US-1 and US-8 genotypes of Phytophthora infestans differentially affect local, proximal and distal gene expression of phenylalanine ammonia-lyase and 3-hydroxy, 3-methylglutaryl CoA reductase in potato leaves

Differential expression of pall and hmgr2 was investigated using northern blot analysis in two potato cultivars (Russet Burbank (RB), susceptible and Kennebec (KB), moderately tolerant) after inoculation with two Phytophthora infestans isolates from the formerly (US-1) and currently predominant genotypes (US-8). The accumulation of pall transcripts was weaker in response to US-8 as compared to US-1 and occurred earlier in KB than in RB. The stronger expression of pall in response to US-1, as compared to US-8, is suggested to be due to defense gene suppression by the latter. No apparent strong accumulation of hmgr2 transcripts was recorded in RB as compared to KB inoculated with either US-1 or US-8. The induction of pall and hmgr2 was first observed in un-inoculated (proximal) close to the inoculated leaflets, then in un-inoculated (distal) leaflets of leaves adjacent to the inoculated leaf, and finally in local inoculated leaflets. The stronger expression of the two genes in proximal and distal leaflets, as compared to the local site of inoculation suggests the translocation of signal(s) from this site to healthy parts of the plant. (c) 2005 Elsevier Ltd. All rights reserved.