By Katharina Schoebi, Checkbiotech

Acidic soil is a notable limiting factor with regards to plant yields, because it inhibits root growth, which in return reduces the uptake of water and nutrients. By genetically engineering barley, researchers have now been able to make it more tolerant to an acidic environment. =

About 40 percent of the earth’s arable land is covered with acidic soils. Plant yields are seriously diminished on these soils, mainly because aluminum is solubilized by the acidity to Al3+. This form of aluminum is toxic to plants and inhibits the growth of roots in acid soils and, by so doing, it decreases the uptake of water and nutrients.

In some cases, plant yields in acidic soils can be maintained by using lime. However, it often takes decades until lime has corrected the acidity of the soil. Another possibility is to use aluminum-tolerant plant species, unfortunately many of the important crop species are not sufficiently aluminum tolerant.

Aluminum tolerant plants are thought to use a mechanism by which the presence of Al3+ activates the release of organic acid anions, such as malate, from the roots that neutralize the toxicity of aluminum.

This phenomenon has been well examined on different wheat lines (Triticum aestivum). Close examination showed that the malate secretion was larger in tolerant lines than in the aluminum sensitive lines. The “malate hypothesis” proposes that the secreted malate binds aluminum in the soil and converts it into a non-toxic form, thus protecting roots from damage.

In 1997, Dr Ryan from the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Canberra collaborated with Professor Tyerman from Adelaide University to show that aluminum activates a channel that is permeable to malate in the cells at the root tips, often referred to by plant scientists as the apex. Seven years later, a team at Okayama University lead by Professor Matsumoto identified a gene that they called ALMT1 and collaborated with Dr Ryan and Dr Delhaize at CSIRO to show that ALMT1 was responsible for the aluminum-activated secretion of malate. On this account, they suggested that ALMT1 would encode the aluminum activated channel.

The groups based at CSIRO and Okayama University have now shown that ALMT1 is capable of conferring aluminum tolerance to barley (Hordeum vulgare). Barley is an economically important crop and does not normally secrete malate in the presence of aluminum and is therefore very sensitive to aluminum. The researchers published their results in the journal Proceedings of the National Academy of Science (PNAS).

In their experiments, the researchers introduced ALMT1 in barley plants via Agrobacterium tumefaciens, a widely used bacterium, which allows for the transfer of new genetic material into a plant. From their results, it was possible to deduce that the expression level of aluminum in transgenic barley lines was comparable with that of aluminum tolerant wheat. They also observed a secretion of malate in the root of transformed plants, whereas plants that were not genetically modified (GM) did not secrete malate.

The researchers determined aluminum tolerance by measuring the elongation of the roots of plants that were grown in acidic soils containing aluminum (Al³⁺). All transgenic barley lines with the ALMT1 gene showed good root growth, even at aluminum concentrations that severely inhibited roots of non-transformed plants. Their work also showed that the roots were unaffected by the aluminum, whereas the root tips of the control plants were severely damaged and malformed.

When the researchers planted the barley in acid soils, the genetically engineered plants exhibited better root growth than the non-modified ones. The next stage of their work is to find out how the plants perform with respect to grain yield, preferably in field trials.

However, in Australia, strict regulations regarding GMO field trials and applications take at least nine months for approval. One of the researcher’s major concerns at the moment is to obtain funding to support field trials.

Dr. Delhaize told Checkbiotech, “I would estimate that field trials could be two or three years away if we obtain financial backing” and indicated that the first trials would be carried out in greenhouses.

The team’s work demonstrated that a single gene is needed to confer aluminum tolerance to an important agricultural crop such as barley allowing farmers more options to manage acid soils.

Although the work has focussed on barley, it could be duplicated in other crops. “We believe the gene has potential in other plant species that are sensitive to aluminium, such as rape (Brassica napus),” Dr. Delhaize told Checkbiotech.

An advantage of ALMT1 in comparison to genes that defend against diseases or provide insect resistance, tolerance to aluminum is not subject to being overcome by mutations in the attacking organism.

However, repeated use of aluminium tolerant plants without neutralizing the acidity of the soil could intensify the acidity to a point where high concentrations of toxic aluminum may ultimately overcome the protection conferred by genes such as ALMT1.

Consequently, an effective management of acidic soils combines the use of lime and aluminium tolerant plants and this will provide the most sustainable solution. With this approach, normally sensitive crops could be grown successfully in acidic soils, the researchers emphasized.

A major concern for the researchers is the need to educate consumers about the benefits that GM technology has to offer to the environment and to society alike.

Katharina Schoebi is a biologist and a Science Writer for Checkbiotech.

Selected publications:
Peter Ryan, et al. (1997) Aluminum activates an anion channel in the apical cells of wheat roots. Proc. Natl. Acad. Sci. 94: 6547-6552
Takayuki Sasaki, et al. (2004) A wheat gene encoding an aluminium-activated malate transporter. Plant Journal 37: 645-653.
Emmanuel Delhaize et al. Engineering high-level aluminum tolerance in barley with the ALMT1 gene. Proceedings of the National Academy of Science. 101 (2004) pp. 15249-15254

Link:
http://www.pnas.org/cgi/reprint/101/42/15249