Effects of wounding and inoculation with Sclerotinia sclerotiorum on isoflavone concentrations in soybean

Abstract

Isoflavones play an important role in the defense response of soybean to pathogen attack. They are involved in nodulation of legumes and are associated with human health benefits including the prevention of heart disease and cancers. Concentrations of the isoflavones daidzein, genistein, and glycitein, the glucoside conjugates daidzin, genistin and glycitin, the acetylglucoside conjugates acetyldaidzin, acetylgenistin, and acetylglycitin, and the malonylglucoside conjugates malonyldaidzin, malonylgenistin, and malonylglycitin were determined in ovendried leaves of 12 soybean cultivars whose stems were (i) non-wounded, non-inoculated (NWNI), (ii) wounded, non-inoculated (WNI), and (iii) wounded and inoculated (WI) with mycelia of Sclerotinia sclerotiorum, the causal fungus of Sclerotinia stem rot of soybean. There were significant differences (P ≤ 0.05) among cultivars in concentrations of isoflavones and their conjugates in all wounding treatments. Concentrations of the aglycones daidzein and genistein were higher (P ≤ 0.05) in WI than in WNI and NWNI plants in all cultivars. Glycitein and its conjugates were detected only in some cultivars in much lower concentrations than daidzein and genistein and their conjugates. Concentrations of total daidzein in the 12 cultivars ranged from 68 to 491, 174 to 781, and 282 to 553 µg g-1 dry weight in NWNI, WNI, and WI plants, respectively. Concentrations of total genistein ranged from 128 to 427, 290 to 840, and 296 to 759 µg g-1 dry weight in NWNI, WNI, and WI plants, respectively. Concentrations of total glycitein ranged from 0 to 44, 0 to 13, and 0 to 24 µg g-1 dry weight in NWNI, WNI, and WI plants, respectively. In NWNI plants, the cultivar Corsoy 79 ranked in the top two (rank 1 = highest concentration, rank 12 = lowest concentration) in concentrations of daidzein and genistein and their conjugates except genistin (ninth rank). In WNI plants, Parker ranked first in concentrations of all conjugates of daidzein and genistein whereas Corsoy 79 consistently ranked in the top four in concentrations of the two isoflavones and their conjugates. In WI plants, Parker ranked first in concentrations of all conjugates of daidzein and genistein except acetyldaidzin (second rank), S19-90 ranked in the top five in concentrations of all conjugates of daidzein and genistein, whereas Corsoy 79 ranked in the top five in concentrations of daidzein and genistein and all of their conjugates. The results from this study suggest that soybean cultivars differ in concentrations of constitutive or induced isoflavones and in their ability to accumulate isoflavones following wounding and/or infection by S. sclerotiorum. Key words: Isoflavones, soybean, leaves, Sclerotinia sclerotiorum, wounding, HPLC