Abstract
Pathogenic and nonpathogenic strains of Xanthomonas campestris pv. glycines (causal agent of bacterial pustule disease of soybean) or a strain of the soybean nonpathogen X. campestris pv. campestris were tested for their ability to grow and induce accumulation of isoflavonoids (glyceollin, coumestrol, daidzein, formononetin and genistein) and isoflavone glucosides (daidzin, genistin and ononin) in leaves of soybean cv. Clark (susceptible to bacterial pustule) and cv. Clark 63 (resistant to bacterial pustule due to the presence of the recessive rpx gene pair). Growth in vivo of nonpathogenic strains of pv. glycines and the strain of pv. campestris was highly restricted in cv. Clark while bacterial growth kinetics of a pathogenic strain of pv. glycines in the susceptible cv. Clark and in the resistant cv. Clark 63 were similar. No or very low levels (<50 μg g −1 fresh weight) of isoflavonoids including glyceollin accumulated by 4 days after inoculation of leaves of cvs Clark or Clark 63 with bacterial strains which showed restricted growth in vivo. In general, higher levels of isoflavone glucosides accumulated, but similar levels of isoflavone glucosides were found after inoculation with bacterial strains which were restricted or not restricted in their growth in vivo. In addition, the isoflavone glucosides were not inhibitory towards the xanthomonads in in vitro bioassays. Additional compounds of unknown identity with antibacterial activity also accumulated in certain interactions, but these compounds did not appear to be involved in resistance either. Resistance to pathogenic strains of pv. glycines conditioned by the rpx gene pair in cv. Clark 63 appears unusual in that bacterial growth is not restricted, but rather symptomology is reduced with a lowering of the number of pustular lesions accompanied by aborted growth and early necrosis of the pustules
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