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 positively correlate with pathogen resistance. Here we test whether in 13 wild Solanum clones and five potato cultivars, basal expression levels of SAR marker genes correlate with resistance to Phytophtora infestans. Most of the examined Solanum plants displayed significant and variable levels of race/isolate-non-specific, partial resistance to five P. infestans isolates of diverse 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. However, no correlation between basal PR mRNA levels and resistance was observed at the genus level. In contrast, significant correlation was found at the species level in S. arnezii x hondelmannii, S. microdontum, S. sucrense and S. tuberosum. In S. tuberosum cultivars, the levels of PR gene expression were the highest in resistant Robijn, intermediate in partially resistant Premiere, Estima and Ehud, and the lowest in susceptible Bintje. These results 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.