Combination of amino acids in the 3a protein and the coat protein of Cucumber mosaic virus determines symptom expression and viral spread in bottle gourd

Summary.

 Bottle gourd plants infected with an isolate of cucumber mosaic virus (CMV-KM) developed severe chronic mosaic symptoms (SCMS) with stunting, but two other isolates (CMV-Y and CMV-D8) did not. CMV-KM and CMV-D8 induced enlarged chlorotic spots and rapidly spread over the inoculated cotyledons, whereas CMV-Y elicited a hypersensitive response (HR) producing pin-point necrotic lesions. Reassortment analysis among the three isolates revealed that the local and systemic symptoms on the plants were regulated by RNA3. Reciprocal recombination and site-directed point mutation analyses of the three RNA3s demonstrated that a combination of genetic information encoded by the movement protein (MP) gene and the coat protein (CP) gene determines the induction of SCMS in bottle gourd. SCMS occurred when Ser⁵¹ in the MP of CMV-D8 was changed to Asn⁵¹, whereas substitution of Ser⁵¹ for Asn⁵¹ in the MP of CMV-KM eliminated its ability to induce SCMS. Furthermore, Ser¹²⁹ in the CPs was shown to be responsible for induction of HR and blocking of efficient cell-to-cell and long-distance movement.