- Boron is a vital soil nutrient for plants but Australian soils can expose wheat to either too much or too little boron
- An Australian wheat-gene discovery now makes it possible to match wheat genetics with local boron levels
Hostile soils have been challenging Australian wheat growing since European settlement. But the advent of the molecular era is finally giving plant breeders molecular tools to efficiently develop varieties that are truly, productively, at home in local soils
Australian soils can differ markedly between being low in boron, mainly in the northern growing region, and excessively high, generally in the south. Having to confront both extremes has long been a challenge for Australian wheat breeders given that adaptation to soil boron levels is important for achieving yield potential.
Boron imbalance symptoms
Both a deficiency and excess of boron can cause nutritional imbalances in wheat. Deficiency typically occurs in highly leached soils in wetter climates. Excessive boron is more common in lower-rainfall regions where limited leaching and shallow soils result in boron accumulation in the subsoil.
- primarily affects pollen development and pollination; and
- sterility can occur without foliar symptoms.
- affects the growth of all plant tissue at all stages of development, particularly the roots leading to root stunting; and
- symptoms show up as regions of chlorosis and necrosis developing from the tips and along the margins of the oldest leaves.
Plants need small quantities of boron to grow, but if too little is available it can lead to symptoms as extreme as plant sterility. At the other end of the scale, excess boron can stunt the growth of plant tissue – particularly the roots – at all stages of development.
Australian plant researchers are well aware of the existence in Australian paddocks of these conflicting extremes and have been on a 30-year quest to better match a cultivar’s genetics to local soil conditions.
This journey recently culminated in an internationally celebrated gene discovery that helps to explain why southern wheat varieties generally do poorly in the north. The discovery also provides new tools to ensure future cultivars can be efficiently bred to thrive whatever the local boron conditions.
Leading that gene discovery, through a joint GRDC and Australian Research Council initiative, the Australian Centre for Plant Functional Genomics (ACPFG) in Adelaide, is the centre’s deputy CEO, Dr Tim Sutton. He and his team of molecular sleuths – Margie Pallotta, Thorsten Schnurbusch, Julie Hayes, Alison Hay and Ute Baumann – were able to dig deep into the wheat genome and isolate the gene that controls how much boron a plant absorbs and retains.
And just as important as identifying the main form of the boron-tolerance gene was the subsequent characterisation of variant forms (with the variants referred to as ‘alleles’ rather than ‘genes’).
The team that searched through the wheat genome to identify the boron-tolerance gene examine wheat plants in a glasshouse at the Plant Accelerator, Waite Campus, University of Adelaide. (Clockwise from centre back) Peter Langridge, Jeff Paull, Margaret Pallotta, Julie Hayes, Alison Hay, Ute Baumann and Tim Sutton (absent: Thorsten Schnurbusch).
PHOTO: Amanda Hudswell
These allelic variants were found to modulate the level of boron tolerance that a cultivar expresses, which now means molecular markers are available that can distinguish between wheat germplasm that has strong, intermediate or weak boron tolerance.
More broadly, the feat brought home to the world that the wheat genome has finally succumbed to the molecular era; its genome is now accessible to gene-based analysis despite its complexity, convoluted genealogy and massive size.
“Finding the gene was a huge job but provides an example of the feasibility of applying gene discovery technology to the wheat genome for key adaptive traits,” Dr Sutton says.
“What helped was access to the resources of the International Wheat Genome Sequencing Consortium, of which the ACPFG is a member. Those resources are paving the way for a new era of wheat genetics and accelerating the search for key adaptive traits to common stresses.”
The gene discovery also made it possible to survey the genetic identity of Australian wheat cultivars – what allele of the boron tolerance gene they carry – and shed light on factors that drive adaptation to regionally different Australian soils.
The difference in root growth between boron-tolerant lines of wheat (left) and boron-intolerant lines of wheat when grown in boron toxic soils.
PHOTO: Courtesy of Nature magazine
The survey confirmed that a strong boron-tolerance allele is already present in Australian wheat varieties, derived from an early Victorian cultivar Currawa (released in 1912) and carried through to the famous South Australian variety Halberd, which dominated SA wheat production in the 1970s and 1980s.
“Halberd is highly adapted to our environments in SA and we know that boron tolerance is one of the key factors behind its stellar adaptation,” Dr Sutton says.
The impact on yields of adaptation to high boron soils was identified in 2012 by a GRDC-funded team led by Dr Glenn McDonald of the University of Adelaide. The team analysed data from more than 230 trials at 68 locations over 12 years and found that boron-tolerant genotypes have up to a 15 per cent yield advantage over intolerant genotypes in southern growing regions where boron is excessive.
“That’s the magnitude of the effect these boron-tolerance alleles can have on adaptation to local environments in the south,” Dr Sutton says.
However, because Currawa’s boron-tolerance allele is now already prevalent in wheat cropped in the south, the celebrity gene discovery is unlikely to add to the yield advantage already achieved.
“Clearly boron toxicity has been a factor that has influenced breeders for many years as they have already selected for the boron-tolerance variant best suited to the southern environment,” Dr Sutton says.
“But what the gene discovery can do is allow breeders to use molecular marker technologies and preferentially select lines containing a boron-tolerance allele that is best suited to different cropping regions.”
A breeders’ ‘toolkit’ containing diagnostic markers that can distinguish functionally important alleles of the boron-tolerance gene with 100 per cent accuracy is now under development in collaboration with the GRDC-funded Australian Wheat and Barley Molecular Marker Program (UA00143), led by Professor Diane Mather from the University of Adelaide.
“Tracing the ancestry of the boron-tolerance gene proved fascinating, showing that the gene originally came through to bread wheat from a cross with a durum landrace,” Dr Sutton says.
“The new molecular tools made it possible to track the boron-tolerance gene to the Mediterranean, in a region known as the Fertile Crescent (the historically fertile deltas around the Nile, Tigris and Euphrates rivers).
However, wheat grown in Australia (and the Americas) is based mostly on temperate, northern-European material and it was problems with climatic adaptation that led breeders back to landraces from environments similar to ours, such as the Middle East and Mediterranean.
“That shows wheat breeding is an international effort, and the importance for some traits of matching functionally different forms of a gene to different agronomic environments.”
The gene findings also partially explain a longstanding observation about growing wheat in Australia, namely that varieties adapted to the southern growing region, such as Halberd, do poorly in northern environments.
Dr Sutton explains that while boron levels are generally high in southern soils, the element tends to be present at much lower concentrations in the north. Wheat carrying the strong form of the boron-tolerance gene (which is common in southern varieties) continues to exclude the element from the cell even if the plant is becoming boron deficient because of low soil concentrations. This means southern-adapted wheat cultivars are inadvertently exacerbating the boron deficiency when grown in northern soils.
Another interesting outcome has been the boron team’s collaboration with experts on the quality defect late-maturity alpha-amylase (LMA) – Dr Daryl Mares and Dr Kolumbina Mrva from the University of Adelaide (see Diagnostic advances help shut down costly wheat defect).
The boron-tolerance gene in Halberd was found to be close to a major gene for resistance to the low falling number caused by LMA. This means that in Halberd-type lines, the two traits are almost always co-inherited.
For regions with low soil boron, a seemingly unrelated selection for non-LMA in Halberd-type material would in fact run a high risk of also carrying over a strong boron-tolerance gene and all the disadvantages that this can cause in boron-deficient soils.
Fortunately, Dr Sutton says there are other sources of resistance to LMA that are not associated with boron tolerance and which breeders can use to develop material suited to all grain-growing regions.
“Identification of the genes controlling LMA in wheat is something ACPFG has been working towards with Daryl’s group. One of the spin-offs from the fine mapping which we needed to do to find the boron-tolerance gene is that we now have resources to look for genes involved in resistance to LMA.”
Current GRDC funding, through projects UA00133 and UA00150 led by Dr Mares, is supporting this work going forward.
Dr Tim Sutton,
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GRDC Project Code
UA00143, UA00133, UA00150