Pseudothionin‐St1, a potato peptide active against potato pathogens

A 5-kDa polypeptide, pseudothionin Solanum tuberosum 1 (Pth-St1), which was active against Clavibacter michiganensis subspecies sepedonicus, a bacterial pathogen of potatoes, has been purified from the buffer-insoluble fraction of potato tubers by salt extraction and HPCL. Pth-St1 was also active against other potato pathogens tested (Pseudomonas solanacearum and Fusarium solani). The N-terminal amino acid sequence of this peptide was identical (except for a N/H substitution at position 2) to that deduced from a previously reported cDNA sequence (EMBL accession number X-13180), which had been misclassified as a Browman-Birk protease inhibitor. Pth-St1 did not inhibit either trypsin or insect alpha-amylase activities, and, in contrast with true thionins, did not affect cell-free protein synthesis or beta-glucuronidase activity. Northern-blot and tissue-print analyses showed that steady-state mRNA levels were highest in flowers (especially in petals), followed by tubers (especially in the epidermal cell layers and in leaf primordia), stems and leaves. Infection of leaves with a bacterial pathogen suspended in 10 mM MgCl2 switched off the gene, whereas mock inoculation with 10 mM MgCl2 alone induced higher mRNA levels.