Australia
February 15, 2011
The scourge of salinity has long been associated with the shocking images of wide stretches of farming country turned white from salt.
What is less recognised is that this spectacular damage is not the greatest threat salinity poses to Australia’s grain crops – it is the low lying salts hidden in the water table beneath the soil surface which are doing the greatest damage.
“Salinity is a big problem nationally, but a lot of it is not the spectacular stuff like you see in places such as Western Australia where there are localised highly saline patches – that’s a land management issue and there’s good people doing good work on that,” said Professor Mark Tester, from the Australian Centre for Plant Functional Genomics at the University of Adelaide and Director of the Australian Plant Phenomics Facility.
“Where there’s a bit of salt in the soil and the yield is getting clipped back a bit - this is a very widespread problem.
“It’s estimated about 70 per cent of the wheat crop around Australia has its yield pegged back by maybe 10pc.”
And, unlike the salt scourges in WA, it’s not huge amounts of salt doing the damage. Prof. Tester says just “modest amounts” – salinity at just one-tenth the strength of sea-water is all that’s needed to start reducing plant yield.
But not all wheat varieties are affected in the same way. Prof. Tester and his team are working their way through the problem of what makes a wheat plant tick, what parts of the plant are active in resisting salt, which parts are susceptible, and which genes trigger those behaviours.
“What we’ve been doing is trying to understand the traits in plants that can contribute to maintaining yields in saline environments,” Prof. Tester said.
“The yield that is being nipped back over a large area of land is because of salt that is down in the subsoil – you can’t see it at the surface. That kind of subsoil salinity is a classic target for crop improvement, trying to improve the ability of plants to withstand that erosion to its yields.”
It’s not a problem exclusive to Australian conditions – salinity affects approximately 5pc of the world’s cultivated land and represents a major limitation to agricultural production at a time when primary production needs to lift its output to meet the globe’s growing population.
However, great leaps forward have been made in addressing the problem in recent years, with scientists around the world improving their understanding of which genes are responsible for the reaction of various crop types to salt pressures.
Here in Australia, Prof. Tester’s work is being supported by the Grains Research and Development Corporation (GRDC), which is investing in pre-breeding research to discover novel genes and deliver germplasm with improved salinity tolerance to plant breeding companies.
These companies can then breed new and better varieties of wheat seed for sale to farmers, who can then grow higher-yielding crops on salt-affected lands.
It sounds like a simple proposition, especially in an era of gene markers and new plant monitoring technology, but the reality is highly complicated.
There are three components of a wheat plant that can help the plant maintain growth in saline soils: its ability to keep salt out of its central shoot; the ability of the shoot to tolerate the salt that does enter (tissue tolerance); and the ability of the plant to handle the negative impact salt has on water availability in the soil (osmotic tolerance).
For the past decade Tester and his team have been working on the first element in trying to identify which genes govern a plant’s ability to exclude salt from its shoot.
Prof. Tester’s team has been investigating the HKT group of genes, which control sodium influx into cells, and some of which are located around the xylem (the pipes that move water through the plant).
In effect, the genes govern the ability of the xylem to suck the salt out of the water before it reaches the leaves.
“It’s like the final purification of water before it gets to the leaves,” Prof. Tester said.
Researchers at the CSIRO have located two of these genes in a single durum line, which provides big breeding possibilities given that most durum lines are very sensitive to salt.
And while the CSIRO has been crossing these genes into durum and bread wheats, Tester’s team has been further investigating the genes’ mechanism of action.
Through the use of transgenic techniques, the researchers are deliberately over-expressing the HKT genes around the xylem vessels to increase the variation beyond what is found naturally.
“The aim is make plants more tolerant by taking a beneficial trait and increasing its influence,” Tester said.
This approach can result in what scientists call the “deleterious pleiotropic effect” – where the increased expression of one positive gene trait triggers a corresponding expression of negative traits – Prof. Tester says that in this instance the other genes activated by the HKT gene group were consistent with the needs of the plant.
The research has found that over-expression of the HKT group led to improved sodium exclusion (up to 37pc less salt in the shoot) and salinity tolerance, with the excess sodium excluded from the shoot stored in the cortical cells of the root.
But more importantly to farmers, plants carrying the HKT gene have been found to result in seedling biomass that is 10pc higher under saline conditions than lines not carrying the genes.
Two intervals in the wheat genome that include members of the HKT group – on the 2A and 6A chromosomes – have now been associated with that result, with 2A the most promising for further breeding work in bread wheats.
But Prof. Tester has not invested all of the industry hopes in just one project, with other areas of investigation also showing promising results.
“I like running quite a lot of programs in parallel and then jumping on the winners – which is a strategic way of operating,” he said.
Some promising results have also been achieved in field trials following the identification of an influential area on chromosome 1 of barley known as HvNax3.
Due to their similarities, the location of key barley genes can act as a guide to the location of similar genes in wheat varieties.
However, the challenge for researchers is that the HvNax3 interval contains hundreds of genes.
“Within that is one particular gene that in model systems has shown to influence salinity tolerance,” Prof. Tester said.
“There are loads of genes that we don’t know the function of, a whole pile of mystery genes, so it might be one of them.
“But we’ve spotted in that list of genes down the end of the chromosome an old friend that has already been published in model systems. So we decided to concentrate on this one.”
So far, trials have shown that HvNax3 reduced shoot sodium accumulation by 10-25 per cent in plants grown in one-third seawater.
Interestingly though, the researchers are not certain of how the gene works, with Prof. Tester speculating that it might help the plant lock salt away in the roots, preventing it from rising up to the shoots.
“We’re still not certain of the actual mechanistic basis for that trait,” he said. “But if the field trials that we were doing this year confirm what we found in the previous year, then there are possibly big effects on yield from this particular chromosome interval.
“But I want to see those results come in for a couple of years yet before getting really excited about it.”
The gene group has been bred into a number of recombinant crosses, involving the South Australian barley cultivar Barque, and if the next round of field trials replicate the early success in bolstering yield in saline conditions, Prof. Tester says numerous opportunities will ensue for identifying the differences between salt-tolerant and insensitive lines.
In the long-term Prof. Tester believes that numerous traits for salinity tolerance will be stacked into wheat varieties to dramatically improve performance.
“While we’re delivering those sodium exclusion discoveries into the farmers’ fields, in parallel with that we’ve got to start making other discoveries on other aspects of salinity tolerance,” he said.
“The obvious ones we’re chasing are osmotic tolerance and the tissue tolerance so we can pile these things on top of one another.
“The more information you have and the more useful it is, the more accurately we can alter traits out in the field and deliver more benefits to the farmer.”
More information on salinity is available at the GRDC website, www.grdc.com.au
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