The peptide snakin-2 (StSN2) has been isolated from potato (Solanum tuberosum cv Jaerla) tubers and found to be active (EC 50 ϭ 1-20 m) against fungal and bacterial plant pathogens. It causes a rapid aggregation of both Gram-positive and Gram-negative bacteria. The corresponding StSN2 cDNA encodes a signal sequence followed by a 15-residue acidic sequence that precedes the mature StSN2 peptide, which is basic (isoelectric point ϭ 9.16) and 66 amino acid residues long (molecular weight of 7,025). The StSN2 gene is developmentally expressed in tubers, stems, flowers, shoot apex, and leaves, but not in roots, or stolons, and is locally up-regulated by wounding and by abscisic acid treatment. Expression of this gene is also up-regulated after infection of potato tubers with the compatible fungus Botritys cinerea and down-regulated by the virulent bacteria Ralstonia solanacearum and Erwinia chrysanthemi. These observations are congruent with the hypothesis that the StSN2 is a component of both constitutive and inducible defense barriers.An important component of plant defense is a diverse set of constitutive and pathogen-inducible antimicrobial compounds that includes the so-called pathogenesis-related proteins, several families of antimicrobial peptides, a variety of chemically diverse organic compounds classified as phytoalexins and phytoanticipins, and certain active oxygen and nitrogen species (Osbourn, 1996(Osbourn, , 1999 Broekaert et al., 1997;Kombrink and Somssich, 1997; García-Olmedo et al., 1998). Accumulation of these compounds and the ability of a given pathogen to deal with them may be decisive for the outcome of the interaction (Titarenko et al., 1997a;Ló pez-Solanilla et al., 1998Miguel et al., 2000; Alamillo and García-Olmedo, 2001;García-Olmedo et al., 2001). Thus, it has been shown that increased levels of certain antimicrobial peptides, either through overexpression of the corresponding genes or by appropriate exogenous treatments, result in enhanced tolerance to particular pathogens (Carmona et al., 1993; Terras et al., 1995; Epple et al., 1997;Molina and García-Olmedo, 1997; Holtorf et al., 1998; Thomma et al., 1998 Thomma et al., , 1999.Furthermore, pathogen mutants that are sensitive to antimicrobial peptides show decreased virulence when inoculated in the plant (Titarenko et al., 1997a; Ló pez-Solanilla et al., 1998).Several families of antimicrobial peptides have been characterized in plants ( García-Olmedo et al., 1992, 1995 Broekaert et al., 1997). The majority of them are Cys-rich and their globular structure is stabilized by disulphide bridges, although linear Gly-/His-rich and macrocyclic Cysknot peptides have also been recently identified (Tam et al., 1999;Park et al., 2000). The peptides are generally encoded by multigenic families in which some genes are developmentally regulated in certain tissues, whereas others are pathogen inducible, and a number of them show both constitutive and pathogen-inducible expression (García-Olmedo et al., 1995 Broekaert et al., 1997).In a previous...

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A new type of antimicrobial peptide, snakin-1 (SN1), has been isolated from potato tubers and found to be active, at concentrations <10 uM, against bacterial and fungal pathogens from potato and other plant species. The action of SN1 and potato defensin PTH1 was synergistic against the bacterium Clavibacter michiganensis subsp. sepedonicus and additive against the fungus Botrytis cinérea. Snakin-1 causes aggregation of both gram-positive and gram-negative bacteria. The peptide has 63 amino acid residues (M r 6,922), 12 of which are cysteines, and is unrelated to any previously isolated protein, although it is homologous to amino acid sequences deduced from cloned cDNAs that encode gibberellin-inducible mRNAs and has some sequence motifs in common with kistrin and other hemotoxic snake venoms. A degenerate oligonucleotide probé based on the internal sequence CCEECKC has been used to clone an SN1 cDNA. With the cDNA used as probé, one copy of the StSNl gene per haploid genome has been estimated and expression of the gene has been detected in tubers, stems, axillary buds, and young floral buds. Expression levéis in petáis and carpels from fully developed flowers were much higher than in sepáis and stamens. The expression pattern of gene StSNl suggests that protein SN1 may be a component of constitutive defense barriers, especially those of storage and reproductive plant organs.Plants and animáis are in cióse contact with widely diverse bacteria and fungi, but only in rare cases does this association result in the development of disease, substantially because of the existence of defense systems. Although considerable differences exist among different types of organisms with respect to their defense mechanisms, the most notable of which is the lack of an adaptive immune response in plants, recent evidence indicates that both plants and animáis share some com-

Four homogeneous proteins (Cw,,, Cw,,, Cw2,. Cw,,) were isolated from etiolated barley leaves by extraction of the insoluble pellet from a Tris-HCI (pH 7.5) homogenate with 1.5 M LiCl and fractionation by reverse-phase high-performance liquid chromatography. All 4 proteins inhibited growth of the pathogen Cfavibacter michiganensis subsp. sepedonicus (E&s = l-3 x IO-' M) and had closely related N-terminal amino acid sequences. The complete amino acid sequences of proteins Cw,, and Cw,, were determined and found to be homologous to previously described, non-specific lipid transfer proteins from plants (32-628 identical positions). The proteins also inhibited growth of the bacterial pathogen Pseudomonas solanacearum (E&s = 3-6 x IO-' M) and the fungus Fusarium solani (EC,,s = 3-20 x 10m6 M). A homologous protein from maize leaves (Cw,,) was purified in a similar manner and also found to have inhibitory properties. A synergistic effect against the fungus was observed when protein Cw,, was combined with thionins. A defense role for non-specific lipid transfer proteins from plants is proposed.Lipid transfer protein; Plant pathogen; Thionin

Plant nonspecific lipid-transfer proteins stimulate the transfer of a broad range of lipids between membranes in vitro. In view of their ability to inhibit bacterial and fungal pathogens, their distribution at high concentrations over exposed surfaces and in the vascular system, and the response of Ltp-gene expression to infection with pathogens, they are now thought to be active plant-defense proteins.