Comparative Analysis for Replication and Movement of Cucumber Mosaic Virus in Cucumis figarei and C. melo.

Abstract

Replication and movement of cucumber mosaic virus (CMV) in CMV-resistant Cucumis figarei (CF) and a CMV-susceptible melon C. melo cv. Earl's Favourite (EF) were compared. Although EF plants were readily infected with CMV at 1.0μg/ml and developed systemic symptoms, leaves of CF plants inoculated with CMV virions (0.5mg/ml) or CMV RNA (50μg/ml) were locally infected. Infected cells within inoculated CF leaves as observed by confocal laser scanning microscopy were notably confined and did not enlarge over time. The number of infected protoplasts isolated from CMV-inoculated CF leaves was significantly lower than that from EF. The amount of CMV antigen detected by ELISA or western blotting in inoculated CF leaves was about 10-fold lower than that in EF leaves. Both (+) and (-) strands of CMV-specific RNAs were detectable from CF leaves, but the amounts of RNAs 1 and 2 were 500- to 1000-fold smaller and those of RNAs 3 and 4 were 100-fold smaller, than those from EF leaves. Protoplasts prepared from healthy CF were as susceptible as those from EF to infection with CMV or CMV RNA. The amount of CMV antigen in RNA-inoculated CF protoplasts detected by ELISA or western blotting similarly increased with time as did that in EF protoplasts. Although both strands of RNAs 3 and 4 similarly accumulated in protoplasts from CF as well as EF, the amounts of RNAs 1 and 2 in CF were suppressed to 1/5 of that in EF. Symptom expression, serology, infectivity, accumulation of CMV antigen in inoculated plants and RNA profile in CMV virions recovered from the infected CF leaves did not noticeably change from those of the original CMV. These results indicate that CF resistance to CMV may be caused in part by interference with viral replication, but the inhibition of virus movement via both cell-to-cell and long-distance pathways is the primary mechanism.