Molecular and genetic analysis of tobamovirus-host interactions (Nicotiana sylvestris, Capsicum, Solanum melongena, Arabidopsis)

The tobacco mosaic tobamovirus (TMV) coat protein (CP) is known to function as an elicitor of the hypersensitive response (HR) in Nicotiana sylvestris (tobacco) and two Capsicum (pepper) species. To compare how different hosts recognize the TMV CP, additional host HRs were identified in Solanum melongena (eggplant) and a previously uncharacterized pepper species (Capsicum annum) that also respond to CP. Further studies aimed at comparing how tobacco, pepper, and eggplant perceive the CP demonstrated that these hosts differed in their ability to localize five tobamovirus strains. To determine the molecular basis for these differences, TMV constructs containing specific mutations designed to alter CP tertiary, quaternary, and surface structures were compared for their ability to elicit the HR in each host. Maintenance of the overall CP three dimensional structure was essential for HR elicitation in tobacco, pepper, and eggplant while differences in recognition among plants resulted from changes in surface features and/or quaternary configurations. Because tobacco, pepper, and eggplant all belong to the family Solanaceae, it is possible that these resistance genes not only function similarly in CP recognition, but are structurally conserved as well.; In addition to the HR, other forms of host resistance may result from the inaccessibility of host factors essential for specific virus functions due to genetic differences between hosts. Identification of host genes that regulate virus functions requires a tractable genetic system. Due to recent advances in Arabidopsis genetics, a number of researchers have developed Arabidopsis based systems to study virus-host interactions. In order to study TMV-host interactions, susceptible and resistant Arabidopsis varieties were identified by screening 14 different ecotypes, for their ability to support TMV systemic movement. Genetic analyses of susceptible and resistant ecotypes showed that more susceptible ecotypes contain dominant traits that are conferred by single dominant genes. Studies aimed at determining the viral functions affected that give rise to susceptible and resistant phenotypes demonstrated that differences result from altered rates of TMV cell-to-cell spread. Furthermore, symptoms exhibited in one highly susceptible ecotype were found to be conferred by a distinct recessive gene. Consequently, multiple host factors control Arabidopsis susceptibility to TMV and molecularly cloning these genes will provide new insights into the mechanisms of TMV cell-to-cell movement and symptom development.