Characterization of two unusual features of resistance to soilborne cereal mosaic virus in hexaploid wheat: leakiness and gradual elimination of viral coat protein from infected root tissues.

Spatiotemporal infection patterns of Soilborne cereal mosaic virus (SBCMV) were compared between resistant and susceptible wheat cultivars to elucidate disease resistance mechanisms. Resistance to SBCMV was manifested by a gradual disappearance of the viral coat protein (CP) from the roots following an initial short period of steady accumulation. Interestingly, viral RNA persisted in the roots of resistant cultivars even after the CP had disappeared. Traces of viral RNA were also detected in the uninoculated leaves of the resistant cv. Cadenza. These findings suggest that the resistance mechanism to SBCMV in wheat involves the efficient disassembly of virus particles and either an inhibition of further synthesis of viral CP or its proteolytic degradation. SBCMV accumulated in the leaves of a small proportion of individual plants of Cadenza and other recognized resistant cultivars, highlighting the leaky nature of the resistance, but the roots of these plants were often devoid of viral CP. Increasing or decreasing the concentration of the inocula had no effect on the incidence rate of such "resistance breakdown"; however, a positive correlation was found between the incidence rate of resistance breakdown and the percentage of systemically infected individuals of recognized susceptible cultivars in each separate experiment.