Effect of hypergravity and phytohormones on isoflavonoid accumulation in soybean (Glycine max. L.) callus

The objective of this study was to explore the potential interaction between gravity and growth hormones on isoflavonoid accumulation. Soybean callus (Glycine max (L.) Merr. cv. 'Acme') was grown in the dark for 16 days at 22 C in a growth medium supplemented with four different combinations of phytohormones and subjected to 4-g and 8-g forces simulated in a centrifuge and 1-g in an adjacent stationary control. Isoflavonoid aglycones and their glycoside concentrations (daidzein, genistein, daidzin, 6″-O-malonyl-7-O- glucosyl daidzein, genistin, 6″-O-malonyl-7-O-glucosyl genistein) were determined in the resulting tissues. Although gravity had no significant impact on callus growth, increasing gravity reduced isoflavonoid accumulation in three out of the four phytohormone-supplemented culture media. The ratio of the auxin naphthalene acetic acid (NAA) to the cytokinin benzylaminopurine (BAP) was found to have profound effect on both callus growth and isoflavonoid accumulation. The cytokinin BAP promoted callus tissue growth, but reduced callus isoflavonoid suggesting the isoflavonoid accumulation was not keeping pace with the cell growth in the elevated concentration of BAP. On the other hand, NAA had little or no effect on callus growth, but greatly enhanced isoflavonoid accumulation. Interactive effects of gravity and hormone on isoflavonoid accumulation were evident and its implication to the mechanism by which gravity exerts the effect on plant secondary metabolites is discussed.