Evidence for participation of RNA 1-encoded elicitor in Cowpea mosaic virus-mediated concurrent protection

The cowpea (Vigna unguiculata) line Arlington, inoculated with Cowpea mosai c virus (CPMV), showed no symptoms, and no infectivity or accumulation of c apsid antigen was detected at several days after inoculation. Coinoculation , but not sequential inoculation, of CPMV with similar concentrations of an other Comovirus; Cowpea severe mosaic virus (CPSMV), resulted in reduced nu mbers of CPSMV-induced lesions. This apparent, CPMV-mediated reduction in n umber of CPSMV-induced infection centers was termed concurrent protection. We report results obtained by inoculating two nearly isogenic cowpea lines derived from a CPMV-susceptible cowpea crossed to Arlington, one line CPMV- susceptible and the other resistant. The CPMV virions B and M, encapsidatin g genomic RNAs 1 and 2, respectively, were extensively purified by gradient centrifugation. In the CPMV-resistant cowpea, either CPMV or CPMV B affect ed concurrent protection against CPSMV and against two distinct non-Comovir uses: Cherry leafroll virus and Southern bean mosaic virus. Adding CPMV M t o the inoculum did not enhance CPMV-B-mediated protection. CPMV B was ineff ective in protecting CPMV-susceptible cowpea. We postulate that CPMV-mediat ed concurrent protection is elicited in CPMV-resistant cowpea by a CPMV RNA -I-encoded factor and acts to reduce accumulation or spread of CPMV and cer tain coinoculated challenging viruses in or from the inoculated cell. Coino culated CPMV did not protect CPMV-resistant cowpea against Tomato bushy stu nt virus or Cucumber mosaic virus. (C) 2000 Academic Press.