Insect effectors and gene-for-gene interactions with host plants

- Important insect pests continue to conform to the gene-for-gene hypothesis.
- New insect NB-LRR-encoding resistance genes have been identified.
- Insect avirulence genes have been cloned.
- Large families of effector-encoding genes are present in gall midge genomes.

Within the context of the four-phase model of plant immunity, gene-for-gene interactions have gained new relevance. Genes conferring resistance to the Asian rice gall midge (Orseolia oryzae) and the small brown planthopper (Nilaparvata lugens) have been cloned in rice (Oryza sativa). Mutations in insect avirulence genes that defeat plant resistance have been identified and cloned. Results are consistent with both the gene-for-gene hypothesis and the new model of plant immunity. Insect resistance genes encode proteins with nucleotide binding sites and leucine-rich repeats. Insects use effectors that elicit effector-triggered immunity. At least seven-percent of Hessian fly genes are effector encoding.

The gene-for-gene interaction is a battle whose outcome depends on genetic variation at specific resistance genes (R/- and r/r) and cognate parasite avirulence genes (A/- and a/a). Parasites use a host of effector proteins, represented as small shapes, to attack specific targets (T1 and T2) in plant cells. In the red panel, resistance proteins (R) that perceive the presence of a cognate effector (E) elicit effector-triggered immunity. In the green panels, perception fails due to the absence of R or E, and susceptibility results.