Replicase and movement protein of Cucumber mosaic virus are symptom determinants in zucchini squash

A pepper strain of Cucumber mosaic virus (Pf-CMV) induces a mild chlorotic spot symptom in zucchini squash at 9 days post-inoculation (dpi), wile Fny strain of CMV causes severe mosaic and stunting symptom at 4 dpi in this host. Pseudorecombinants were constructed between the two strains, and assessments of symptom severity were indicated that both RNA2 and RNA3 were responsible for both mildness and the slow appearance of symptom elicited by Pf-CMV in zucchini squash. With various RNA2 and RNA3 chimeras between two strains of CMV, the genetic symptom determinants of phenotype of Pf-CMV were mapped to Tyr residue at positions amino acid 267 in 2a protein and at positions amino acid 168 in 3a movement protein (MP). Chimeras changed the sequences (both changed Tyr to lie) in the codons of both amino acid 168 of 3a MP and amino acid 267 of 2a protein were resulted in the high RNA accumulation, severity of symptom, and the rapid systemic spread, suggesting that 2a replicase as well as MP is involved in virus movement. The RNA accumulation pattern of all pseudorecombinants and chimeras are identical in protoplast of zucchini squash, indicating the virus movement is responsible for the phenotypes of two CMV strains rather than virus replication.