Chloroplast‐targeted antioxidant protein protects against necrotrophic fungal attack

Plant pathogens have various strategies to overcome plant defenses. Biotrophs use a stealth strategy, living within the host and acquiring nutrients from it but without killing it; necrotrophs kill the infected cells and consume nutrients. There are also pathogens called hemi-biotrophs, which start out using a biotrophic strategy and then later become necrotrophs. In turn, plants have various strategies to resist attack. To deal with pathogens, plants often exhibit a hypersensitive response (HR), i.e. accumulating reactive oxygen species (ROS), inducing expression of pathogenesis-related proteins, and promoting localized cell death (LCD) at the infection site, thereby stopping the spread of the pathogen to healthy parts of the plant. However, these defenses (ROS, LCD), which are successful against biotrophs, are not useful against necrotrophs, since these defenses only make it easier for necrotrophs – they want dead tissue. ROS can be generated in various compartments of the plant cell, and chloroplasts are the main ROS source in illuminated leaves. The Carrillo group previously showed (Zurbriggen et al., 2009) that expressing a chloroplast-targeted flavodoxin (Fld) specifically reduced plastid-generated ROS and consequently inhibited LCD upon inoculation with a hemi-biotrophic bacterial pathogen, Xanthomonas campestris, albeit a non-host interaction where infection wasn’t expected. In this highlighted article (Rossi et al., 2017), the groups of N estor Carrillo and Fernando Pieckenstain demonstrate that this Fld approach is effective against a necrotrophic fungus, Botrytis cinerea. How did this work unfold? N estor decided that collaboration with an expert in the lifestyle and virulence of necrotrophic microorganisms was needed. Although the plant science community in Argentina is relatively small, and N estor and Fernando knew of each other’s work, they had not met until they worked out a plan for collaborating, when they both attended a science meeting. They chose Botrytis cinerea as a model necrotrophic pathogen, as it represents a significant threat to a wide range of crop plants, and at the same time there is plenty of information about its pathogenicity mechanisms, range of hosts and plant responses. Photosynthesis is an early target of both biotic and abiotic stresses. Inhibiting photosynthesis leads to increased ROS, resulting in a self-propagating (vicious) cycle of ROS production and cell damage (Tognetti et al., 2006). Light harvesting with limited possibilities of efficient photochemistry represents the main ROS source in illuminated leaves, because most ROS are produced as byproducts of photosynthetic electron transport (PET). Differential preservation of photosynthetic activities is therefore a sensitive physiological marker of low ROS levels and tissue vitality: relevant for both biotrophs and necrotrophs, but especially important for necrotrophs that feed on dead tissue. They accordingly measured photosynthetic flows and quantum yields of photochemical processes at the level of photosystem II (PSII), which mostly reflects the efficiency of lineal electron flow, i.e. the pathway responsible for NADPH and ATP synthesis. A fraction of the photons absorbed by PSII are trapped as excitons and used for photochemistry, while the rest are dissipated in various energy forms (fluorescence, heat). Fld expression affects photosynthetic performance even under mock conditions, by increasing the number of active reaction centers per leaf cross-section. Botrytis infection inactivated these reaction centers, leading to progressive energy overloading of those still active and a general decline of photosynthetic efficiency. Consequently, ROS production increases and photosynthesis is further inhibited (Figure 1a). By providing an alternative electron sink at the acceptor end of the PET chain, Fld drives reducing equivalents away from oxygen and into productive pathways (Figure 1b), protecting reaction centers from inactivation and improving photosynthetic efficiency. What’s next? Their plans include RNAseq analysis of WT and pfld plants inoculated with B. cinerea to identify differentially-expressed genes whose products might be implicated in signaling tissue death during fungal infection, especially those subject to redox modulation. These genes could be targeted to improve resistance to Botrytis. They will evaluate if establishing systemic resistance is affected in pfld lines. They are also interested in determining what signals exit plastids in response to changes in redox status