Evidence of calycosin-7-O-β-D-glucoside's role as a major antioxidant molecule of Astragalus membranaceus Bge. var. mongholicus (Bge.) Hsiao plants under freezing stress

It was previously demonstrated that a significant increase in calycosin-7-O-beta-D-glucoside (CG), the major isoflavonoid in roots of Astragalus membranaceus Bge. var. mongholicus (Bge.) Hsiao, is induced under low temperatures. To reveal its physiological functions under stress, seedlings with different isoflavonoid levels were established using a phenylalanine ammonia lyase (PAL) enzyme inhibitor, L-alpha-aminooxy-beta-phenylpropionic acid (AOPP). Changes in the content of malondialdehyde (MDA), activities of antioxidant enzymes and levels of isoflavonoids were monitored in leaves and roots of this plant during 72 h freezing and H2O2 treatments. Cold pretreatment significantly enhanced the freezing tolerance of this species. The plants with higher CG content showed both lower MDA content and a lower percentage of injury under freezing stress. A marked decrease in CG level over a 72 h freezing treatment in all experimental groups strongly indicated that CG was transformed and might play a key role against freezing stress. At the same time, plants with a higher level of CG also showed more superoxide dismutase (SOD) and catalase (CAT) activity. These results implied that the significant promotion of antioxidant capacity in this species might be associated with the remarkable accumulation of CG after cold pretreatment. Our results provided the first evidence that a type of isoflavonoid, CG, might play a very important role against freezing stress in vivo. This report also provides a basis for future studies to reveal the mechanisms of CG's role in freezing tolerance. (C) 2014 Elsevier B.V. All rights reserved.