Molecular cloning of a potato leaf cDNA encoding an aspartic protease (StAsp) and its expression after P. infestans infection.

Aspartic proteinases (EC 3.4.23) are widely distributed in the plant kingdom, and a number of cDNAs have been isolated from different plants. Here we report the isolation an expression analysis of a cDNA from Solanum tuberosum L. (cv. Pampeana) named StAsp. The StAsp cDNA clone was obtained using a reverse transcriptase-polymerase chain reaction (RT-PCR) and degenerated primers encoding to plant aspartic proteinases conserved domains. The coding region of the gene is 1494 bp long encoding 497 amino acids of a predicted 54 kDa molecular mass and with a pI of 5.5. The gene shares a high homology with an aspartic proteinase cDNA of tomato, 97% and 94% homology on the level of DNA and protein, respectively. The deduced amino acid sequence contains the conserved features of plant aspartic proteinases, including the plant specific insert. Northern blot analysis indicated that StAps transcripts are differentially accumulated in potato leaves after Phytophthora infestans infection in two potato cultivars with different degree of field resistance to this pathogen. In the resistant cultivar (Pampeana), induction was higher and more durable than in the susceptible cultivar (Bintje), suggesting that the StAsp level expression are associated with the resistance degree of potato cultivars to P. infestans. Results obtained previously about the induction of StAP proteins in stress conditions and these results suggest that potato aspartic proteinases are components of the plant defense response.