USE OF A GENE-EXPRESSION SYSTEM BASED ON POTATO-VIRUS-X TO RAPIDLY IDENTIFY AND CHARACTERIZE A TOMATO PTO HOMOLOG THAT CONTROLS FENTHION SENSITIVITY

A novel transient gene expression system was used to study both the to mato disease resistance gene Pto and a Pto homolog designated Fan. The gene expression system was based on potato virus X (PVX). Tomato plan ts that were both susceptible to strains of Pseudomonas syringae pv to mato carrying the corresponding avirulence gene awPto and insensitive to the insecticide fenthion were infected with in vitro-generated tran scripts of PVX derivatives containing either Pto or Fan. Expression of the Pto gene from the virus genome failed to elicit P. s. tomato resi stance, indicating that the PVX system is not suitable for the study o f Pto. However, expression of the Fen gene resulted in sensitivity to fenthion. The utility of the PVX gene expression system was further de monstrated through structure/function studies of the Fen gene. A corre lation was shown between Pen protein kinase activity and the ability o f this protein to confer fenthion sensitivity to tomato, Furthermore, it was demonstrated that mutation of a putative N-terminal myristoylat ion site, proposed to be involved in membrane targeting, rendered the Fen protein inactive, Analysis of a Pto Fen chimeric gene allowed the fenthion sensitivity domain to be localized to the C-terminal part of the Fen protein. Interestingly, expression of the Fen kinase from the PVX genome in Nicotiana spp resulted in a fenthion-independent necroti c response, Our results support the involvement of the Fen gene in a s ignal transduction pathway(s).