peruvianum is a systemin receptor-mediated process (5, 12). The MAPK activation in the cells of L. peruvianum in response to systemin, chitosan, and pmg elicitor was inhibited by suramin (Fig. 5), thereby supporting the role of suramin as an inhibitor of receptor function.
Heparin, a glycosaminoglycan of the animal extracellular matrix with sulfonic acid groups such as found in suramin, and suramin both were reported to inhibit binding of 125I-basic fibroblast growth factor to its binding sites (27). However, heparin had no effect on the medium alkalinization of suspension-cultured tomato cells in response to systemin (Table 2). Thus, the negative charges alone cannot account for the suramin effect on the systemin receptor. The alkalinization of cell medium by carbohydrate elicitors was, however, partially inhibited by heparin (Table 2). This inhibition may reflect different structural features of the different binding sites that can interact with suramin and heparin. NF 007, 8-(3-Nitrobenzamido)-naphtalenetrisulfonic acid, another sulfonic acid-containing molecule, is a derivative of suramin. Unlike suramin, NF 007 interacts with cells of L. peruvianum as an inducer rather than an inhibitor of medium alkalinization. Thus, common structural features of suramin and NF 007 might be important for binding to an elicitor binding site, but additional determinants are required for suramin to act as an inhibitor. These properties together with the low IC50s of suramin observed for the inhibition of the alkalinization response, induced by all three elicitors, suggest that suramin may recognize some characteristic structural features that might be common among the different elicitor receptors. The identity of sites involved in suramin interactions with the membrane-associated systemin and elicitor receptors could provide more specific information concerning the possible structural relationship between plant receptors and perhaps between plant and animal receptors as well.
Based on its polyanionic hydrophilic character, it has been assumed that suramin is not able to pass the membrane barrier and enter the cell where it can interfere with intracellular signaling processes, e.g., through inhibition of tyrosine phosphatases, G proteins, and DNA polymerases (41, 42 and 43). However, in some animal cell systems, suramin seems to enter cells via endocytosis (44). Because endocytotic mechanisms are also known in plants (45), it was important to determine whether suramin can pass the plasma membrane barrier and interfere with intracellular signaling processes leading to the alkalinization response. Calyculin A, a protein phosphatase 1 and 2a inhibitor, and erythrosin B, a membrane-permeable inhibitor of the plasma membrane H+-ATPase, both activate the alkalinization response in suspension-cultured tomato cells (9, 46). The roles of these inhibitors in activating the defense response is not known, but it is likely that they inhibit enzymes that are negative regulators of the response (9). Suramin did not inhibit calyculin A- or erythrosin B-induced alkalinization responses (Fig. 6). These data indicated that suramin acts upstream of the targets of the inhibitors, supporting its role in interfering extracellularly with binding of systemin and oligosaccharide elicitors to their receptors.
Taken together, the results of this study further support a role for the receptors of systemin, chitosan, and pmg elicitor in activating the intracellular events leading to defense gene activation in response to herbivores and pathogens. Suramin provides a tool to investigate receptor-mediated defense responses in plant cells. It may prove to be useful in elucidating the early events in plant defense signaling and in exploring the analogies between signaling pathways of plants and animals further (Fig. 7).
We thank Sue Vogtman and Tom Koehler for growing and maintaining plants. This research was supported in part by Washington State University College of Agriculture and Home Economics (Project No. 1791), the National Science Foundation (Grant No. IBN 9601099), and the United States Department of Agriculture Competitive Grants Program (Grant No. 9801502).
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