An Inhibitor of Polyprotein Processing with the Characteristics of a Natural Virus Resistance Factor. Fernando Ponz. Department of Plant Pathology, College of Agricultural and Environmental Sciences, University of California, Davis, California 95616, U.S.A.. Christopher B. Glascock, and George Bruening. Department of Plant Pathology, College of Agricultural and Environmental Sciences, University of California, Davis, California 95616, U.S.A.. MPMI 1:25-31. Accepted 7 August 1987. Copyright 1987 The American Phytophathological Society.

Seedlings of cowpea (Vigna unguiculata cv. Arlington) are operationally immune to cowpea mosaic virus (CPMV), giving no detected increase of inoculated virus, but they are susceptible to another comovirus, cowpea severe mosaic virus (CPSMV). Arlington-derived immunity against CPMV is controlled by a single dominant locus in crosses to susceptible line Blackeye 5. Previous results revealed an inhibitor of the proteolytic processing of a CPMV polyprotein in extracts of Arlington cowpea protoplasts, and the inhibitor was postulated to effect the observed immunity. We found this inhibitory activity in partially fractionated extracts of Arlington cowpea leaves. Two other activities also were candidates for immunity factor(s) by virtue of their greater potency in Arlington than in Blackeye 5 cowpea leaf extracts: inhibitor(s) of the translation of CPMV RNAs and proteinase(s) that degrade CPMV-encoded proteins. The proteinases degraded CPSMV and CPMV proteins equally well, and the activities were not coinherited with immunity against CPMV in progeny of cowpea crosses. The inhibitor of CPMV polyprotein processing possessed both the virus specificity and the coinheritance that are expected for an agent conveying immunity to CPMV. The inheritance of the translation inhibitor activities was complex, and our results show that one or more of these activities may contribute to the immunity against CPMV.