Pseudothionin‐St1, a potato peptide active against potato pathogens

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Abstract
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 Bowman‐Birk protease inhibitor. Pth‐St1 did not inhibit either trypsin or insect α‐amylase activities, and, in contrast with true thionins, did not affect cell‐free protein synthesis or β‐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.