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

Abstract

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. The amount of citrate secreted during the 24‐h Al treatment period increased with increasing concentration of Al (0–70 μM). We analysed citrate secretion basically under 3 conditions: (1) by varying light‐exposure, (2) with intact or excised shoots and (3) by using a divided chamber technique. Further, the content of organic acids in the tissue and the activity of enzymes involved in organic acid metabolism were analysed and evaluated. The results indicate that high rate of citrate secretion in soybean requires a 4‐h induction period. Al had a continuous effect on the citrate secretion when Al was removed from the treatment solution. Citrate secretion increased steadily under exposure to continuous light. However, when the shoots were excised the citrate secretion rate dropped to 3–6 times that of their control counterparts. Results of root manipulation experiments revealed that citrate secretion required the direct contact of Al. In other words, only the Al‐treated root portions secreted citrate. All these observations suggest that the shoots play a role in Al‐induced citrate secretion. Although shoots may not supply citrate for the secretion upon Al treatment, it seems that they may provide the carbon source and/or energy for citrate synthesis in the root. On the other hand, the root organic acid content (1‐cm apex) indicated that malate might contribute to citrate secretion by keeping the balance between citrate synthesis and release in the root apices. Quantification of enzymes involved in organic acid metabolism showed only a 16% increase in citrate synthase activity upon Al treatments (6 h) with no differences in other enzymes. Hence, we could not rule out completely the potential contribution of citrate from shoots and the results are discussed in the light of shoots contributing either energy or citrate itself for enhanced citrate secretion in the Al‐tolerant plant roots.

Immobilization of Aluminum with Phosphorus in Roots Is Associated with High Aluminum Resistance in Buckwheat

“…This differs from the earlier report by Ma et al (1997) who found that the rate of secretion continued to increase above 50 mM. This disparity might result from experimental conditions, since Yang et al (2001) demonstrated that even light intensity can affect the Al-induced secretion of citrate from soybean (Glycine max) roots. Finally, the variation in Al-induced oxalic acid efflux along the root was similar in both cultivars, showing a 10-fold greater efflux from the apical 0-to-5-mm region than from the 5-to-10-mm region (Fig.…”

Citrate Secretion Coupled with the Modulation of Soybean Root Tip under Aluminum Stress. Up-Regulation of Transcription, Translation, and Threonine-Oriented Phosphorylation of Plasma Membrane H+-ATPase

“…In contrast to a number of previous studies where organic acid secretion from roots was measured in a simple salt solution containing CaCl 2 only (Li et al, 2000;Yang et al, 2001;Piñ eros et al, 2002), we used a complete nutrient solution for soybean seedlings and Murashige and Skoog medium for Arabidopsis seedlings. To avoid contaminating the plasma membrane isolates with endoplasmic membranes, we examined the activity of various inhibitor-sensitive ATPases in both the microsomal and plasma membrane fractions (Table I).…”

“…Citrate secretion from intact roots of soybean was measured using HPLC according to the method of Yang et al (2001). Citrate secretion from Arabidopsis roots was determined by the method of Delhaize et al (1993), with some modifications.…”

“…Many plants were found to secrete organic acids in response to Al stress. For instance, citrate is released from the roots of soybean (Glycine max; Yang et al, 2001) and maize (Zea mays;Piñ eros et al, 2002), oxalate from buckwheat (Fagopyrum esculentum;Ma et al, 1997), and malate is released from the roots of Al-tolerant genotypes of wheat (Triticum aestivum;Delhaize et al, 1993;Ryan et al, 1995). However, it remains unclear how Al activates organic acid secretion and how this process is regulated .…”

“…However, it remains unclear how Al activates organic acid secretion and how this process is regulated . Recent studies indicated that the Al-dependent secretion of organic acids in soybean, wheat, and rye is poorly associated with changes in internal organic acid concentrations or with the activities of enzymes that synthesize these acids in the root cells (Ryan et al, 1995;Yang et al, 2001;Hayes and Ma, 2003). The Al-dependent secretion of malate from wheat roots and citrate from maize roots occurs via anion channels on the plasma membrane of the root cells (Ryan et al, 1997;Zhang et al, 2001;Piñ eros et al, 2002;Sasaki et al, 2004).…”