Plant Executor Genes.

Xifeng Chen,Kaijun Zhao,Wei Guo,Chunlian Wang,Zhiyuan Ji

doi:10.3390/ijms23031524

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 resistance. 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.

Highlights

Over the course of their lifetimes, plants are exposed to microorganisms in the environment and are faced with continuous hostilities from pathogenic microbes [1]
The single amino acid mutation attenuates the interaction between transcription activation-like effectors (TALEs) and OsTFIIAγ5, and it leads to a reduction in the TALE-dependent induction of the downstream target genes. xa5 confers rice resistance to various strains of Xanthomonas oryzae (Xoo and Xanthomonas oryzae pv. oryzicola (Xoc))
The activation mode of an E gene is ideal for balancing the immunity and yields in crop breeding, an area for which the E gene family members have bright prospects [67]

Summary

Introduction

Over the course of their lifetimes, plants are exposed to microorganisms in the environment and are faced with continuous hostilities from pathogenic microbes [1]. Proteins encoded by resistance (R) genes are fundamental to the innate immunity of plants because they perceive pathogen effectors and switch on the immune response through various mechanisms [3]. TALEs (transcription activation-like effectors) in pathogens are the natural triggers for E-gene-mediated disease resistance. E genes are “silenced” in the absence of a pathogen attack, and the induction of E gene expression can lead to programmed cell death (PCD) in multiple plant and mammalian cells, implying that the launched immune response is activated through some conserved mechanism [7]. The members in the Group 2 E proteins (G2EPs) have some common features (Figure 1): (1) G2EPs have multiple potential hydrophobic membranespanning domains (22~23 aa), with imperfect L(I)-X(4-9)-L(I) motifs, probably forming ion (probable Ca2+) channels in membranes; (2) The C-terminal domain contains the ED.

Interaction between E Genes and Cognate Avirulence Proteins

Executor Genes in Rice–Xanthomonas oryzae Interaction

Genetically Engineered E Genes for Combating Xanthomonas Diseases

Discussion