Abstract
Aluminum (Al) affects numerous physiological processes in plants. However, Al tolerance mechanisms mediated by increased synthesis of organic acids (OAs) have been outlined recently. In this study, we examined the role of OAs in the short (1–8 h) and long-term (4 days) Al tolerance in maize seedlings. Exposure to Al stress for 4 days results in a rapid inhibition of root growth. Al induced morphological changes in the maize roots, especially at a higher solution of Al concentration (1,000 μM Al). The increase in Al accumulation in roots, including strongly elevated levels of Al accumulated in root cell walls suggests that Al tolerance in maize is mediated in part by higher accumulation of Al in the roots. The enhanced citrate exudation, which was only observed at 1,000 μM Al may lead to detoxification of Al by formation of OA–Al complexes in the root apoplast. This mechanism has been suggested to play a significant role in Al resistance response in maize. The short-term responses underlying internal detoxification via OA-chelators were also investigated. Succinate, malate, citrate and total root OA contents decreased markedly, 2 h after the Al exposure. At 4 and 8 h time points, OA contents increased or remained unchanged, except for that of malate which decreased. The level of OAs in shoots, on the other hand, showed alterations that were less pronounced in response to Al. Specifically, the citrate and total OA concentrations significantly increased at 4 h, but showed a pronounced decrease at the 8 h time point. Based on our findings, we propose that multiple responses, including Al exclusion by Al accumulation in root cells and citrate efflux, may contribute towards higher Al resistance in maize. The rapid OA changes in responses to short-term Al treatment may not be responsible for Al tolerance. However, increased OA synthesis observed in this study may be involved in diminishing the stress triggered by Al. The molecular aspects underlying Al resistance mechanism via Al-induced expression of the enzymes catalyzing OA synthesis and metabolism remain to be elucidated.
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Acknowledgments
This paper is dedicated to Professor Brahim Marzouk (Biotechnology Center, Borj Cedria Technopark, Hammam-Lif, Tunisia) on the occasion of his retirement, in recognition of his significant contribution to plant biochemistry and physiology. We greatly appreciate the significant contribution of Dr. Sami Ben Jamaa during writing of the manuscript, and Mrs Najeh Galai for her technical assistance. This work was financially supported by the Ministry of Higher Education, Scientific Research, and Technology of Tunisia.
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Chaffai, R., Marzouk, B. The role of organic acids in the short- and long-term aluminum tolerance in maize seedlings (Zea mays L.). Acta Physiol Plant 31, 805–814 (2009). https://doi.org/10.1007/s11738-009-0295-2
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DOI: https://doi.org/10.1007/s11738-009-0295-2