Analysis of the pH Effect on Infectivity of Cucumber Mosaic Virus. A Possible Role of Ribonuclease.

Abstract

The infectivity of Cucumber mosaic virus (CMV) and CMV RNA on cowpea and tobacco leaves after mechanical inoculation was lower when the inoculum was suspended in buffers at pH 6.0 as opposed to pHs 7.0 or 8.0, with or without EDTA. When leaves of both plants were inoculated with CMV suspended in water and immediately treated with buffers at the above pHs, infectivity did not vary among the treatments, suggesting that pH did not affect cell susceptibility, but did affect the inoculum. Before inoculation, no detectable structural changes in the CMV particles were observed at this pH range. Thus the effect on the inoculum of exuded substances such as degradative enzymes in epidermal cell sap after inoculation was examined on tobacco leaves. The effect of pH on this interaction, as well as the behavior of ribonuclease (RNase) in the epidermal sap correlated well with the infectivities of CMV and CMV RNA. CMV RNA was degraded more by the epidermal RNase at pH 6.0 than at pHs 7.0 or 8.0. The RNase also bound more easily with CMV virions at pH 6.0 than at pH 8.0 without attacking the inner RNA, thereby maintaining their activity; when virions treated with epidermal sap at pH 6.0 were dissociated into the coat protein (CP) and RNA, the RNA was significantly degraded. Thus, RNase exuded from wounded epidermal cells may bind easily with CMV CP at pH 6.0, disrupting virus multiplication in the cells at the infection sites.