Ten thousand years ago, somewhere in the ‘fertile crescent’ near modern day Turkey, several small but amazing events kick-started the spread of farming, the birth of civilisation and ultimately changed the world.
Although we are still learning about the precise nature of these events, we know that at this time people began to collect seeds from local wild grasses to grow them for food, selecting the best seeds to grow in subsequent seasons. During this process of selection and cultivation the wild grasses cross-bred, or hybridised, leading to domesticated forms of ancient wheat such as einkorn and emmer. Selection and cultivation continued, giving rise to both modern bread wheat and durum wheat, used for making pasta and couscous. Wheat is now the most cultivated crop in the world and forms the staple food for 35% of the world’s population. However, thousands of years of repeated selection and crossing to obtain the best yields and quality has significantly narrowed wheat’s gene pool.
For a team of Australian researchers looking at the problem of salinity tolerance in durum wheat, the solution was clear: look at the ancestors and wild relatives of modern wheats for tolerance to salt and re-introduce these genes into modern wheat lines.
“It was some pretty big thinking about 15 years ago by our collaborators at CSIRO that started this work,” says Dr Matthew Gilliham of the University of Adelaide and the ARC Centre for Plant Energy Biology. Matthew is senior author on a paper recently published in Nature Biotechnology announcing the development of a line of durum wheat which is salt tolerant under commercial farming conditions.
Salinity affects over 20% of the world’s agricultural land and is a major issue in Australia’s prime wheat-growing areas, with nearly 70% of this land susceptible to salinity. “Through the years, wheat has lost genetic diversity for things such as tolerance to harsh environmental conditions. That’s why we need to go back in time, get some genes from wild relatives and ancestors that grow in these harsh conditions and cross them back in.”
To find genes for salt tolerance, researchers from Australia’s CSIRO looked at Triticum monococcum, also known as einkorn. It is not a direct ancestor of bread wheat or durum, but it is closely related to the grasses that were, and it still grows in some parts of the world today. It can also grow in salty soil.
When the initial crosses between durum and the T. monococcum were made using traditional plant breeding methods, whole pieces of chromosomes containing thousands of genes were introduced. More years of crossing and selection were needed to reduce the number of genes from the T. monococcum in the durum lines and by 2009, researchers were trialling durum wheat lines with increased tolerance to salinity. But what where the genes and how did they work?
In salty soils, sodium ions from salt enter wheat plants via the roots. From there they enter the plant’s water-transport system from where they can be taken to the leaves. “The hypothesis we were working on is that salinity tolerance in cereal crops, especially wheat, is related to the ability to exclude sodium ions from the leaves. If you build up sodium levels in leaf cells you start to inhibit essential life processes like photosynthesis, so wheats that exclude salt from their leaves grow better in salty soils” explained Matthew.
“Our group, including researchers from the Australian Centre for Plant Functional Genomics, used a range of molecular and physiological tests to work out that the important gene in this story was the sodium transporter gene TmHKT1;5-A. We worked out where the gene was turned on, and what it did. This gene makes a protein that acts as a sodium selective transporter, which prevents the sodium from entering the shoots by filtering it out at the root level, it essentially turns the roots into a sodium selective sponge. Compared to the shoots, the build up of sodium in root cells does not inhibit cellular metabolism very much at all.”
Although the understanding of the function of the sodium transporter involved transgenic (genetic modification) techniques, the introduction of the genes into the durum lines did not, meaning that the lines of wheat could be tested under commercial conditions without going through Australia’s strict regulatory framework for genetically modified organisms.
The durum line was trialled on a variety of field sites across Southern Australia including a commercial farm near Moree in northern New South Wales, These trials were led by CSIRO researchers Richard James and Rana Munns. Farmers in this area usually harvest about 2.5 tonnes per hectare, a typical and profitable yield for broad-acre, rain-fed (non-irrigated) cropping in semi-arid areas.
However, like many farms in the grain producing areas of Australia, salinity is beginning to affect yields. On this farm, a commercial durum variety and the line with the introduced sodium transporter genes had the same performance on normal soil. But at the highest salinity level, the new line outperformed the commercial variety by approximately 25%. This means farmers can use varieties developed from the improved line across their farms, even in paddocks only partly affected by salinity with a significant yield advantage over the current varieties.
“Our research is the first to show that sodium exclusion genes increase grain yield in the field” said Matthew, which is why the group’s work is attracting a lot of attention, including publication in the prestigious Nature Biotechnology. But the team’s work is not over yet. They have already identified other genes from ancient relatives that may be useful in improving salinity tolerance further, highlighting the huge potential for improving modern wheat using the diversity already present in nature. “There are other aspects to the salt- tolerance story and more genes to identify and characterise” adds Matthew. “We haven’t solved the problem, we have just put one piece back in the puzzle.”