THE PLANT CELL, Vol 9, Issue 8 1425-1433, Copyright © 1997 by American Society of Plant Biologists
Specific Binding of the Syringolide Elicitors to a Soluble Protein Fraction from Soybean Leaves
C. Ji, Y. Okinaka, Y. Takeuchi, T. Tsurushima, R. I. Buzzell, J. J. Sims, S. L. Midland, D. Slaymaker, M. Yoshikawa, N. Yamaoka and N. T. Keen
Department of Plant Pathology, University of California, Riverside, California 92521
Syringolides are glycolipid elicitors produced by Gram-negative bacteria
expressing Pseudomonas syringae avirulence gene D. The syringolides mediate
gene-for-gene complementarity, inducing the hypersensitive response only in
soybean plants carrying the Rpg4 disease resistance gene. A site(s) for
125I-syringolide 1 was detected in the soluble protein fraction from
soybean leaves, but no evidence for ligand-specific binding to the
microsomal fraction was obtained. The Kd value for syringolide 1 binding
with the soluble fraction was 8.7 nM, and binding was 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 125I-syringolide 1 and several
structural derivatives demonstrated a direct correlation between binding
affinity to the soluble fraction and elicitor activity. However,
differential competition binding studies disclosed no differences in
syringolide binding to soluble fractions from Rpg4/Rpg4 or rpg4/rpg4
soybean leaves. Thus, the observed binding site 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 binding, possibly in intracellular signal
transduction.
