Effect of K-252a and abscisic acid on the ef ̄ux of citrate from soybean roots

The Al-induced release of organic acid has been suggested as an important mechanism for Al resistance in plants. In this study, the effect of K-252a and abscisic acid (ABA) on the ef ̄ux of citrate was investigated in soybean (Glycine max L.) roots. Al initiated citrate ef ̄ux from the root apices 30 min after the addition of Al. The Al-triggered ef ̄ux of citrate was sensitive to metabolic inhibitors and anion channel inhibitors. Pretreatment or treatment with K-252a, an inhibitor of protein kinase, severely inhibited the Alinduced ef ̄ux of citrate accompanying an increase in Al accumulation and intensi®ed Al-induced root growth inhibition. Al-treatment increased the endogenous level of abscisic acid (ABA) in soybean roots in a doseand time-dependent manner, while K-252a failed to inhibit the Al-induced increase in endogenous ABA. Exogenous application of ABA increased the activity of citrate synthase (EC 4.1.3.7) by 26.2%, and decreased Al accumulation by 32.3%, respectively. ABA-induced increases in citrate ef ̄ux and root elongation were suppressed by K-252a, while ABA could not reverse the K-252a effects. Taken together, these results suggest that ABA is probably involved in the early response, after which K-252asensitive protein kinases play a key step in regulating the activity of an anion channel, through which citrate is released from the apical cells of soybean