Expression of genes for phenylalanine ammonia-lyase and 3-hydroxy-3-methylglutaryl CoA reductase in aged potato tubers infected with Phytophthora infestans

In comparison with tissues of unaged potato tubers, tissues of aged tubers rapidly express features of resistance, such as hypersensitive cell death, browning and the accumulation of phytoalexins in response to infection by an incompatible race of Phytophthora infestans. Expression of two putative defense genes, namely, genes for 3-hydroxy-3-methylglutaryl CoA reductase (HMGR) and phenylalanine ammonia-lyase (PAL), and the activities of the corresponding enzymes were determined in aged, inoculated potato discs. The level of HMGR activity was higher after inoculation with an incompatible than with a compatible race of the pathogen. The levels of the transcripts in the total RNA were, however, nearly identical in both cases. When we examined polysomal RNA, we found an enhanced level of transcripts for HMGR only in tissues that had been infected with the incompatible race. Furthermore, comparison of the patterns of distribution of HMGR mRNA associated with polysomes in two gradients revealed modulation of HMGR at the level of initiation of translation. These results suggest that the activity of HMGR in aged potato tubers that have been attacked by P. infestans might be regulated, in part, at the post-transcriptional level in a race-cultivar-specific manner. By contrast, the levels of PAL mRNA in both the total and the polysomal RNA fractions, as well as the enzymatic activity, were higher in the incompatible than in the compatible interaction. The results suggest that the activity of PAL is regulated differentially at the transcriptional level during the interactions.