Characterization of aluminium-induced citrate secretion in aluminium-tolerant soybean (Glycine max) plants

Recently, we showed that secretion of citrate in an aluminium (Al) tolerant cultivar soybean (Glycine max) (cv. Suzunari) is a specific response to Al stress [Yang et al. (2000) Physiol Plant 110: 72-77]. Here we investigated the intrinsic mechanisms behind the secretion of citrate induced by Al. Th e amount of citrate secreted during the 24-h Al treatment period increased with increasing concentration of Al (0-70 muM). We analysed citrate secreti on basically under 3 conditions: (1) by varying light-exposure, (2) with in tact or excised shoots and (3) by using a divided chamber technique. Furthe r, the content of organic acids in the tissue and the activity of enzymes i nvolved in organic acid metabolism were analysed and evaluated. The results indicate that high rate of citrate secretion in soybean requires a 4-h ind uction period. Al had a continuous effect on the citrate secretion when Al was removed from the treatment solution. Citrate secretion increased steadi ly under exposure to continuous light. However, when the shoots were excise d the citrate secretion rate dropped to 3-6 times that of their control cou nterparts. Results of root manipulation experiments revealed that citrate s ecretion required the direct contact of Al. In other words, only the Al-tre ated root portions secreted citrate. All these observations suggest that th e shoots play a role in Al-induced citrate secretion. Although shoots may n ot supply citrate for the secretion upon Al treatment, it seems that they m ay provide the carbon source and/or energy for citrate synthesis in the roo t. On the other hand, the root organic acid content (1-cm apex) indicated t hat malate might contribute to citrate secretion by keeping the balance bet ween citrate synthesis and release in the root apices. Quantification of en zymes involved in organic acid metabolism showed only a 16% increase in cit rate synthase activity upon Al treatments (6 h) with no differences in othe r enzymes. Hence, we could not rule out completely the potential contributi on of citrate from shoots and the results are discussed in the light of sho ots contributing either energy or citrate itself for enhanced citrate secre tion in the Al-tolerant plant roots.