Pepper Mottle Virus and Its Host Interactions: Current State of Knowledge

Pepper mottle virus (PepMoV) is a destructive pathogen that infects various solanaceous plants, including pepper, bell pepper, potato, and tomato. In this review, we summarize what is known about the molecular characteristics of PepMoV and its interactions with host plants. Comparisons of symptom variations caused by PepMoV isolates in plant hosts indicates a possible relationship between symptom development and genetic variation. Researchers have investigated the PepMoV–plant pathosystem to identify effective and durable genes that confer resistance to the pathogen. As a result, several recessive pvr or dominant Pvr resistance genes that confer resistance to PepMoV in pepper have been characterized. On the other hand, the molecular mechanisms underlying the interaction between these resistance genes and PepMoV-encoded genes remain largely unknown. Our understanding of the molecular interactions between PepMoV and host plants should be increased by reverse genetic approaches and comprehensive transcriptomic analyses of both the virus and the host genes.

Double-stranded RNA confers resistance to pepper mottle virus in Nicotiana benthamiana

RNA interference (RNAi) is a regulatory mechanism of gene expression mediated by small RNAs. By using the RNAi technique, exogenous double-stranded RNA (dsRNA) designed to target mRNA, suppresses target gene expression levels in plants. In this study, we adopted the RNAi mechanism as a tool to protect plants from viruses. We designed and synthesized several dsRNAs targeting the pepper mottle virus (PepMoV) genes HC-Pro and NIb. When used on Nicotiana benthamiana plants, these dsRNAs protected the plant against viral infection over a specific period. By optimizing dsRNA and virus injection time, the protection efficiency of dsRNA by targeting virus genes could be maximized. It seems that exogenous dsRNA-derived RNA-induced silencing complex was able to defend the host against viral infection instantly. Furthermore, each dsRNA designed to target different regions within a transcript had varying levels of effects on virus survival in the host plants. When targeting the middle part of both the NIb and HC-Pro genes using the dsRNAs, the highest viral growth inhibitory effect was observed. An RLM-5′ RACE was performed using plant leaves infected with PepMoV after dsRNA treatment and it was observed that most of the mRNA cleavages occurred close to the 3′ part within the dsRNA target position on the mRNA. These results suggest that the dsRNA tool can be used as a plant vaccine platform for crop protection.

Tobamoviruses of two new species trigger resistance in pepper plants harbouring functional L alleles

Tobamoviruses are often referred to as the most notorious viral pathogens of pepper crops. These viruses are not transmitted by invertebrate vectors, but rather by physical contact and seeds. In this study, pepper plants displaying mild mottle and mosaic symptoms were sampled in four different regions of Peru. Upon double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) tests, seven samples cross-reacted weakly with antibodies against pepper mild mottle virus (PMMoV), suggesting the presence of tobamoviruses. When employing RT-PCR, conserved primers amplified cDNA fragments of viruses from two putative new tobamovirus species in the samples. The complete genome of two representative isolates were, therefore, sequenced and analysed in silico. These viruses, which were tentatively named yellow pepper mild mottle virus (YPMMoV) and chilli pepper mild mottle virus (CPMMoV), shared highest nucleotide genome sequence identities of 83 and 85 % with bell pepper mottle virus (BpeMV), respectively. Mechanical inoculation of indicator plants with YPMMoV and CPMMoV isolates did not show any obvious differences in host ranges. These viruses were also inoculated mechanically on pepper plants harbouring different resistance L alleles to determine their pathotypes. Pepper plants carrying unfunctional L alleles (L 0) to tobamoviruses were infected by all isolates and presented differential symptomatology for YPMMoV and CPMMoV. On the other hand, pepper plants carrying L 1, L 2, L 3 and L 4 alleles were resistant to all isolates, indicating that these viruses belong to pathotype P0.