Induction of a proteinase K inhibitor in a potato cultivar with a high degree of field resistance against Phytophthora infestans

A proteinase EI inhibitor (PLPKI) was isolated from a potato cultivar with a high level of held resistance (Solanum tuberosum L. cv. Pampeana INTA), a fter 24 h of infection with Phytophthora infestans, when inhibitory activit y was markedly increased. Purification was performed by beat treatment, gel filtration chromatography and affinity chromatography, A size of 60 kDa ma s estimated by SDS-PAGE in partially denaturing conditions and by gel filtr ation. It is multimeric and the monomer has a molecular mass of 8.5-9.0 kDa . PLPKI is highly active against proteinase K (EC 3.4.21.14) but poorly inh ibits two serine proteinases of animal origin, trypsin (EC 3.4,21,4) and ch ymotrypsin (EC 3,4.21,1), A differential expression (determined by activity and immunoblotting assays) of PLPKI was observed between two potato cultiv ars with different degrees of field resistance to P. infestans. In the resi stant cultivar (cv. Pampeana INTA) PLPKI induction (19-fold with respect to healthy leaves) occurred 21 h after infection and remained over basal leve ls after 48 h infection. By contrast, in the susceptible cultivar (cv. Bint je), no induction was observed.