A Single Amino Acid in Cucumber Mosaic Virus Determines Systemic Infection in Legumes: Species-Specific Differences in Key Residue Locations

Adzuki bean (<i>Vigna angularis</i>), a major grain legume in Asia, is susceptible to infection by cucumber mosaic virus (CMV), which threatens crop productivity. Here, we characterized the CMV-Pa3 isolate from adzuki bean and investigated the role of specific amino acid residues in the viral 2a protein influencing systemic infection of legumes. Phylogenetic analysis demonstrated that CMV-Pa3 is genetically distinct from other legume-infecting isolates. Inoculation assays revealed that CMV-Pa3 causes systemic infection in adzuki bean, cowpea, soybean, and pea, whereas the control isolate CMV-Rs1 is restricted to inducing local necrotic lesions in cowpea and adzuki bean. Site-directed mutagenesis targeted two conserved amino acid positions (631 and 641) in the 2a protein of CMV-Rs1. Functional analysis showed that residue 631 (Tyr) facilitates systemic infection across all tested legumes, while alteration at position 641 (Ser) alone enables systemic infection in cowpea and pea. These findings identify amino acid determinants in the CMV 2a protein critical for overcoming host restrictions and mediating systemic infection in various leguminous species. This work offers new insights into the molecular mechanisms underlying CMV pathogenicity and host specificity.