How salt tolerance works
Salt-tolerant plants possess the ability to either tolerate high levels
of sodium or to exclude sodium from their system. This is achieved by
either preventing sodium from entering the water transport system (xylem)
and being taken to the leaves, or by isolating the sodium in special vacuoles
within the leaves where it is not able to interfere with metabolic processes
such as photosynthesis.
Cereal crops differ in their salt tolerance. For example, barley is naturally
more salt tolerant than wheat because it can exclude salt from its
leaves by storing it in vacuoles. Modern bread wheat is naturally more
salt tolerant than durum wheat because it has a gene that helps exclude
sodium from the xylem system, which stops it from reaching the leaves.
The absence of this gene in durum wheat means it takes up about five times
more sodium than its bread wheat cousins. This sodium enters the leaves and
interferes with photosynthesis and grain fill.
Research is now underway to assess the ability of the salt-exclusion gene from
Triticum monococcum to enhance the natural salt tolerance of bread wheat and
extend the arable land on which bread wheats can be produced.
Australian researchers have also identified other genes from ancient cereal
relatives that could prove useful in further improving the salinity tolerance
of modern crops.
World-renowned crop physiologist Professor Rana Munns, from CSIRO, has topped off a 40-year research career with the discovery and successful field-testing of a gene conferring salt tolerance in wheat.
It is the first time that salt-exclusion genes have been shown to increase grain yield in the field. The findings have significant potential for durum wheat production on saline land in Australia and internationally.
“Our results pave the way for growers to have profitable wheat options on saline land that might otherwise be left unsown or used for less valuable crops,” Professor Munns says.
She presented the results of the GRDC-funded research at the 2013 Hector and Andrew Stewart Memorial Public Lecture at the University of Western Australia in April.
CSIRO scientists, in collaboration with the durum breeder Dr Ray Hare (retired) from the New South Wales Department of Primary Industries, detected the presence of a salt-exclusion gene in an ancient strain of wheat more than a decade ago. It has taken all the intervening years to unravel how the gene works and to breed it successfully into wheat lines for field-testing. Collaboration with colleagues at the University of Adelaide and the Australian Centre for Plant Functional Genomics has also been critical to the success of the project.
The gene effectively turns the roots into a sodium-selective sponge, preventing too much sodium from reaching the wheat’s shoots.
“Excessive sodium in the leaves and shoots interferes with photosynthesis and inhibits the production of carbohydrates for grain fill,” Professor Munns says. However, a build up of sodium in root cells does not inhibit cellular metabolism significantly.
She says the gene allows just enough sodium into the durum leaves and shoots to maintain their osmotic balance and keep them turgid and fully functioning.
To find genes for salt tolerance, the CSIRO researchers examined an ancient cousin of modern durum and bread wheats, Triticum monococcum, which still grows in some parts of the world today and can also grow in saline soil.
Using conventional breeding methods, a gene from T. monococcum was inserted into the commercial durum wheat cultivar Tamaroi, which was then grown under commercial conditions in northern NSW.
The Tamaroi durum line containing the salt-exclusion gene yielded 25 per cent more on salt-affected areas than the same line without the gene. Similar yield results were obtained in South Australian trials conducted by Dr Tony Rathjen, a durum breeder at the University of Adelaide.
CSIRO researchers Professor Rana Munns and Dr
Richard James inspect a durum wheat line containing
a salt-exclusion gene that enables it to yield 25 per
cent more on salt-affected areas than the same line
without the gene.
PHOTO: Carl Davies
Significantly, there was no yield penalty incurred by the salt-exclusion gene on non-saline soil. Yields of the Tamaroi line with the gene were the same or higher than the Tamaroi line without the gene (about 2.5 tonnes per hectare).
“As most salt-affected fields are not uniformly saline, it is important that any introduced gene for salt exclusion brings no penalty on the best soils as most of the grain yield comes from these higher-producing areas,” Professor Munns says.
With significant potential to lift grain yields on saline land worldwide, the pre-breeding research has stimulated strong interest among national and international plant breeding companies.
Since concluding the research, Professor Munns and her CSIRO colleagues have provided all Australian wheat breeders and 20 international breeders from countries including the US, India, Pakistan, Bangladesh, Tunisia and Argentina with durum and bread wheat lines containing the salt-tolerant gene.
“Commercial release of our salt-tolerant durum lines now lies in the hands of the breeders,” Professor Munns says.
Professor Rana Munns,
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