Restricted systemic movement of cowpea chlorotic mottle virus in soybean with nonnecrotic resistance

Resistance in soybean PI 346304 to cowpea chlorotic mottle virus was related to restricted virus movement. When unifoliolate leaves were mechanically inoculated, both PI 346304 and susceptible cultivar Davis developed local chlorosis and accumulated similar quantities of virions in inoculated leaves. However, PI 346304 greatly restricted virus movement and subsequent accumulation in uninoculated sections of inoculated leaves and in uninoculated leaves and roots. Nonnecrotic resistance in PI 346304 was shown not to involve induced resistance, inactivation of virions, or resistance to symptom development. In immunocytochemical studies, virus antigen was found in most cells of mechanically inoculated leaves of both Davis and PI 346304, indicating that cell-to-cell movement was largely unrestricted. However, antigen was rarely found within vascular tissue of PI 346304 but occurred frequently in similar Davis tissue. We suggest that restriction of virus entry and/or exit from the vascular tissue is responsible for nonnecrotic resistance in PI 346304