Does basal PR gene expression in Solanum species contribute to non-specific resistance to Phytophthora infestans?

Systemic acquired resistance (SAR) occurs in many plant species, including potato. SAR can be induced by various signals, but also basal levels of SAR may vary between plants. In Arabidopsis mutants, basal SAR levels positive ly correlate with pathogen resistance. Here we test whether in 13 wild Sola num clones and five potato cultivars, basal expression levels of SAR marker genes correlate with resistance to Phytophtora infestans. Most of the exam ined Solanum plants displayed significant and variable levels of race/isola te-non-specific, partial resistance to five P. infestans isolates of divers e origin. Constitutive mRNA levels of the pathogenesis-related genes PR-1, PR-2 and PR-5 in non-infected leaves varied between the Solanum clones. How ever, no correlation between basal PR mRNA levels and resistance was observ ed at the genus level. In contrast, significant correlation was found at th e species level in S. arnezii x hondelmannii, S. microdontum, S. sucrense a nd S. tuberosum. In S. tuberosum cultivars, the levels of PR gene expressio n were the highest in resistant Robijn, intermediate in partially resistant Premiere, Estima and Ehud, and the lowest in susceptible Bintje. These res ults suggest that constitutive expression of PR genes may contribute to non -specific resistance to P. infestans in Solanum. Therefore, PR mRNAs could serve as molecular markers in potato breeding programs. (C) 2000 Academic Press.