Abstract: The reduction in the ratio of red light (R) to far-red light (FR) is an important signal of shade stress.In the present study,an experiment was conducted to study the effect of normal and low R/FR irradiation on the isoflavone contents of two soybean varieties:high shadetolerance species ‘ND12’ and low shadetolerance species ‘C103’.Relative expression levels of key genes,which encoded isoflavone synthetic enzymes in leaves,hypocotyls and roots were also studied.The results indicated that compared with seedlings under normal light,low R/FR increased total contents of isoflavone and genistein significantly in the leaves of ‘ND12’,but decreased in hypocotyls;while the opposite result was obtained in ‘C103’.Expression of isoflavone synthesis key genes,including IFS2, 4CL, C4H and  CHI correlated with isoflavone accumulation under low R/FR irradiation.Shading signal induced isoflavone accumulation particularly in leave of shadetolerance varieties’.This study preliminary clarified the shading signal (low R/FR) that how to control the rules of isoflavone accumulation in soybean seedlings.This study laid a foundation to research regulation mechanism of soybean isoflavone response to shading signal.

Key words: shading, soybean, isoflavone, metabolism, gene expression