SPECIFIC BINDING OF THE SYRINGOLIDE ELICITORS TO A SOLUBLE-PROTEIN FRACTION FROM SOYBEAN LEAVES

Syringolides are glycolipid elicitors produced by Gram-negative bacter ia expressing Pseudomonas syringae avirulence gene D. The syringolides mediate gene-for-gene complementarity, inducing the hypersensitive re sponse only in soybean plants carrying the Rpg4 disease resistance gen e. A site(s) for I-125-syringolide 1 was detected in the soluble prote in fraction from soybean leaves, but no evidence for ligand-specific b inding to the microsomal fraction was obtained. The K-d value for syri ngolide 1 binding with the soluble fraction was 8.7 nM, and binding wa s greatly reduced by prior protease treatment or heating. A native gel assay was also used to demonstrate ligand-specific binding of labeled syringolide 1 with a soluble protein(s), Competition studies with I-1 25-syringolide 1 and several structural derivatives demonstrated a dir ect correlation between binding affinity to the soluble fraction and e licitor activity. However, differential competition binding studies di sclosed no differences in syringolide binding to soluble fractions fro m Rpg4/Rpg4 or rpg4/rpg4 soybean leaves, Thus, the observed binding si te fulfills several criteria expected of an intracellular receptor for the syringolides, but it is most likely not encoded by the Rpg4 gene, Instead, the Rpg4 gene product may function subsequent to elicitor bi nding, possibly in intracellular signal transduction.