Robust anti‐oxidant defences in the rice blast fungusMagnaporthe oryzaeconfer tolerance to the host oxidative burst

Abstract

SummaryPlants respond to pathogen attack via a rapid burst of reactive oxygen species (ROS). However, ROS are also produced by fungal metabolism and are required for the development of infection structures in Magnaporthe oryzae.To obtain a better understanding of redox regulation in M. oryzae, we measured the amount and redox potential of glutathione (E GSH ), as the major cytoplasmic anti-oxidant, the rates of ROS production, and mitochondrial activity using multi-channel four-dimensional (x,y,z,t) confocal imaging of Grx1-roGFP2 and fluorescent reporters during spore germination, appressorium formation and infection.High levels of mitochondrial activity and ROS were localized to the growing germ tube and appressorium, but E GSH was highly reduced and tightly regulated during development. Furthermore, germlings were extremely resistant to external H 2 O 2 exposure ex planta. E GSH remained highly reduced during successful infection of the susceptible rice cultivar CO39. By contrast, there was a dramatic reduction in the infection of resistant (IR68) rice, but the sparse hyphae that did form also maintained a similar reduced E GSH .We conclude that M. oryzae has a robust anti-oxidant defence system and maintains tight control of E GSH despite substantial oxidative challenge. Furthermore, the magnitude of the host oxidative burst alone does not stress the pathogen sufficiently to prevent infection in this pathosystem.

Reactive oxygen species and plant resistance to fungal pathogens

“…This elegant approach showed the importance of intracellular redox differences between infecting hyphae or in appressorial structures during fungal invasion (Heller et al, 2012). Similar observations were made in M. grisea lines where glutathione and ROS production where determined by live cell imaging using Grx1-roGFP2 and fluorescent markers (Samalova et al, 2014).…”

“…Confocal imaging using fluorescent reporters was used to characterise the cytosolic glutathione redox potential during spore germination, appressorium formation and infection. Results show that M. grisea is endowed with solid antioxidative defences even during reduced penetration of the fungus in resistant hosts and ROS produced by the host are unlikely to be a direct toxic barrier for the fungus (Samalova et al, 2014). YAP-1 of Ustilago maydis is a homolog of the yeast AP-1-like protein that regulates the response to oxidative stress.…”

Investigating chitin deacetylation and chitosan hydrolysis during vegetative growth in Magnaporthe oryzae

“…, 2014) and incubated for specified times in the growth chamber. For imaging of mycelial growth, conidia were incubated into liquid CM and incubated in the dark at 24 °C for ~72 hr, with shaking at 150 rpm.…”

Reactive oxygen species metabolism and plant-fungal interactions

“…This co-evolution between plant and pathogens serves as the basis of the zig-zag model of plant pathology (Hillmer et al, 2017;Jones and Dangl, 2006). However, oxidative bursts are not effective in preventing proliferation of the necrotrophic fungi B. cinerea and Sclerotinia sclerotiorum, and can be tolerated by the hemibiotrophs, Septoria tritici and M. oryzae (Govrin and Levine, 2000;Shetty et al, 2007;Samalova et al, 2014;Marroquin-Guzman et al, 2017). How do fungi deal with host ROS and maintain redox balance during host infection?…”

“…To eliminate ROS (and RNS), fungal pathogens have developed robust antioxidation systems involving superoxide dismutases (SODs), catalases, peroxidases, glutathione and thioredoxin. Interestingly, although ROS was thought to act as a toxifying component in hosts against plant pathogens, it is not produced in amounts sufficient to overcome these pathogens (Marroquin-Guzman et al, 2017;Samalova et al, 2014) and instead is emerging as a signaling component of the plant defense response.…”