A spotlight on non-host resistance to plant viruses

Abstract

Plant viruses encounter a range of host defenses including non-host resistance (NHR), leading to the arrest of virus replication and movement in plants. Viruses have limited host ranges, and adaptation to a new host is an atypical phenomenon. The entire genotypes of plant species which are imperceptive to every single isolate of a genetically variable virus species are described as non-hosts. NHR is the non-specific resistance manifested by an innately immune non-host due to pre-existing and inducible defense responses, which cannot be evaded by yet-to-be adapted plant viruses. NHR-to-plant viruses are widespread, but the phenotypic variation is often not detectable within plant species. Therefore, molecular and genetic mechanisms of NHR need to be systematically studied to enable exploitation in crop protection. This article comprehensively describes the possible mechanisms of NHR against plant viruses. Also, the previous definition of NHR to plant viruses is insufficient, and the main aim of this article is to sensitize plant pathologists to the existence of NHR to plant viruses and to highlight the need for immediate and elaborate research in this area.

Keywords: Broad spectrum resistance; Co-evolution; Plant immunity; Virus adaptation; Virus vectors.

Figure 1. A generalized overview of the potential molecular mechanisms of non-host resistance to viruses.

(A) The incompatible interaction between viral factors and host proteins or the members of non-functional viral susceptibility proteins, such as eIF, translation initiation factor; NLR, nucleotide-binding domain leucine-rich repeat; and RDR, RNA-dependent RNA polymerase, can provide resistance to viruses by inhibiting viral multiplication. (B) A simplified mechanism showing antiviral silencing in which RDR creates an amplification loop in the process of RNA degradation, mediated by the dsRNA-specific endo-ribonuclease Dicer-like (DCL) family of proteins. DCLs detect and process dsRNA into small interfering RNAs (siRNAs). Argonaute (AGO) proteins play a major role in RNA silencing which further functions distinctively in transcriptional and post-transcriptional gene silencing. Viral suppressors for RNA silencing (VSR) repress RNA silencing, having evolved to be encoded by plant viruses to neutralize resistance mechanisms by antiviral silencing in plants. (C) Salicylic acid (SA) significantly induces the expression of RDR in the presence of TMV infections, which shows a critical link between RNA silencing and the nonexpressor of PR-1 (NPR1)-dependent SA signaling pathway. Abbreviations: COI, CORONATINE INSENSITIVE-1; JAZ, jasmonate ZIM domain; NSP, non-structural proteins.