Scientists One Step Closer to Developing Crops that Thrive in Problematic Saline Soils

A team of researchers from the University of Adelaide in Australia has developed a new approach in developing plants that can thrive in saline soils, bringing salt-tolerant crops a step closer to reality.

A farmer tends to her rice field in Vietnam

The Earth is one salty planet. About 70 percent of its surface is covered with water, and more than 95 percent of that water contains 35 grams of sodium chloride per liter. The accumulation of this salt in cultivated fields has been a problem since the beginning of agriculture. While irrigation has made it possible to extend agriculture to semi-arid and arid areas of land, it has also resulted in large-scale water logging and salinity. Evaporation of irrigated water leaves behind salt which accumulates over time. Land degradation due to increased salinity presently affects more than 20% of all irrigated land in at least 100 countries.

High soil salinity negatively affects the growth of many crops. Salt, for example, decreases the availability of water in the soil. Accumulation of excess salt ions in plant cells is also fatal. These ions can impair the activity of plant enzymes, inhibit photosynthesis and damage the cell membrane. Development of crop varieties resistant to salinity is an important strategy to sustain food production in many parts of the world.

Recently, a team of researchers from the Australian Center for Plant Functional Genomics (ACPFG) and the University of Adelaide's School of Agriculture, Food and Wine, developed salt-tolerant plants using a novel approach, bringing salt-tolerant crops a step closer to reality.

Their work appears in the current issue of the journal Plant Cell.

"More than 800 million hectares of land throughout the world are salt affected," said Mark Tester, leader of the study and professor at the University of Adelaide. "This amount accounts for more than 6 percent of the world's total land area."

Tester and colleagues focused on a transporter, a membrane-embedded protein that moves ions in and out of the plant cell. This particular transporter, called HKT1;1, mediates salinity tolerance by retrieving sodium ions (Na⁺) from the transpiration stream, therefore reducing the levels of Na⁺ in the shoot. The gene that codes for the transporter is particularly expressed around the plant's water conducting pipes. Mutants that lack this gene were found to be salt sensitive.

They developed Arabidopsis plants that over-express HKT1;1 in the pericycle and vascular bundle of the stele of mature roots. Na⁺ transport was monitored using radiolabeled sodium (²²Na⁺).

The scientists found that over-expression of HKT1;1 in the stele reduced sodium accumulation in shoot by up to 64 percent. By contrast, they found that plants constitutively expressing the gene accumulated high levels of sodium in the shoot and grew poorly. When grown in a medium supplied with 100 mM of NaCl, the transgenic plants that over-express HKT1;1 in stelar root cells continued to thrive, whereas their non-transgenic counterparts as well as plants that constitutively express the transporter gene exhibited signs of salt stress.

The study demonstrates that manipulating transport processes in specific plant cells may be more effective in modifying the accumulation of solutes in the plant than manipulating these processes indiscriminately.

According to Tester, the same approach has been used to increase nitrogen use efficiency of crops. "We have also successfully used this approach to increase the delivery of iron and zinc to the endosperm of rice grains," says Tester. "I think it could also be used to increase the efficiency of phytoremediation."

The team is now in the process of applying this approach to develop salt-tolerant cereal crops.

"We appear to have been successful in rice and field trials are the next step. We would be pleased to move this into other crops, such as millet and wheat, where there is a clear agronomic advantage. Key is identifying the correct promoters to control the gene expression."

Tester explains that although there is natural variation for the stelar expression of these HKT1 subfamily of genes in cereals, which will confer some level of salinity tolerance by non-transgenic means, the extent of the alterations will always be limited by the natural variation available.

"This is limited or absent in some species, and could always be increased, too," says Tester. "So there is likely to be room for a GM approach in many situations."

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Møller, I.S., Gilliham, M., Jha, D., Mayo, G.M., Roy, S.J., Coates, J.C., Haseloff, J., and Tester, M. (2009). Shoot Na⁺ exclusion and increased salinity tolerance engineered by cell type-specific alteration of Na⁺ transport in Arabidopsis. Plant Cell http://dx.doi.org/10.1105/tpc.108.064568 (Open Access Paper)

Munns, R. and Tester, M. (2008). Mechanisms of Salinity Tolerance. Annu. Rev. Plant Biol. 59:651–81. http://dx.doi.org/10.1146/annurev.arplant.59.032607.092911

For more information, read the Pocket K on Biotechnology with Salinity for Coping in Problem Soils at http://www.isaaa.org/kc/inforesources/publications/pocketk/default.html#Pocket_K_No._31.htm