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A Secreted SPRY Domain-Containing Protein (SPRYSEC) from the Plant-Parasitic Nematode Globodera rostochiensis Interacts with a CC-NB-LRR Protein from a Susceptible Tomato

Esophageal gland secretions from nematodes are believed to include effectors that play important roles in plant parasitism. We have identified a novel gene family encoding secreted proteins specifically expressed in the dorsal esophageal gland of Globodera rostochiensis early in the parasitic cycle, and which contain the B30.2/SPRY domain. The secondary structure of these proteins, named the secreted SPRY domain-containing proteins (SPRYSEC), includes highly conserved regions folding into β-strands interspersed with loops varying in sequence and in length. Mapping sequence diversity onto a three-dimensional structure model of the SPRYSEC indicated that most of the variability is in the extended loops that shape the so-called surface A in the SPRY domains. Seven of nine amino acid sites subjected to diversifying selection in the SPRYSEC are also at this surface. In both yeast-two-hybrid screening using a library from a susceptible tomato and in an in vitro pull-down assay, one of the SPRYSEC interacted with the leucine-rich repeat (LRR) region of a novel coiled-coil nucleotide-binding LRR protein, which is highly similar to members of the SW5 resistance gene cluster. Given that the tomato cultivar used is susceptible to nematode infection, this SPRYSEC could be an evolutionary intermediate that binds to a classical immune receptor but does not yet, or no longer, triggers a resistance response. Alternatively, this SPRYSEC may bind to the immune receptor to downregulate its activity.