Reduced PAL gene suppression in Verticillium-infected resistant tomatoes.

In tomato, resistance to the wilt fungus Verticillium albo-atrum is determined primarily by the Ve locus. When two tomato near-isolines which differ at this locus and in their susceptibility to the pathogen were compared, more rapid suberin coating in the xylem of resistant plants correlated closely with a more rapid increase in the activity of phenylalanine ammonia-lyase (PAL; EC 4.3.1.5), an enzyme which is essential to the suberization process. In contrast, levels of mRNA did not increase proportionally to the measured enzyme activities; rather, there was a substantial suppression of mRNA levels in the susceptible tomato line, consistent with a much lower elevation of PAL activity and significantly less vascular coating. The suppression was absent or substantially reduced in the resistant line. The results indicate that the pathogen can suppress defense genes in susceptible plants but suggest that their expression is altered in resistant hosts and that post-transcriptional regulation plays a significant role.