CHARACTERIZATION OF CAULIFLOWER MOSAIC-VIRUS (CAMV) RESISTANCE IN VIRUS-RESISTANT ECOTYPES OF ARABIDOPSIS

Two Arabidopsis ecotypes are resistant to systemic infection by caulif lower mosaic virus (CaMV), a plant pararetrovirus, Arabidopsis ecotype Enkheim-2 (En-2) is highly resistant to CaMV infection while Bla-14 i s more weakly resistant. CaMV resistance in En-2 can be largely attrib uted to the action of a single semidominant gene called cauliflower mo saic virus resistance1 (CAR1), located at a locus on chromosome 1, Res istance in Bla-14 is tightly linked to CAR1 and may be due to a weak a llele at the same locus or another gene in a gene cluster, A quantitat ive polymerase chain reaction assay in conjunction with replication- a nd movement-incompetent viral mutants was used to determine whether vi rus replication or movement is affected in the resistant ecotypes, The pattern of accumulation of the wild-type virus in the resistant ecoty pe, En-2, was similar to that of a movement-incompetent CaMV mutant, s uggesting that CAR1 interferes with or fails to support CaMV movement. CaMV-inoculated En-2 plants do not show visible signs of a hypersensi tive response, However, indicators of an induced defense response do a ppear in CaMV-infected En-2 plants, such as the activation of pathogen esis-related protein gene expression and the production of camalexin, an Arabidopsis phytoalexin. Defense responses induced chemically or by mutation in the susceptible ecotypes delayed and reduced the severity of a CaMV infection, These findings suggest that CAR1 acts either in the susceptible ecotype to support virus movement or in the resistant ecotype to signal a defense response.