Functional characterization of the mutations in Pepper mild mottle virus overcoming tomato tm-1-mediated resistance

Viral sequences required for efficient viral infection differ between two Chinese pepper mild mottle virus isolates

Pepper mild mottle virus (PMMoV) causes mosaic symptoms and malformation on both leaf and fruit of pepper, reduces considerable economical yields and poses threats to human health. In this study, infectious clone of PMMoV Huludao (HLD) isolate (pCB-PMMoV-HLD) was constructed and its infectious ablility in Nicotiana benthamiana was confirmed by virions observation and Northern blot analysis. The mutant PMMoV (HLD-fsCP) that cannot express coat protein (CP) showed reduced viral accumulation but can systemically infect N. benthamiana. We constructed several chimeric mutant viruses (ZA-HB-HC, HA-ZB-HC, HA-HB-ZC and HA-ZB-ZC) by sequences substitution between PMMoV-HLD and PMMoV Zhejiang isolates (PMMoV-ZJ) and analyzed their infectious abilities in N. benthamiana and Capsicum annuum. The results showed that the chimera virus expressed by pCB-ZA-HB-HC, pCB-HA-HB-ZC and pCB-HA-ZB-ZC, but not by pCB-HA-ZB-HC, exhibited reduced infectious ability compared with wild-type PMMoV-ZJ and PMMoV-HLD, which indicated that RNA sequences required for efficient infection of PMMoV differ between the two virus isolates. The differential requirement of viral RNA sequences for efficient PMMoV infection provided theoretical value to further understand the infection and pathogenesis of PMMoV.

[Virus resistance genes in plants]

Plants defend themselves from virus infection by RNA silencing and resistance (R) gene-mediated mechanisms. Many dominant R genes encode nucleotide-binding site and leucine-rich repeat (NB-LRR)-containing proteins. NB-LRR proteins are also encoded by R genes against bacteria or fungi, suggesting a similar mechanism underlies defense systems to diverse pathogens. In contrast, several non-NB-LRR-type R genes have recently been cloned, each of which differs from others in sequences and functions. In this review, we introduce a diversity of R gene-mediated plant defense systems against viruses. Tm-1, JAX1, and Scmv1, resistance genes against tomato mosaic virus, potexviruses, and sugarcane mosaic virus, respectively, inhibit virus multiplication at a single cell level. The RTM1, RTM2, RTM3 genes of Arabidopsis thaliana inhibit systemic transport of potyviruses through the phloem. STV11 of rice against rice stripe virus and Ty-1 and Ty-3 genes of tomato against tomato yellow leaf curl virus allow low level virus multiplication and confer tolerance. The wide diversity of plant defense systems against viruses implies their recent emergence. We suggest that plants evolved new defense systems to counter infection by viruses that had overcome pre-existing defense systems (RNA silencing and NB-LRR-type R gene-mediated systems).