Towards an understanding of salt tolerance in plants
| Date: 09 Feb 2011
One of the major issues affecting Australian agriculture is increasing salinity of highly productive land. Soil salinity refers to a high concentration of salts in the landscape, mainly common salt or sodium chloride (NaCl), which has a negative impact on soil and water quality. Salinity in the landscape occurs naturally and indirectly as a result of agricultural irrigation and/or land clearing. These latter activities can lead to rising groundwater levels, which carries salt into the root zone of crop and pasture species (Munns & Tester, 2008).
Salinity leads to sodium (Na) accumulation to toxic levels in plant cells, and is a major limitation to plant growth. The sodium Na+ ions enter the root cells passively via non-selective cation channels, and move with water into the cells, eventually reaching the xylem (small ‘pipes’ that conduct water and salts from the roots to the shoots). The Na+ ions are then transported throughout the plant and eventually deposit in shoots and leaves, where they accumulate to toxic levels (Munns & Tester, 2008).
At a molecular level, plant cells maintain safe intracellular sodium concentrations with sodium-proton antiporters. These are special ‘sodium pumps’ – large protein molecules that can remove Na+ ions from the cell by exchanging them with H+ ions. The pumps are located either in the outer cell membranes (plasma membrane) and result in the removal of the Na+ ions from the cells, or in an internal intracellular membrane, resulting in Na+ immobilization or sequestration (Hasegawa et al., 2000).
There are two closely related sodium pumps, the proteins NHX5 and NHX6, which act together to control cellular ion levels and plant development. A related protein in the test plant Arabidopsis, AtNHX1, has a very well-characterised role in salt tolerance. Also there is a closely related protein from tomato, which has been shown to increase salt tolerance (Rodríguez-Rosales et al., 2008). While the role of AtNHX5 and AtNHX6 in salt tolerance is not so clear, recent work in the Gendall laboratory at La Trobe University has indicated that AtNHX5 and AtNHX6 may be involved in salt tolerance of plant cells.
We expect that NHX proteins will have a role in producing transgenic plants with enhanced tolerance to saline environments. However it will be critical to understand the mechanisms which regulate their activity and the protein interactions critical to their functions. My PhD project will investigate the feedback regulation of the NXH ion pumps, and determine the functional effect of NXH6 upregulation in nhx5 mutant plants under stressed and non-stressed conditions. Work will also be undertaken to understand the downstream (MEANING) effects of NHX activity to ensure their safe use in transgenic plants.
My project will continue the identification and development of other novel genes that are vital for salt tolerance in Arabidopsis, and by extension to other major cropping species. However, it appears likely that the AtNHX5 and AtNHX6 protein pumps will have a significant role in increasing salt tolerance of crops. A deeper understanding of the mechanisms underlying this salt tolerance will be critical to the use of NHX proteins in producing transgenic salt-tolerant crop species. They will allow saline lands to be cultivated productively while a long term solution to remediating saline lands is implemented.
References:
Hasegawa, P. M., Bressan, R. A., Zhu, J. K. & Bohnert, H. J. 2000. PLANT CELLULAR AND MOLECULAR RESPONSES TO HIGH SALINITY. Annu Rev Plant Physiol Plant Mol Biol, 51, 463-499.
Munns, R. & Tester, M. 2008. Mechanisms of salinity tolerance. Annu Rev Plant Biol, 59, 651-81.
Rodríguez-Rosales, M. P., Jiang, X., Gálvez, F. J., Aranda, M. N., Cubero, B. & Venema, K. 2008. Overexpression of the tomato K+/H+ antiporter LeNHX2 confers salt tolerance by improving potassium compartmentalization. New Phytologist, 179, 366-377.
Contact details
Joanne Ernest, La Trobe University
jrernest@students.latrobe.edu.au
03 94792221
Salinity leads to sodium (Na) accumulation to toxic levels in plant cells, and is a major limitation to plant growth. The sodium Na+ ions enter the root cells passively via non-selective cation channels, and move with water into the cells, eventually reaching the xylem (small ‘pipes’ that conduct water and salts from the roots to the shoots). The Na+ ions are then transported throughout the plant and eventually deposit in shoots and leaves, where they accumulate to toxic levels (Munns & Tester, 2008).
At a molecular level, plant cells maintain safe intracellular sodium concentrations with sodium-proton antiporters. These are special ‘sodium pumps’ – large protein molecules that can remove Na+ ions from the cell by exchanging them with H+ ions. The pumps are located either in the outer cell membranes (plasma membrane) and result in the removal of the Na+ ions from the cells, or in an internal intracellular membrane, resulting in Na+ immobilization or sequestration (Hasegawa et al., 2000).
There are two closely related sodium pumps, the proteins NHX5 and NHX6, which act together to control cellular ion levels and plant development. A related protein in the test plant Arabidopsis, AtNHX1, has a very well-characterised role in salt tolerance. Also there is a closely related protein from tomato, which has been shown to increase salt tolerance (Rodríguez-Rosales et al., 2008). While the role of AtNHX5 and AtNHX6 in salt tolerance is not so clear, recent work in the Gendall laboratory at La Trobe University has indicated that AtNHX5 and AtNHX6 may be involved in salt tolerance of plant cells.
We expect that NHX proteins will have a role in producing transgenic plants with enhanced tolerance to saline environments. However it will be critical to understand the mechanisms which regulate their activity and the protein interactions critical to their functions. My PhD project will investigate the feedback regulation of the NXH ion pumps, and determine the functional effect of NXH6 upregulation in nhx5 mutant plants under stressed and non-stressed conditions. Work will also be undertaken to understand the downstream (MEANING) effects of NHX activity to ensure their safe use in transgenic plants.
My project will continue the identification and development of other novel genes that are vital for salt tolerance in Arabidopsis, and by extension to other major cropping species. However, it appears likely that the AtNHX5 and AtNHX6 protein pumps will have a significant role in increasing salt tolerance of crops. A deeper understanding of the mechanisms underlying this salt tolerance will be critical to the use of NHX proteins in producing transgenic salt-tolerant crop species. They will allow saline lands to be cultivated productively while a long term solution to remediating saline lands is implemented.
References:
Hasegawa, P. M., Bressan, R. A., Zhu, J. K. & Bohnert, H. J. 2000. PLANT CELLULAR AND MOLECULAR RESPONSES TO HIGH SALINITY. Annu Rev Plant Physiol Plant Mol Biol, 51, 463-499.
Munns, R. & Tester, M. 2008. Mechanisms of salinity tolerance. Annu Rev Plant Biol, 59, 651-81.
Rodríguez-Rosales, M. P., Jiang, X., Gálvez, F. J., Aranda, M. N., Cubero, B. & Venema, K. 2008. Overexpression of the tomato K+/H+ antiporter LeNHX2 confers salt tolerance by improving potassium compartmentalization. New Phytologist, 179, 366-377.
Contact details
Joanne Ernest, La Trobe University
jrernest@students.latrobe.edu.au
03 94792221
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