Cowpea mild mottle virus (CPMMV; genus Carlavirus) can be a destructive pathogen of soybean but there is little information about its distribution on soybean in China. Here, we collected soybean plants with virus-like symptoms from 11 fields widely scattered within China, and used high-throughput sequencing to determine their virome. Most samples (8/11) were co-infected by the well-studied potyvirus soybean mosaic virus (SMV) and CPMMV, and the remaining three samples were singly infected with CPMMV. The near-complete genome sequences of the 11 CPMMV isolates were determined and phylogenetic analysis showed that they constituted a new genetic clade. One recombination event was detected among the CPMMV sequences, and the isolate CPMMV_JL_CC was identified as recombinant. In mechanical inoculation assays, co-infection by CPMMV and SMV resulted in an enhancement of disease symptoms, but decreased the expression level of the genomic RNAs and CP of CPMMV, without significantly affecting SMV accumulation. The interaction between these viruses needs further investigation.
The movement of some plant RNA viruses is mediated by triple gene block (TGB) proteins, which cooperate to transfer the viral genome from cell to cell through plasmodesmata. Here, we investigated the function of the TGB proteins of cowpea mild mottle virus (CPMMV; genus Carlavirus, family Betaflexiviridae), which causes severe damage to soybean production. Subcellular localization experiments demonstrated that TGBp1 and TGBp3 were localized to the endoplasmic reticulum (ER), plasmodesmata (PD) and nucleus in Nicotiana benthamiana leaves. TGBp2 was unusually localized to PD. In protein interaction assays TGBp2 significantly enhanced the interaction between TGBp3 and TGBp1. Interaction assays using deletion mutants showed that the C-terminal transmembrane (TM) domain of TGBp2 is critical for its localization to PD and for its interaction with TGBp1 and TGBp3.
Soybean mosaic virus (SMV; Potyvirus, Potyviridae) is one of the most prevalent and destructive viral pathogens in the world. The P1 protein is the first N-terminal product in the potyvirus genome and shows a high sequence variability that may be related to virus adaptation to hosts. In this work, we focused on the different functions of P1 proteins in two SMV isolates SMVGZL and SMVNB during their infection of plants. Isolate SMVGZL induced weaker symptoms than SMVNB in mechanical inoculation assays, and the accumulation level of SMV CP in SMVGZL-infected leaves was lower than that in leaves infected with SMVNB, especially at the late stage of infection. The isolates SMVGZL and SMVNB had a high similarity in genome sequence except for the P1 region. P1GZL induced a higher salicylic acid (SA) response than P1NB in Nicotiana benthamiana, which may explain the lower virus titers in plants infected with SMVGZL. Our results suggest that the divergence in the P1 proteins of these SMV isolates influenced their virulence via differentially regulating SA signaling.
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