Plant Executor Genes

Abstract

Executor (E) genes comprise a new type of plant resistance (R) genes, identified from host–Xanthomonas interactions. The Xanthomonas‐secreted transcription activation‐like effectors (TALEs) usually function as major virulence factors, which activate the expression of the so‐called “susceptibility” (S) genes for disease development. This activation is achieved via the binding of the TALEs to the effector‐binding element (EBE) in the S gene promoter. However, host plants have evolved EBEs in the promoters of some otherwise silent R genes, whose expression directly causes a host cell death that is characterized by a hypersensitive response (HR). Such R genes are called E genes because they trap the pathogen TALEs in order to activate expression, and the resulting HR prevents pathogen growth and disease development. Currently, deploying E gene resistance is becoming a major component in disease resistance breeding, especially for rice bacterial blight re-sistance. Currently, the biochemical mechanisms, or the working pathways of the E proteins, are still fuzzy. There is no significant nucleotide sequence homology among E genes, although E proteins share some structural motifs that are probably associated with the signal transduction in the effector‐triggered immunity. Here, we summarize the current knowledge regarding TALE‐type avirulence proteins, E gene activation, the E protein structural traits, and the classification of E genes, in order to sharpen our understanding of the plant E genes.