Overcoming the difficulty of breeding drought tolerant wheat
Conjure up the image of drought in your mind. You’re probably seeing dry, barren earth with the mirage-like shimmer of baking temperatures in the distance.
Unfortunately for CSIRO plant geneticist Dr Rudy Dolferus, hot and dry conditions are only half the challenge when creating new varieties of drought tolerant wheat.
“Wheat is a winter crop and faces a variety of environmental challenges to produce grain for farmers,” says Dr Dolferus.
“When flowering early, the crop has enough soil moisture to produce grain but faces the prospect of being frosted. When flowering later, frost damage can be avoided but soil moisture is depleted, so heat and drought stress affect grain yield,” he says.
“Achieve a trifecta of resilience against drier soils, heat and frost, and you have true drought tolerance.”
Dr Dolferus is working on a project funded by the Grains Research and Development Corporation (GRDC) that is looking to breed drought resilience into Australian crops like wheat, which is worth some $6 billion annually to the Australian economy.
To attain that goal, he’s having to contend with a complex interplay between genetics and environmental factors. However, as Australia faces arguably its worst ever drought conditions, the good news is that advances in genetics mean that pre-breeding trials are underway with new lines of drought tolerant wheat currently being assessed.
Changing conditions face genetic hurdles
In recent years, winter rainfall has been significantly lower and drought stress has moved to earlier in the season. Erratic changes in the climate have caused frost events to occur later in the season, while droughts and heat spells have started to occur earlier.
With climate change modelling showing increasingly common periods of drought, farmers don’t have time to wait for their plants to adapt either.
One challenge with breeding wheat is its sheer genetic complexity. The wheat genome is five times larger than the human genome, and while it has been mapped, it hasn’t been fully understood.
“The response of plants to environmental stresses such as drought is controlled by large gene networks of redundant genes, and unravelling the genetic function of these networks is challenging.”
As researchers like Dr Dolferus put familial lines of wheat through their paces, it has become clear that the endless race for higher yields come harvest time may have come at a cost. Many varieties have a spectacular ability to grow and produce high yields in good conditions, but their environmental resilience is low.
“Domesticated varieties have lost something in the way they adapt to adverse environmental change, and we’re looking backwards to find an answer,” he says.
“There are ancestral varieties, as well as old wheat varieties that are more resilient that we can go back to, and I’m working on a line from 1973 as my current candidate. Older lines are lower yielding, but more resilient.”
Breakthrough in gene marker development
The difficulty of breeding drought tolerance into crops like wheat is two-fold.
Firstly, taking plants out of a stable and safe greenhouse setting for field trials exposes them to a plethora of different variables.
From different soils, to varying inputs like fertiliser, exposure to pests and diseases, and the very environmental factors they are being tested against – a lack of moisture, varying heat and frost. So understanding what exactly caused a crop to succeed or fail (or if they would conversely fail or succeed in other conditions) is incredibly hard to pinpoint.
Secondly, going back to the laboratory and identifying what genes are responsible for what studied traits has also been difficult. The complexity of interacting genes involved in response to drought and other stresses means that it cannot be pinpointed to a single responsible gene.
However, it’s here that Dr Dolferus has found a clue. Plants that are higher yielding under water stress conditions are better at maintaining growth and that is reflected in the fact that they keep their stomata (which are like skin pores in humans) open longer when stressed during flowering.
Sensitive plants close those stomata quickly under stress and stop growing. Stomata control transpiration of water (the uptake of water from the soil) and photosynthesis (the conversion of energy), and thereby regulate growth and grain production.
“Simply put, we’ve been looking for plants that don’t do that. Some are resilient and keep developing grain, but others fail,” he says.
With a reference genome in the Chinese Spring wheat variety, Dr Dolferus can sequence the genes of his varieties and individual plants of interest and compare them to the reference genome.
That identifies genetic differences between tolerant and sensitive lines, which has given the researchers targets to place genetic markers around, allowing marker-assisted breeding to take place. This knowledge can also be applied to other crops like canola and chickpeas, which also suffer in drought from the same stressors.
Crops in the ground
If it sounds like it takes a long time for this breeding process to occur, it’s because it does. It can take over 10 years for a new variety to reach the market. That’s not too dissimilar from the journey required for a human drug trial to leave the lab and arrive on the pharmacy shelf.
However, these genetic advances are offering traditional breeding techniques a helping hand. And while GMO techniques would allow for quicker editing of genes, it wouldn’t solve the initial problem of identifying targets, nor testing them in real world conditions to validate their suitability.
Dr Dolferus now has crops that have been planted and are being harvested at the New South Wales Department of Primary Industries’ Yanco research site, undergoing evaluation before crop breeders further tweak varieties for commercial release.
The difficulty has been to show that wheat lines with known drought tolerance and sensitivity (from controlled environment work) are performing the same way in the field where they are also challenged by other stresses such as frost and heat.
Predictably, some wheat lines have succeeded, while others have failed. However, this all aids the further identification of genes that are either favourable or unfavourable for drought tolerance. This process will need to be repeated in other environments where frost or heat stress are also part of the equation, and progress is being made on how to analyse field data for the combined effect of different stresses.
However, the limits of how far crops can be bred to grow in a water-limited drought environment – while still providing a profitable yield – are unknown.
“A question needs to be asked whether the industry would be happier to hedge its bets with varieties that are hardier and more resilient, but which produce lower yields. If conditions continue to worsen, they may have no choice,” he says.
“However, we’re successfully working towards a solution and future-proofing the Australian cropping industry.”