Acid soils significantly limit crop production worldwide because approximately 50% of the world's potentially arable soils are acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring tolerance to acid soil stress has been a focus of intense research interest over the past decade. The primary limitations on acid soils are toxic levels of aluminum (Al) and manganese (Mn), as well as suboptimal levels of phosphorous (P). This review examines our current understanding of the physiological, genetic, and molecular basis for crop Al tolerance, as well as reviews the emerging area of P efficiency, which involves the genetically based ability of some crop genotypes to tolerate P deficiency stress on acid soils. These are interesting times for this field because researchers are on the verge of identifying some of the genes that confer Al tolerance in crop plants; these discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of these tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving crop Al tolerance via both molecular-assisted breeding and biotechnology.

How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorous efficiency.

Fast root growth responses, root exudates, and internal detoxification as clues to the mechanisms of aluminium toxicity and resistance: a review

Aluminum (Al) toxicity is the primary factor limiting crop production on acidic soils (pH values of 5 or below), and because 50% of the world’s potentially arable lands are acidic, Al toxicity is a very important limitation to worldwide crop production. This review examines our current understanding of mechanisms of Al toxicity, as well as the physiological, genetic and molecular basis for Al resistance. Al resistance can be achieved by mechanisms that facilitate Al exclusion from the root apex (Al exclusion) and/or by mechanisms that confer the ability of plants to tolerate Al in the plant symplasm (Al tolerance). Compelling evidence has been presented in the literature for a resistance mechanism based on exclusion of Al due to Al-activated carboxylate release from the growing root tip. More recently, researchers have provided support for an additional Al-resistance mechanism involving internal detoxification of Al with carboxylate ligands (deprotonated organic acids) and the sequestration of the Al-carboxylate complexes in the vacuole. This is a field that is entering a phase of new discovery, as researchers are on the verge of identifying some of the genes that contribute to Al resistance in plants. The identification and characterization of Al resistance genes will not only greatly advance our understanding of Al-resistance mechanisms, but more importantly, will be the source of new molecular resources that researchers will use to develop improved crops better suited for cultivation on acid soils.

https://pubag.nal.usda.gov/download/15460/pdf

The physiology, genetics and molecular biology of plant aluminum resistance and toxicity.

Trace element behaviour at the root-soil interface: Implications in phytoremediation

Aluminum (Al) is the most abundant metal in the earth's crust, while its soluble ionic form (Al(3+)) shows phytotoxicity, which is characterized by a rapid inhibition of root elongation. Aluminum targets multiple cellular sites by binding, resulting in disrupted structure and/or functions of the cell wall, plasma membrane, signal transduction pathway, and Ca homeostasis. On the other hand, some plant species have evolved mechanisms to cope with Al toxicity both externally and internally. The well-documented mechanisms for external detoxification of Al include the release of organic acid anions from roots and alkalination of the rhizosphere. Genes encoding transporters for Al-induced secretion of organic acid anions have been identified and characterized. Recent studies show that ABC transporters are involved in Al resistance. The internal detoxification of Al in Al-accumulating plants is achieved by the formation of nontoxic Al complexes with organic acids or other chelators and sequestration of these complexes in the vacuoles. In some plant species, Al shows beneficial effects on plant growth under particular conditions, although the exact mechanisms for these effects are unknown.

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Syndrome of aluminum toxicity and diversity of aluminum resistance in higher plants.

It was reported that differential Al tolerance between two cultivars of taro (Colocasia esculenta [L.] Schott) did not correspond to the amount of oxalic acid in Al medium (Ma and Miyasaka 1998: Plant Physiol., 118, 861-865). We compared the amount of organic acids, especially, citric and malic acids, in Al media of randomly selected seven plant species (AI tolerance order: Brachiaria brizantha, rice, tea > maize > pea, Cassia tora L. > barley) and of two Al-tolerant and AI-sensitive cultivars each in five plant species (rice, maize, wheat, pea, and sorghum). A larger amount of citric acid was found in the Al medium than in the medium without Al in all the plants except for Brachiaria brizantha and the AI-sensitive cultivar of pea. The largest amount of citric acid was detected in the Al medium of C. tora among the plant species, but its growth was inhibited considerably by both short- and long-term treatments with Al. In maize, wheat, and pea, a larger amount of citric and malic acids was detect...

https://www.tandfonline.com/doi/pdf/10.1080/00380768.2000.10409141

Ryouichi Sasaki

Papers

Comparison of the amount of citric and malic acids in Al media of seven plant species and two cultivars each in five plant species