Aluminum-induced secretion of both citrate and malate in rye

Aluminum (Al)-resistant mechanisms responsible for Al-induced secretion of organic acids are poorly understood. In this study, we characterized the Al-induced secretion of both citrate and malate from rye (Secale cereale L. cv. King). Secretion of organic acids increased with increasing concentration (10, 30 and 50 μM) and duration of Al treatments. Neither phosphorous (P) deficiency up to 15 days nor addition of 50 μM lanthanum, 50 μM lead, 10 μM cadmium, or 200 μM manganese caused secretion of organic acids, suggesting that this secretion was a specific response to Al stress. Aluminum activated citrate synthase, the main enzyme for the synthesis of citrate, but its activation occurred only in the root tip. The elongation of roots of an Al-sensitive cultivar of wheat (Tritium aestivum L. cv. Scout 66) was not inhibited by 50 μM Al in the presence of externally applied 50 μM citrate or 400 μM malate. The secretion of citrate and malate from intact rye roots exposed to 50 μM Al corresponded to 31.3 ± 1.7 μM and 11.5 ± 2.5 μM, respectively, in the rhizosphere based on an assumption of a 2 mm thick unstirred layer around root tips. This result indicated that Al-resistance in rye was achieved by the Al-induced synthesis of citrate in root apices followed by Al-induced specific secretion of citrate from root tips.