From Player to Pawn: Viral Avirulence Factors Involved in Plant Immunity.

Changjun Huang

doi:10.3390/v13040688

Abstract

In the plant immune system, according to the ‘gene-for-gene’ model, a resistance (R) gene product in the plant specifically surveils a corresponding effector protein functioning as an avirulence (Avr) gene product. This system differs from other plant–pathogen interaction systems, in which plant R genes recognize a single type of gene or gene family because almost all virus genes with distinct structures and functions can also interact with R genes as Avr determinants. Thus, research conducted on viral Avr-R systems can provide a novel understanding of Avr and R gene product interactions and identify mechanisms that enable rapid co-evolution of plants and phytopathogens. In this review, we intend to provide a brief overview of virus-encoded proteins and their roles in triggering plant resistance, and we also summarize current progress in understanding plant resistance against virus Avr genes. Moreover, we present applications of Avr gene-mediated phenotyping in R gene identification and screening of segregating populations during breeding processes.

Highlights

Viruses consume a substantial amount of host resources; subsequently, disease symptoms develop as a consequence of disruptions of the cellular machinery required for plant physiology and natural growth, and these disruptions eventually result in developmental abnormalities and other phenotypic manifestations
Given the importance of suppressing RNA silencing for virus survival, it is not unexpected that direct and indirect interactions would occur between RNA Silencing Suppressor (RSS) and R genes to elicit effector-triggered immunity (ETI), which acts as a second plant defense layer [107]
In contrast to a simple classification in which extreme resistance (ER) caused by viral elicitors falls into the four categories above, research within the past few years shows that ER determinants are largely spread throughout the viral genome [118,119]

Summary

Introduction

Plant viruses contain single-stranded or double-stranded RNA or DNA genomes and vary substantially in their genome structure and organization. More and more evidence has shown that CPs of all plant viruses are multifunctional and have various roles during different replication stages, ranging from early to late events in the infection cycle The diversity of these functions in different viral systems includes virus transmission by specific vectors, translation of viral RNA, regulation of intercellular and systemic movement of the virus, suppression of both post transcription gene silencing (PTGS) and transcription gene silencing (TGS), as well as determination of symptomatology and pathogenesis [24,25]. L gene alleles encode CC-NB-LRR type resistance proteins with the ability to elicit resistance responses to different tobamoviral CP Avr effectors [37,38] Since both N0 and its Nicotiana orthologues and the L alleles from pepper mediate resistance against tobamoviruses by recognizing the CP [33,34], it seems that these genes have evolved from a common Solanum ancestor. Another possibility is that the single CPs of CMV and TCV, or their possible complexes with other host components (e.g., guardee or decoy factor) may have highly similar CP protein folding domains [44]

Replication-Related Proteins

Other Proteins

Application of Viral Avr Factors in Resistance Studies

Conclusions and Perspectives