Molecular basis for plant susceptibility and resistance

The interaction between wheat and Hessian fly [Mayetiola destructor (Say)] is governed by two interacting processes: induced susceptibility and induced resistance. Induced susceptibility involves the activation of susceptibility (S) genes upon insect attack. The activation of S genes induces the formation of nutritive cells and a carbon sink at the feeding site, and causes elevated conversion of carbon-compounds into nitrogen-compounds. Silence of S genes prevents the formation of nutritive cells and carbon sink, resulting in “resistant” plants that do not carry any effective resistance (R) genes. Induced susceptibility is likely caused by a complex of effector proteins (or so called elicitors) that are injected into host plants by the insect. Multiple members of elicitor-like gene families are clustered within short chromosome regions in the Hessian fly genome. Diversifying selection on the cluster members has resulted in proteins with diverse functions. While these elicitors can manipulate the metabolic pathways of host plants, resulting in induced susceptibility, they can also be recognized by the plant’s surveillance system to initiate defense reaction, resulting in induced resistance. Induced resistance, mediated by specific resistance genes that operate in a gene-for-gene fashion, involves in suppression of S genes in addition to the launch of toxic defense.