Originally published as Genetics Published Articles Ahead of Print on November 15, 2004.

Genetics, Vol. 169, 1009-1019, February 2005, Copyright © 2005
doi:10.1534/genetics.103.025098

Selection on Glycine ß-1,3-Endoglucanase Genes Differentially Inhibited by a Phytophthora Glucanase Inhibitor Protein

J. G. Bishop^*,1, D. R. Ripoll, S. Bashir^,2, C. M. B. Damasceno, J. D. Seeds^* and J. K. C. Rose

^* School of Biological Sciences, Washington State University, Vancouver, Washington 98686-9600
Computational Biology Service Unit, Cornell Theory Center
Department of Plant Biology, Cornell University, Ithaca, New York 14853

¹ Corresponding author: School of Biological Sciences, Washington State University, 14204 NE Salmon Creek Ave., Vancouver, WA 98686.
E-mail: bishop{at}vancouver.wsu.edu

Plant endo-ß-1,3-glucanases (EGases) degrade the cell wall polysaccharides of attacking pathogens and release elicitors of additional plant defenses. Isozymes EGaseA and EGaseB of soybean differ in susceptibility to a glucanase inhibitor protein (GIP1) produced by Phytophthora sojae, a major soybean pathogen. EGaseA, the major elicitor-releasing isozyme, is a high-affinity ligand for GIP1, which completely inhibits it, whereas EGaseB is unaffected by GIP1. We tested for departures from neutral evolution on the basis of partial sequences of EGaseA and EGaseB from 20 widespread accessions of Glycine soja (the wild progenitor of soybean), from 4 other Glycine species, and across dicotyledonous plants. G. soja exhibited little intraspecific variation at either locus. Phylogeny-based codon evolution models detected strong evidence of positive selection on Glycine EGaseA and weaker evidence for selection on dicot EGases and Glycine EGaseB. Positively selected peptide sites were identified and located on a structural model of EGase bound to GIP1. Positively selected sites and highly variable sites were found disproportionately within 4.5 Å of bound GIP1. Low variation within G. soja EGases, coupled with positive selection in both Glycine and dicot lineages and the proximity of rapidly evolving sites to GIP1, suggests an arms race involving repeated adaptation to pathogen attack and inhibition.

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