Suramin inhibits initiation of defense signaling by systemin, chitosan, and a beta-glucan elicitor in suspension-cultured Lycopersicon peruvianum cells

Systemin-mediated defense signaling in tomato (Lycopersicon esculentum) pla nts is analogous to the cytokine-mediated inflammatory response in animals. Herein, we report that the initiation of defense signaling in suspension-c ultured cells of Lycopersicon peruvianum by the peptide systemin, as well a s by chitosan and beta-glucan elicitor from Phytophtora megasperma, is inhi bited by the polysulfonated naphtylurea compound suramin, a known inhibitor of cytokine and growth factor receptor interactions in animal cells. Using a radioreceptor assay, we show that suramin interfered with the binding of the systemin analog I-125-Tyr-2,Ala-15-systemin to the systemin receptor w ith an IC50 of 160 mu M. Additionally, labeling of the systemin receptor wi th a photoaffinity analog of systemin was inhibited in the presence of sura min. Receptor-mediated tyrosine phosphorylation of a 48-kDa mitogen-activat ed protein kinase and alkalinization of the medium of suspension-cultured c ells in response to systemin and carbohydrate elicitors were also inhibited by suramin. The inhibition of medium alkalinization by suramin was reversi ble in the presence of high concentrations of systemin and carbohydrate eli citors. Calyculin A and erythrosin B, intracellular inhibitors of phosphata ses and plasma membrane proton ATPases, respectively, both induce medium al kalinization, but neither response war inhibited by suramin, The polysulfon ated compound heparin did not inhibit systemin-induced medium alkalinizatio n, NF 007, a suramin derivative, induced medium alkalinization, indicating that neither NF 007 nor heparin interact with elicitor receptors like suram in. The data indicate that cell-surface receptors in plants show some commo n structural features with animal cytokine and growth factor receptors that can interact with suramin to interfere with ligand binding.