Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2006
Publication Date: 4/28/2006
Citation: Liao, H.S., Wang, X.S., Shaff, J., Wan, H., Yan, X., Kochian, L.V. 2006. Phosphorus and aluminum interactions in soybean in relation to al tolerance: exudation of specific organic acids from different regions of the intact root system. Plant Physiology. 141:674-684. Interpretive Summary: Large areas of land within the U.S. and over 40% of the world’s arable lands are acidic. In these acid soils, aluminum (Al) toxicity and phosphorous (P) deficiency both limit crop production. To date, there have not been studies looking at the interactions of Al tolerance and P nutrition for crop genotypes differing in acid soil tolerance. In this study, we employed 2 acid soil tolerant/P efficient and 2 acid soil sensitive/P inefficient soybean lines. A divided root chamber containing nutrient solutions was also used to mimic the real world acid soil conditions of elevated P levels in the simulated upper soil horizon, and Al toxicity/P deficiency in the lower horizon. We found there were significant interactions between P nutrition and Al tolerance, particularly in the more acid soil tolerant, P efficient soybean lines. When higher levels of P were provided to the more shallow lateral roots, and more acidic, Al toxic solutions were provided to the more deeply situated tap root, it was found that the more tolerant genotypes exhibited a stimulated tap root Al tolerance. These findings show, for the first time, that plants have evolved specialized physiological roles for different roots within an entire root system, such that the more shallow roots can more efficiently obtain the scarcely available P from the upper soil horizons, which improves P nutrition for the more deeply situated roots. These more deeply situated tap roots, in turn, then exhibit enhanced tolerance to Al toxicity and thus can grow into the acidic subsoils to provide more water and nutrients for the plant.
Technical Abstract: Aluminum (Al) toxicity and phosphorus (P) deficiency often coexist in acid soils to severely limit crop growth and production. Understanding the mechanisms underlying plant Al and P interactions is necessary to facilitate the development of acid tolerant crops. Here we studied the effects of Al and P interactions on soybean Al tolerance, P efficiency, and root organic acid exudation. In homogenous nutrient solutions, P addition significantly increased Al tolerance, particularly in 2 P efficient soybean genotypes, compared with two P-inefficient genotypes. Analysis of root exudates indicated that Al induced citrate exudation, P deficiency triggered oxalate exudation, and malate release was induced by both treatments. To more closely mimic low P acid soils, a divided root chamber/nutrient solution approach was employed to impose elevated P conditions in the simulated upper soil horizon, and Al toxicity/P deficiency in the lower horizon. Under these conditions, we found that the two P efficient genotypes were more Al tolerant during the early stages of the experiment than the P inefficient lines. Although the same three organic acids were exuded by roots in the divided chamber experiments, their exudation patterns were very different from those in the homogeneous solution system. The two P-efficient genotypes secreted more malate and citrate from the tap root tip, suggesting that improved P nutrition may enhance exudation of organic acids in the root regions dealing with the greatest Al toxicity, thus enhancing Al tolerance. These findings demonstrate for the first time that P efficiency may play a role in Al tolerance in soybean through stimulated exudation of different Al-chelating organic acids in specific roots and root regions.