JXB Advance Access originally published online on May 28, 2003
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Journal of Experimental Botany, Vol. 54, No. 388, pp. 1753-1759,
July 1, 2003
© 2003 Oxford University Press
Al-induced efflux of organic acid anions is poorly associated with internal organic acid metabolism in triticale roots
Received 4 December 2002; Accepted 4 April 2003
,
Faculty of Agriculture, Kagawa University, Ikenobe 2393, Miki-cho, Kita-gun, Kagawa 761-0795, Japan
* Present address: Environmental Biology, School of Earth and Environmental Sciences, Adelaide University, SA 5005, Australia. To whom correspondence should be addressed. Fax: +81 87 891 3137. E-mail: maj{at}ag.kagawa-u.ac.jp
Abbreviations: Al, aluminium; CS, citrate synthase; MDH, malate dehydrogenase; NADP-ICDH, NADP-isocitrate dehydrogenase; PEPC, phosphoenolpyruvate carboxylase.
The secretion of organic acid anions from roots has been identified as a mechanism of resistance to Al. However, the process leading to the secretion of organic acid anions is poorly understood. The effect of Al on organic acid metabolism was investigated in two lines of triticale (xTriticosecale Wittmark) differing in Al-induced secretion of malate and citrate and in Al resistance. The site of Al-induced secretion of citrate and malate from a resistant line was localized to the root apices (terminal 5 mm). The levels of citrate (root apices and mature root segments) and malate (mature segments only) in roots increased during exposure to Al, but similar changes were observed in both triticale genotypes. The in vitro activities of four enzymes involved in malate and citrate metabolism (citrate synthase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and NADP-isocitrate dehydrogenase) were similar for sensitive and resistant lines in both root apices and mature root segments. The response of these enzymes to pH did not differ between tolerant and sensitive lines or in the presence and absence of Al. Moreover, cytoplasmic and vacuolar pH were not affected by exposure to Al in either line. Together, these results indicate that the Al-dependent efflux of organic acid anions from the roots of triticale is not regulated by their internal levels in the roots or by the capacity of the root cells to synthesize malate and citrate.
Key words: Aluminium resistance, citrate, malate, organic acid anion efflux, organic acid metabolism, triticale.
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