Opposing Functions for Plant Xanthine Dehydrogenase in Response to Powdery Mildew Infection: Production and Scavenging of Reactive Oxygen Species

Xanthine dehydrogenases are fascinating enzymes. Their basic function is in purine catabolism, catalyzing the conversion of hypoxanthine to xanthine and xanthine to uric acid. In mammals, xanthine dehydrogenases, which use NAD⁺ as electron acceptor to produce uric acid, can be posttranslationally modified to become xanthine oxidases, which use O₂ as electron acceptor to produce reactive oxygen species (ROS). Because uric acid is a scavenger of ROS, the resulting influence of xanthine dehydrogenase on ROS status is likely responsible for its importance in a remarkably wide range of processes in mammals, including immunity (reviewed in Vorbach et al., 2003). Plant xanthine dehydrogenases, like those of chicken and Drosophila melanogaster, have not been reported to undergo an analogous posttranslational modification, but do appear capable of using both O₂ and NAD⁺ as electron acceptors, as well as of producing high levels of ROS (Zarepour et al., 2010). Now, Ma et al. (2016) report that xanthine dehydrogenase is important in defense responses in Arabidopsis thaliana, likely through roles in both production of ROS in epidermal cells and scavenging of ROS in mesophyll cells.

The powdery mildew fungus Golovinomyces cichoracearum triggers defense responses in Arabidopsis mediated by the R gene RPW8.2. In a screen for mutants defective in RPW8.2-related resistance to powdery mildew, Ma and coworkers identified three plants with point mutations in XANTHINE DEHYDROGENASE1 (XDH1), including two that altered residues strictly conserved among xanthine dehydrogenases. The mutants showed impaired resistance to powdery mildew and accumulated less H₂O₂ in the haustorial complex in epidermal cells invaded by the fungus. These point mutations decreased the activity of recombinant XDH1 proteins, in terms of both dehydrogenase activity and ROS production. Thus, XDH1 activity appears to be important for RPW8.2-mediated powdery mildew resistance.

Interestingly, the authors observed autofluorescent objects (AFOs) within mesophyll cells of the mutant plants. The AFOs were highly enriched in xanthine, and the authors cleverly used AFO formation as a marker for xanthine accumulation with both spatial and temporal resolution. They found that xanthine accumulation in the mutants was further induced by pathogen inoculation, suggesting that there is increased purine catabolism (mediated at least in part by XDH1) upon infection. Importantly, the authors observed both local and systemic induction of AFOs (i.e., purine catabolism) in response to powdery mildew infection. This finding supports the authors’ conclusion that upregulation of purine catabolism is part of the defense response.

Cell death during the hypersensitive response is generally preceded by production of ROS and is often confined to site of infection. Ma et al. propose that plants spatially constrain the hypersensitive response by upregulating purine catabolism to produce uric acid (via XDH1 activity) as a ROS scavenger. Indeed, the authors found that feeding uric acid suppressed the H₂O₂ accumulation normally observed in mesophyll cells of infected xdh1 mutant plants. Together, the data in this work support a model in which XDH1 in epidermal cells contributes to the ROS burst to help restrict the area of fungal infection while XDH1 in underlying mesophyll cells serves to protect chloroplasts from oxidative damage by producing ROS scavengers (see figure). Thus, XDH1 appears to be an important tool allowing plants to harness and direct the power of ROS.

XDH1 plays opposing roles to harness ROS in resistance to powdery mildew fungus. In invaded epidermal cells, XDH1 produces superoxide, contributing to the ROS burst that constrains the fungal haustorium. In mesophyll cells, XDH1 produces uric acid to scavenge ROS and protect chloroplasts from oxidative damage. (Adapted from Figure 9 of Ma et al. [2016].)