Apoplastic Venom Allergen-like Proteins of Plant Parasitic Nematodes Modulate the Activation of Plant Innate Immunity by Cell Surface Receptors

Despite causing considerable damage to host tissue during parasitism, nematodes establish persistent infections in both animals and plants. It is thought that an elaborate repertoire of nematode effectors suppress damage-triggered immune responses of the host. However, the nature and mode of action of most of nematode immunomodulatory compounds are not well understood. We have recently discovered that the effector venom allergen-like proteins (VAPs) of plant-parasitic nematodes selectively suppress host immunity during the onset of parasitism. VAPs are uniquely conserved in secretions of all animal- and plant-parasitic nematodes, but their role in parasitism has remained elusive. Knocking-down the expression of Gr-VAP1 severely hampered the infectivity of Globodera rostochiensis. By contrast, heterologous expression of Gr-VAP1 and VAPs from Heterodera schachtii in plants resulted in the loss of basal immunity to multiple pathogens. Surprisingly, these VAPs only affect the defense responses mediated by surface-localized immune receptors. The modulation of basal immunity by ectopic VAPs involves extracellular protease-based host defenses and chloroplast-localized non-photochemical quenching. The delivery of VAPs into host tissue coincides with the enzymatic breakdown of plant cell walls by migratory nematodes. We, therefore, conclude that parasitic nematodes most likely utilize VAPs to suppress the activation of defenses by immunogenic breakdown products in damaged host tissue.