Plant-virus interactions

Exploring the Diversity of Mechanisms Associated With Plant Tolerance to Virus Infection

Tolerance is defined as an interaction in which viruses accumulate to some degree without causing significant loss of vigor or fitness to their hosts. Tolerance can be described as a stable equilibrium between the virus and its host, an interaction in which each partner not only accommodate trade-offs for survival but also receive some benefits (e.g., protection of the plant against super-infection by virulent viruses; virus invasion of meristem tissues allowing vertical transmission). This equilibrium, which would be associated with little selective pressure for the emergence of severe viral strains, is common in wild ecosystems and has important implications for the management of viral diseases in the field. Plant viruses are obligatory intracellular parasites that divert the host cellular machinery to complete their infection cycle. Highjacking/modification of plant factors can affect plant vigor and fitness. In addition, the toxic effects of viral proteins and the deployment of plant defense responses contribute to the induction of symptoms ranging in severity from tissue discoloration to malformation or tissue necrosis. The impact of viral infection is also influenced by the virulence of the specific virus strain (or strains for mixed infections), the host genotype and environmental conditions. Although plant resistance mechanisms that restrict virus accumulation or movement have received much attention, molecular mechanisms associated with tolerance are less well-understood. We review the experimental evidence that supports the concept that tolerance can be achieved by reaching the proper balance between plant defense responses and virus counter-defenses. We also discuss plant translation repression mechanisms, plant protein degradation or modification pathways and viral self-attenuation strategies that regulate the accumulation or activity of viral proteins to mitigate their impact on the host. Finally, we discuss current progress and future opportunities toward the application of various tolerance mechanisms in the field.

Emerging role of SGT1 as a regulator of NB-LRR-receptor nucleocytoplasmic partitioning

Plant nucleotide-binding (NB) and leucine-rich repeat (LRR) receptors mediate effector-triggered immunity. Two major classes of NB-LRR proteins are involved in this process, namely, toll-interleukin receptor (TIR)-NB-LRR and coiled coil (CC)-NB-LRR proteins. Recent reports show that some of the TIR-NB-LRRs and CC-NB-LRRs localize to the cytoplasm and nucleus. Equilibrium between these pools is required for full resistance, suggesting tight regulation of nucleocytoplasmic receptor shuttling. We recently showed that SGT1, a protein that controls NB-LRR receptor stability and activity, facilitates nuclear import of N protein, which is a TIR-NB-LRR receptor. In this addendum, we show that the subcellular localization of Rx, a CC-NB-LRR protein, reflects the positions of SGT1 ectopic variants in the cell. This suggests that SGT1 might have a general role in maintaining the nucleocytoplasmic balance of NB-LRR receptors. We discuss these results in light of differences in the N and Rx systems of effector-triggered immunity.

Emerging role of SGT1 as a regulator of NB-LRR-receptor nucleocytoplasmic partitioning

Plant nucleotide-binding (NB) and leucine-rich repeat (LRR) receptors mediate effector-triggered immunity. Two major classes of NB-LRR proteins are involved in this process, namely, toll-interleukin receptor (TIR)-NB-LRR and coiled coil (CC)-NB-LRR proteins. Recent reports show that some of the TIR-NB-LRRs and CC-NB-LRRs localize to the cytoplasm and nucleus. Equilibrium between these pools is required for full resistance, suggesting tight regulation of nucleocytoplasmic receptor shuttling. We recently showed that SGT1, a protein that controls NB-LRR receptor stability and activity, facilitates nuclear import of N protein, which is a TIR-NB-LRR receptor. In this addendum, we show that the subcellular localization of Rx, a CC-NB-LRR protein, reflects the positions of SGT1 ectopic variants in the cell. This suggests that SGT1 might have a general role in maintaining the nucleocytoplasmic balance of NB-LRR receptors. We discuss these results in light of differences in the N and Rx systems of effector-triggered immunity.

Transgenic expression of Tobacco mosaic virus capsid and movement proteins modulate plant basal defense and biotic stress responses in Nicotiana tabacum

Plant viruses cause metabolic and physiological changes associated with symptomatic disease phenotypes. Symptoms involve direct and indirect effects, which result in disruption of host physiology. We used transgenic tobacco expressing a variant of Tobacco mosaic virus (TMV) coat protein (CP(T42W)) or movement protein (MP), and a hybrid line (MP×CP(T42W)) that coexpresses both proteins, to study the plant response to individual viral proteins. Findings employing microarray analysis of MP×CP(T42W) plants and silenced mp×cp(T42W)* controls revealed that altered transcripts were mostly downregulated, suggesting a persistent shut-off due to MP×CP(T42W) expression. Next, we showed that MP triggered reactive oxygen species (ROS) accumulation, reduction of total ascorbate, and expression of ROS scavenging genes. These effects were enhanced when both proteins were coexpressed. MP and MP×CP(T42W) plants showed increased levels of salicylic acid (SA) and SA-responsive gene expression. Furthermore, these effects were partially reproduced in Nicotiana benthamiana when GMP1 transcript was silenced. CP(T42W) seems to be playing a negative role in the defense response by reducing the expression of PR-1 and RDR-1. MP and MP×CP(T42W) transgenic expression promoted a recovery-like phenotype in TMV RNA infections and enhanced susceptibility to Pseudomonas syringae and Sclerotinia sclerotiorum. The individual effects of viral proteins may reflect the ability of a virus to balance its own virulence.