1.12.4S
GENETIC STRATEGIES TO IDENTIFY COMPATIBILITY AND INCOMPATIBILITY FUNCTIONS IN THE TEV-ARABIDOPSIS SYSTEM

JC CARRINGTON, S WHITHAM, KD KASSCHAU, MC SCHAAD, S MAHAJAN and S CHISHOLM

Institute of Biological Chemistry, Washington State University, Pullman, WA 99164-6340, USA

Background and objectives
Whether or not a plant is systemically infected by a given virus depends on the net effects of both compatible and incompatible interactions. Compatible interactions are those required for genome expression or replication, cell-to-cell movement and long-distance movement, whereas incompatible interactions are those that result in virus recognition and arrest. We are investigating viral and host functions that contribute to these interactions.

Results and conclusions
Through genetic, biochemical and cell biological analyses we have identified a number of tobacco etch potyvirus (TEV)-encoded functions required for genome replication and virus movement. Four TEV proteins are required for cell-to-cell and long-distance movement: capsid protein (CP), Cl helicase, NIa and HC-Pro [1]. The CP and Cl proteins appear to be necessary as structural components of the intercellular transport apparatus, whereas the NIa protein is necessary for host genotype-specific long-distance movement. HC-Pro, on the other hand, may stimulate systemic movement indirectly through either stimulation of host factors involved in compatibility, or suppression of defence responses that limit infection [2]. To identify cellular factors that affect compatibility and incompatibility, we have designed a number of genetic selections and screens for altered TEV susceptibility mutants of Arabidopsis thaliana. Selectable viruses that express either herbicide resistance or proherbicide sensitivity have been generated to select for gain-of-susceptibility or loss-of-susceptibility mutants. Several gain-of-susceptibility mutants have been isolated. Combined with natural ecotype genetic variation, two interdependent loci (RTM1 and RTM2) which condition a specific restriction of long-distance movement of TEV have been identified and mapped. In contrast, RTM1 and RTM2 have no effect on establishment and spread of TEV within inoculated leaves. The genetic strategies employed here have high potential to reveal both cellular and viral factors that affect compatibility and incompatibility.

References
1. Carrington JC, Kasschau KD, Mahaian S, Schaad MC, 1996. Plant Cell 8, 1669-1681.
2. Kasschau KD, Cronin S, Carrington JC, 1997. Virology 228, 251-262.