Syringopeptins, Pseudomonas syringae pv. syringae phytotoxins, resemble syringomycin in closing stomata D. DI GIORGIO a , L. CAMONI a , K. A. MOTT b , J. Y. TAKEMOTO b and A. BALLIO a * a Dipartimento di Scienze Biochimiche ‘A. Rossi Fanelli’ e Centro di Biologia Molecolare del CNR, Universita ‘La Sapienza’, P. le Alde Moro 5, 00185 Rome, Italy b Department of Biology, Utah State University, Logan, UT, USA The recent finding that syringomycin (SR) and syringotoxin (ST)-producing isolates of Pseudomonas syringae pv. syringae also synthesize syringopeptins (SPs), another class of phytotoxic lipodepsipeptides, prompted studies of the biological properties and comparisons of the activities of the two groups of metabolites. The present paper reports the effects of two forms of SP on stomatal movement in detached leaves of Xanthium strumarium and in epidermal strips of Vicia faba and shows that these phytotoxins, as does the previously investigated SR, promote stomatal closure. SPs are at least 10-fold more efficient than SR in both tissues. In epidermal strips, the toxin-induced closure was not reversed by fusicoccin, a fungal metabolite that reversed the closing effects of abscisic acid. As reported in previous papers, these amphiphilic toxins affect several functions of biological membranes. A marked decrease in the rate of photosynthesis recorded in X. strumarium leaves treated with SPs is consistent with effects on biological membranes. INTRODUCTION In recent years, bioactive lipodepsipeptides have been isolated from plant-pathogenic strains of Pseudomonas syringae pv. syringae. Several have been purified and their structures elucidated, namely syringomycins (SRs) from various strains of this bacterium (Gross et al., 1977; Ballio et al., 1988; Segre et al., 1989; Fukuchi et al., 1990b; Fukuchi et al., 1992b), syringotoxin (ST) from strains pathogenic for citrus trees (Gross et al., 1977; Ballio et al., 1990; Fukuchi et al., 1990a; Fukuchi et al., 1992b), and syringostatins (SSs) from a lilac isolate (Isogai et al., 1989; Fukuchi et al., 1992a; Fukuchi et al., 1992b). Recent additions to the above lipodepsipeptides are the pseudo- mycins (Harrison et al., 1991; Ballio et al., 1994), bioactive metabolites of P. syringae MSU 16H, which are able to protect elms against infection by Ceratocystis ulmi (Lam et al., 1987). All these metabolites have many structural features in common, the only differences concerning the size of the lipid moiety (always a 3-hydroxy fatty acyl) and the nature of five of nine amino acid residues (Fig. 1a). Recently, it has been observed that the strains mentioned above also produce some larger and less polar lipodepsipeptides with a higher amino acid content, which have been named syringopeptins (SPs; Fig. 1b); so far SPs with 22 (SP-22) and 25 (SP-25) amino acid residues have been described (Ballio et al., 1991). The phytotoxic and antifungal activities of most of these metabolites have been investigated, and their effects on physiological processes in yeast and higher plants examined to some extent. It has been shown that SR affects ion transport across the plasma membrane (Takemoto, 1992) and stimulates phosphorylation of membrane pro- teins with the involvement of Ca 2 (Suzuki et al., 1992). Furthermore, SR induces closure of sto- mata, its effects being similar to those of abscisic acid (ABA) (Mott & Takemoto, 1989). As for SPs, it has been observed that they have a more pronounced phytotoxicity and a lower anti- biotic activity than SR and ST (Iacobellis et al., 1992); in some plant systems they behave qualita- tively like the nonapeptides, but frequently with higher effectiveness (Di Giorgio et al., 1994). A marked difference between the two classes of lipodepsipeptides has been found in their effective- ness upon some ATPases. In fact, the enzyme Plant Pathology (1996) 45, 564–571 * To whom correspondence should be addressed. Accepted 28 December 1995.