As more variable rainfall patterns consolidate into recurring yield drags for the grains industry, researchers are making progress endowing bread wheat with greater resilience to dry terminal conditions.
There is no simple genetic solution, however. Years of research worldwide have found that genes and traits conferring 'drought resistance' can often introduce yield penalties in the financially crucial good years.
Instead, impressive progress building the required resilience into cereals is being made using strategies that allow for greater genetic complexity.
One such breakthrough has been achieved by Dr Jack Christopher and his team of the Queensland Alliance of Agriculture and Food Innovation (QAAFI) - with GRDC, University of Queensland (UQ) and Queensland Government investment over six years.
The team has developed an accelerated technique to knead many subtle genetic differences into the genomes of elite wheat varieties.
"The process retains the elite characteristics of popular wheat varieties, but multiple genetic differences are introduced by this technique that enhance resilience and yields under water stress and with no concurrent yield penalty in good years," Dr Christopher says.
A focus on selection
The team targeted three popular Australian bread wheat varieties - Mace(PBR), Scout(PBR) and Suntop(PBR) - that are adapted to growing conditions in western, southern and northern growing regions respectively.
The process retains the elite characteristics of popular wheat varieties, but multiple genetic differences are introduced by this technique that enhance resilience and yields under water stress and with no concurrent yield penalty in good years.
Dr Christopher says some of the best lines from the resulting germplasm stands a greater than 85 per cent chance of out-yielding the three parent varieties in dry years (affected by terminal drought), while creating no yield drag in the financially important - but increasingly rare - good years.
In the environment where Dr Christopher works at the Leslie Research Centre in Toowoomba, in the northern grains region, terminal water stress is a serious constraint in 70 to 80 per cent of all years.
"With good years making up just 10 to 12 per cent of the total, the pressure increases to earn more from the majority of seasons when crops are water-stressed," he says.
"The better lines amongst the germplasm we developed were able to do that when field tested at 21 sites across the western, southern and northern growing regions and uptake by commercial breeders means that the lead lines and their progeny could start reaching growers in the near future."
Anchoring the genetic innovation was the prior discovery of wheat genotypes that yield 10 to 20 per cent more under water-stressed conditions in the northern region, where crops often rely heavily on water stored in deep cracking clay soils from summer-dominant rainfall.
The plants had a noticeable difference that contributed to the yield increase.
The leaves delayed dying-off at the end of the season, staying green for longer - which allows the plant to better set seed and fill the grain under dry conditions.
"We reasoned that if the plants can stay green despite the common lack of in-season rainfall, then they have to be getting water from somewhere," Dr Christopher says.
"We hypothesised they must have more roots at depth later in the season in order to get at moisture deeper in the soil profile."
We reasoned that if the plants can stay green despite the common lack of in-season rainfall, then they have to be getting water from somewhere.
To characterise the roots, wheat plants were grown to maturity in large root chambers with clear sides that allowed the imaging of root architecture.
Imaging of the root architecture showed they have a narrower overall spread, with less roots to the side but a greater proportion of the roots directly underneath the plant and growing deep into the soil.
"Overall, the plants are producing about the same root length and biomass, but placing them directly under the plant, not to the sides," Dr Christopher says.
Narrower overall root architecture is also associated with narrow seedling root angle - with the pattern allowing wheat to get more water later in the northern season when conversion of water to grain is very efficient.
The 'stay-green' leaves and narrow root phenotypes were each underpinned by a number of contributing traits.
Each trait was also found to be under complex genetic control, making selection of these characteristics during a standard breeding program exceptionally difficult.
For example, root architecture can be underpinned by:
- narrow seedling root angle;
- increased seedling root number;
- the ability to grow more roots at depth late in the season after flowering;
- or a combination of these - and possibly other - traits.
Adding to the genetic complexity was the discovery that in addition to various traits contributing to root architecture and 'stay-green', these phenotypes also share some common sub-traits.
"We were faced with a large number of genetic regions that need to be co-inherited or combined to achieve the most favourable phenotype," Dr Christopher says.
"That is a hard target to breed for, so we developed an alternative approach called a multiple reference parent (MR) nested association mapping (NAM) population, which we abbreviate to MR-NAM population."
Making this population involved crossing each reference parent - Mace(PBR), Scout(PBR) or Suntop(PBR) - in such a way as to integrate genetics from 12 additional parents that served as the donors of root architecture and 'stay-green' traits.
The output is three interrelated populations of about 500 lines - one for each of the major cropping regions - that express various mixes of root and 'stay-green' traits. The overall material of more than 1500 lines forms the MR-NAM population.
Genetically, that means that at any one place in the genome, there are potentially 15 different genetic variants possible in the overall MR-NAM population.
"That level of genetic diversity makes it much more amenable when it comes to breeding for complex traits," Dr Christopher says.
Facilitating the intensive pre-breeding work was QAAFI's speed-breeding facilities that allowed the development (and in-breeding) of genetically fixed MR-NAM lines in just 18 months.
The MR-NAM population was initially screened using a high-throughput method that measures seedling root angle and number in clear-sided pots.
The 'stay-green' traits were screened in the field in a wide range of environments using a hand held GreenSeeker®. This is a sensor that was adapted from the WeedSeeker® technology used on spraying rigs and exploits the ability to detect the greenness of weeds for selectively spraying herbicide.
The GreenSeeker® was used to measure the level of greenness of the crop canopy of the MR-NAM population in the field.
Repeated measurements from before flowering through to maturity provided a measure of the timing and rate of canopy senescence (leaf browning).
The best performing stay-green lines delayed the onset of leaf death the longest. This is one of the most important 'stay-green' traits in wheat.
"We found that the time between flowering and the onset of senescence varied between the lines," Dr Christopher says.
"The best performing 'stay-green' lines delayed the onset of leaf death the longest. This is one of the most important 'stay-green' traits in wheat."
A yield gain of 1 to 2 per cent was observed for every extra day that leaves stayed green under water stress.
The best performing 27 lines from the MR-NAM population were released for evaluation to breeders in 2017 in all of the major wheat growing regions.
Some of those lines have now progressed to being used as parents in breeding crosses.
Dr Christopher has also set his sights on making additional gain by crossing the lines with the favourable traits within each of the three reference NAM populations to combine traits and increase the yield benefits.
This additional material has been developed since 2016 and a subset made available for testing by commercial breeders during the 2019 season.
In addition, a statistical method has been developed that can grapple with the complex genetics of the root and 'stay-green' traits within the interrelated genetic structure of the MR-NAM population.
That work was undertaken by the GRDC's Statistics for the Australian Grains Industry (SAGI) program and has identified clusters of DNA markers associated with various contributing traits.
The markers could ultimately liberate commercial breeders from relying on phenotyping methods and services provided by Dr Christopher's team when selecting for 'stay-green'.
"The statistics teams also analysed yield performance in 21 different field environments to identify highly adaptable lines that do well in all environments and have a high probability of consistently exceeding the yield of the three original starting varieties," Dr Christopher says.
While the project has now been completed, the MR-NAM population can potentially continue to produce benefits as it constitutes a new platform for making rapid gain relative to harsh growing conditions.
"Moving forward, the MR-NAM population provides a gateway for the rapid transfer of additional traits into this elite material and into new elite cultivars as they come on line," Dr Christopher says.
"Already, researchers at QAAFI are setting their sights on donors that can add into the MR-NAM population enhanced heat tolerance and water use efficiency through improved transpiration efficiency."
More information: Jack Christopher, QAAFI, firstname.lastname@example.org