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Parasitic Worms Stimulate Host NADPH Oxidases to Produce Reactive Oxygen Species That Limit Plant Cell Death and Promote Infection

Reactive oxygen species restrict cell death at sites of parasitic nematode infection and support nurse cell formation in plant roots. Promoting Parasitism with ROS Some species of nematode worms can invade the roots of plants and establish a feeding site composed of a large syncytial plant cell. This biotrophic lifestyle requires that the worms find a way to suppress plants’ immune responses. One aspect of plant immunity is the production of reactive oxygen species (ROS) that damage pathogens and promote plant cell death to limit the spread of infection. Siddique et al. found that deleting the enzymes that produce ROS in Arabidopsis thaliana plants responding to infection by Heterodera schachtii worms prevented the worms from establishing syncytia and growing within roots, suggesting that the worms have co-opted plant ROS as a means of promoting parasitism. Thus, plant ROS can play both positive and negative roles during infection. Plants and animals produce reactive oxygen species (ROS) in response to infection. In plants, ROS not only activate defense responses and promote cell death to limit the spread of pathogens but also restrict the amount of cell death in response to pathogen recognition. Plants also use hormones, such as salicylic acid, to mediate immune responses to infection. However, there are long-lasting biotrophic plant-pathogen interactions, such as the interaction between parasitic nematodes and plant roots during which defense responses are suppressed and root cells are reorganized to specific nurse cell systems. In plants, ROS are primarily generated by plasma membrane–localized NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidases, and loss of NADPH oxidase activity compromises immune responses and cell death. We found that infection of Arabidopsis thaliana by the parasitic nematode Heterodera schachtii activated the NADPH oxidases RbohD and RbohF to produce ROS, which was necessary to restrict infected plant cell death and promote nurse cell formation. RbohD- and RbohF-deficient plants exhibited larger regions of cell death in response to nematode infection, and nurse cell formation was greatly reduced. Genetic disruption of SID2, which is required for salicylic acid accumulation and immune activation in nematode-infected plants, led to the increased size of nematodes in RbohD- and RbohF-deficient plants, but did not decrease plant cell death. Thus, by stimulating NADPH oxidase–generated ROS, parasitic nematodes fine-tune the pattern of plant cell death during the destructive root invasion and may antagonize salicylic acid–induced defense responses during biotrophic life stages.