Acid rich soils are generally not considered healthy for plant growth as they lack nutrition and are rich in mineral toxins. Plants that are not adapted to grow in acid soils when grown in them will possess only small root system due to the inhibition of root elongation by high concentrations of aluminium. Therefore, the plants will not be able to access water and nutrients much. Plants have different capacities in tolerating acid soils. Many studies on the genetics and physiology that can control this variation were conducted. The mechanisms and the genes responsible for controlling aluminium resistance in mutants and segregating populations were analyzed in the species of Arabidopsis, sorghum, rice, barley and wheat. It is understood that organic anion release from the roots is an indication of plant resistance development.

Aluminium resistance is found to be governed by genes that belong to two different gene families which encode for transport proteins. Enough data is currently available for increasing aluminium resistance of crop species with the help of biotechnology. This study focuses on the progress made in the research related to aluminium resistance in plants, the genes that are controlling relevant processes and the genetic engineering approaches to increase aluminium resistance in crop plants.

Function of aluminium in soil

The growth of food crops is affected if the soil pH is less than 5.5. Aluminium is usually present in the soil as silicates and oxides. These compounds dissolve in the acid soil easily and release aluminium. At low pH, trivalent aluminium exists abundantly while in high pH, bivalent forms exist more. Gibbsite, a trivalent aluminium can exist in high pH as well. Trivalent aluminium is observed as toxic to the plants.

Soluble trivalent aluminium at very low concentrations has the ability to block the growth of roots. There are some exceptions where the plants adapt to acid soils and aluminium in the soil stimulates plant growth by accumulating in the leaves. Plant growth is inhibited at lower concentrations of trivalent aluminium, while at its high concentrations there was no effect observed. Many plants accumulate aluminium in the apoplast, which prevents the solute flow and affects the function of the membrane by attaching to the proteins and lipids of the membrane. High aluminium concentrations in apoplast are known to stimulate production of callose. Aluminium will block the nutrient uptake by inhibiting the ion channels that are associated with potassium and calcium influx.

Aluminium resistance
High concentrations of aluminium accumulate in the roots and leaves of some plants while some of the members of the Triticeae are resistant to aluminium. The plants sensitive to aluminium retain this mineral at the root apices, which is 10 times greater than the resistant variety. The resistance genes are mapped and cloned from a group of species classifying the resistance genes into three categories. The resistance genes isolated by analyzing segregating populations explaining the genotypic variation. The second type is identified by mutant analysis and it does not explain genotypic variation. The third type are those genes, which need additional backup information.
The genes lying behind the aluminium resistance are isolated and these can be used for generating transgenic plants having increased aluminium resistance. The organic anion transporter genes of the root are the aluminium resistance genes that can be isolated from plants. The first aluminium resistance gene isolated was a wheat gene called TaALMT1 which is involved in malate transport activated by aluminium. The promoter of TaALMT1 showing polymorphisms is associated with aluminium resistance.

The second gene, MATE or multi-drug and toxicity extrusion gene that is involved in aluminium resistance was cloned in sorghum. The sbMATE gene encodes citrate transport protein. This protein is made to express in the root apices of resistant sorghum plant which increases the citrate transport. The coding sequence of sbMATE gene in resistant and sensitive plants was almost same with a small change in the intron region. Other genes of aluminium resistance are HvAACT1, AtMATE, AtALMT1, ZmMATE1, TaMATE1. But, the ability of aluminium resistance for the genes TaMATE1, BnALMT1 and BnALMT2 have to be investigated yet.

By mutant analysis, two genes in rice called STAR1 and STAR2 were found to create plant sensitivity to aluminium toxicity, when they are removed. These two genes are known to create a functional ATP binding cassette transporter which is localized in the vesicles of the root cells. The STAR1 and 2 genes involvement in aluminium resistance is not clear and it might be associated with the released substances that modify the cell wall at the time of aluminium stress.

It was established long back that organic anion efflux is correlated with aluminium resistance. The homozygous tobacco plants expressing citrate synthase gene could accumulate citrate 10 times higher than that of the wild type plants. The synthesis of citrate will lead to the transport of this organic anion from the cytosol, across plasma membrane increasing aluminium resistance. The citrate efflux in transgenic plants was increased by four times than the wild type, in turn increasing the aluminium resistance. Enhanced aluminium resistance was reported in transgenic papaya, tobacco, Arabidopsis and alfalfa. In transgenic tobacco, aluminium resistance was about 4.5 times higher than the wild type, when the rice CS gene was expressed in it, while the increase was just marginal in all other transgenic plants.

Gaofeng Zhou, Emmanuel Delhaize, Meixue Zhou and Peter R Ryan. Biotechnological solutions for enhancing the aluminium resistance of crop plants.

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