additional sources of pathogen resistance and improve crop health. M. oryzae infects hosts by first forming specialized infection structures, appressoria, at the tips of germ tubes emerging from spores adhered to the leaf surface. 4,5 A thin penetration peg emerging from an unmelanized patch on the base of the appressorium 6 is forced through the rice leaf cuticle under hydrostatic turgor pressure 1 . In the first penetrated cell, the peg differentiates into primary hyphae then bulbous invasive hyphae (IH) that are surrounded by the plant-derived extra-invasive hyphal membrane (EIHM). Branching IH fill the first invaded cell before spreading into neighboring living rice cells at around 44 h post-inoculation. 7,8 This biotrophic growth phase progresses for 4-5 days before M. oryzae enters its necrotrophic phase.To colonize rice cells, M. oryzae must first suppress or avoid triggering two types of plant innate immunity that protect against microbial attack [9][10][11] : pathogen-associated molecular pattern (PAMP) triggered immunity (PTI), which can be suppressed by microbial effectors, and effector-triggered immunity (ETI), if effectors are detected. The biotrophic interfacial complex (BIC), a host membrane-derived structure, is formed behind M. oryzae IH in each in...

Aerobic metabolism generates biologically challenging reactive oxygen species (ROS) by the endogenous autooxidation of components of the electron transport chain (ETC). Basal levels of oxidative stress can dramatically rise upon activation of the NADPH oxidase-dependent respiratory burst. To minimize ROS toxicity, prokaryotic and eukaryotic organisms express a battery of low-molecular-weight thiol scavengers, a legion of detoxifying catalases, peroxidases, and superoxide dismutases, as well as a variety of repair systems. We present herein blockage of bacterial respiration as a novel strategy that helps the intracellular pathogen Salmonella survive extreme oxidative stress conditions. A Salmonella strain bearing mutations in complex I NADHdehydrogenasesisrefractorytotheearlyNADPHoxidasedependent antimicrobial activity of IFN␥-activated macrophages. The ability of NADH-rich, complex I-deficient Salmonella to survive oxidative stress is associated with resistance to peroxynitrite (ONOO ؊ ) and hydrogen peroxide (H 2 O 2 ). Inhibition of respiration with nitric oxide (NO) also triggered a protective adaptive response against oxidative stress. Expression of the NDH-II dehydrogenase decreases NADH levels, thereby abrogating resistance of NO-adapted Salmonella to H 2 O 2 . NADH antagonizes the hydroxyl radical (OH ⅐ ) generated in classical Fenton chemistry or spontaneous decomposition of peroxynitrous acid (ONOOH), while fueling AhpCF alkylhydroperoxidase. Together, these findings identify the accumulation of NADH following the NO-mediated inhibition of Salmonella's ETC as a novel antioxidant strategy. NO-dependent respiratory arrest may help mitochondria and a plethora of organisms cope with oxidative stress engendered in situations as diverse as aerobic respiration, ischemia reperfusion, and inflammation.Oxidative stress engendered by the sustained synthesis of NO mediates cytotoxicity against a variety of eukaryotic and prokaryotic cells (1-3). Because of its unpaired electron, NO directly reacts with metal prosthetic groups of cytochromes in the electron transport chain (ETC) 2 and [Fe-S] clusters of dehydratases (4, 5). Alternatively, reactive nitrogen species (RNS) generated through the interaction of NO with O 2 and superoxide (O 2 . ) indirectly mediate cytotoxicity of this diatomic radical.

By remodeling the phagosomal membrane, the type III secretion system encoded within the Salmonella pathogenicity island-2 (SPI2) helps Salmonella thrive within professional phagocytes. We report here that nitric oxide (NO) generated by IFNγ-activated macrophages abrogates the intracellular survival advantage associated with a functional SPI2 type III secretion system. NO congeners inhibit overall expression of SPI2 effectors encoded both inside and outside the SPI2 gene cluster, reflecting a reduced transcript level of the sensor kinase SsrA that governs overall SPI2 transcription. Down-regulation of SPI2 expression in IFNγ-treated macrophages does not seem to be the result of global NO cytotoxicity, because transcription of the housekeeping rpoD sigma factor remains unchanged, whereas the expression of the hmpA-encoded, NO-metabolizing flavohemoprotein is stimulated. Because of the reduced SPI2 expression, Salmonella-containing vacuoles interact more efficiently with compartments of the late endosomal/lysosomal system in NO-producing, IFNγ-treated macrophages. These findings demonstrate that inhibition of intracellular SPI2 transcription by NO promotes the interaction of Salmonella phagosomes with the degradative compartments required for enhanced antimicrobial activity. Transcriptional repression of a type III secretion system that blocks phagolysosome biogenesis represents a novel mechanism by which NO mediates resistance of IFNγ-activated phagocytes to an intracellular pathogen.

The Lyme disease bacterium Borrelia burgdorferi survives diverse environmental challenges as it cycles between its tick vectors and various vertebrate hosts. B. burgdorferi must withstand prolonged periods of starvation while it resides in unfed Ixodes ticks. In this study, the regulatory protein DksA is shown to play a pivotal role controlling the transcriptional responses of B. burgdorferi to starvation. The results suggest that DksA gene regulatory activity impacts B. burgdorferi metabolism, virulence gene expression, and the ability of this bacterium to complete its natural life cycle.