The Study Of Avirulence Gene And Motifs Of Cucumber Mosaic Virus That Induces A Hypersensitive Response In Cowpea

Resistance in cowpea to infection with strains of Cucumber mosaic virus (CMV) involves a local hypersensitive response (HR).Previous studies indicated that the2a replicase of CMV is involved in HRinduction. In this study, we confirmed and extended this observation by demonstrating that the nonviralexpression of the2a protein encoded by CMV is able to induce a cell death response in cowpea plants,whereas no other CMV-encoded proteins elicit such response. The2a single-amino acid mutant, F631Y, no longer induces the necrosis response, yet the A641S mutant still induces cell death. The2a doublemutant, F631Y and A641S, does not induce HR. However, the three2a mutants have comparable replicaseactivities in a fluorescence reporter assay. The2aD610A mutant that alters the highly conserved GDDmotif abolishes the replicase activity; however, it does not affect HR induction in cowpea. The2a301-778aa fragment introduced with the same D610A mutation in the GDD motif is also capable of inducing HR in cowpea.Collectively, these findings suggest that the2a protein of CMV is sufficient to induce HR incowpea independently of its replicase activity.