Hessian fly Avirulence gene loss‐of‐function defeats plant resistance without compromising the larva's ability to induce a gall tissue

Plant pathogen effectors encoded by Avirulence (Avr) genes benefit the pathogen by promoting colonization and benefit plants that have a matching resistance (R) gene by constituting a signal that triggers resistance. The Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), resembles a plant pathogen in showing R/Avr interactions. Because of these interactions, a wheat plant with the H13 resistance gene can be resistant or susceptible depending on the genotype of the larva that attacks the plant, being resistant if attack comes from a larva with a functional vH13 gene, but susceptible if attack comes from a larva with a non‐functional vH13 gene. In this study we asked: does this susceptible interaction involving plants with H13 look like susceptible interactions with plants lacking H13? Possibly, the H13 plant attacked by a larva with a non‐functional vH13 is induced to partial rather than complete resistance. Or the larva, lacking its vH13‐encoded effector, is compromised in its ability to induce susceptibility, which includes forcing the plant to create a gall nutritive tissue. Responses of epidermal cells to larval attack were explored using imaging techniques (light microscopy, scanning and transmission electron microscopy). Whole‐organism responses were investigated by measuring the growth of plants and larvae. No evidence was found for partial resistance responses by H13 plants or for a compromise in the ability of vH13 loss‐of‐function larvae to induce susceptibility. It appears that disrupting vH13 function is sufficient for overcoming the induced resistance mediated by the H13 gene.