R-gene-mediated resistance to plant viruses

Human parvovirus B19 virus-like particle formation in Nicotiana benthamiana

There has been a surge in the interest to utilize plants as hosts for producing vaccine antigens. In this study, we demonstrated the successful expression of the human parvovirus B19 (B19V) capsid protein (VP2) in Nicotiana benthamiana cells. The B19V VP1 and VP2 genes were cloned under the control of estrogen-inducible promoters and transiently expressed in N. benthamiana leaves using the agroinfiltration method. The addition of estrogen significantly boosted the expression of VP2. Furthermore, codon optimization of the VP2 sequence resulted in over a 30-fold increase in its expression compared with that of the wild-type. Analysis of negatively stained samples by sucrose density gradient ultracentrifugation and electron microscopy revealed that the expressed VP2 proteins formed spherical particles with diameters of approximately 20 nm. Immunostaining analysis of protoplasts derived from VP2-expressing N. benthamiana leaves indicated that VP2 signals were predominantly localized in the cytoplasm. These findings strongly suggested that B19V VP2 assembles and formed virus-like particles (VLPs) within the cytoplasm of N. benthamiana cells, presenting a promising method for producing B19V VLPs in plant systems.

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.