'Redesigned' wheat roots to be driven deeper to tap late season water

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‘Redesigned’ wheat roots to be driven deeper to tap late season water
Irrespective of whether conventional, transgenic or genomic technology is used, when it comes to water productivity all breeders face ‘the phenotyping problem’ – the need to assess plant traits for yield lifts under field conditions. Now a new India–Australia collaboration is providing the chance to do just that … with wheat roots the target
By Dr Gio Braidotti

Key points
• Wheat needs deeper roots to reach late-season moisture
• Drought tolerance must mean yield gain, not just plant survival
• High expectations for India–Australia collaboration

When it comes to breeding wheat better able to cope with water scarcity, there is one particular problem that has been taunting agricultural scientists for years. Just as crops are about to flower and set seed, residual water in the soil is frequently just out of reach of the deepest roots of current wheat varieties.
Breeders know that if they could change the roots’ ‘architecture’ they could make that water available for grain production.
Until recently the insurmountable problem was the technical complexity of measuring and assessing – or ‘phenotyping’ – the impact on yield of different types of root systems.
From 2009, however, a collaboration brokered by the Australian Centre for International Agricultural Research (ACIAR) is allowing Australian and Indian scientists to pool resources in an unprecedented push to crack the phenotyping problem as it affects water scarcity in dryland wheat farming systems.
Taking part from Australia are Dr Richard Richards’s CSIRO Plant Industry team, with Dr Michelle Watt serving as project leader for the ACIAR project.
The team ranks among the world’s most successful at developing phenotyping technology to make gains in water productivity. Their work has resulted in the development of wheat lines used by commercial breeders in Australia – most recently in the LongReach Scoutpbrlogo variety – and overseas, including at the International Maize and Wheat Improvement Center (CIMMYT) in Mexico.
Dr Richards says the phenotyping problem forms the main bottleneck for breeders dealing with complex traits such as drought tolerance. The problem recurs irrespective of the breeding strategy adopted – whether conventional, transgenic or genomic.
“Ultimately all these strategies need to characterise their germplasm under real field conditions and ensure they achieve yield gains as opposed to gains in the ability of the plant to merely survive,” Dr Richards says.
In India, matching funds are being provided by the Indian Council of Agricultural Research to support researchers at the Directorate of Wheat Research, the Indian Agricultural Research Institute and the Agharkar Research Institute.
The Indian collaborators have field sites that are ideal for water productivity studies. They are located in the central and peninsular states, where wheat is grown entirely on soil-stored moisture acquired during the monsoon. With no rainfall to confuse the study, the conditions are ideal to screen for variation in root architecture in wheat germplasm and evaluate the impact on yield.
“A deep-root project on this scale has never before been attempted,” Dr Watt says. “That is the great thing about this collaboration with India – they have the best field conditions to correlate root architecture with water productivity and CSIRO has the phenotyping expertise. And both countries hold some really interesting germplasm. Together we can realise the potential of these resources.”
During the 2010 season and for several years to come, hundreds of wheat lines – including a wide range of elite wheat varieties, Indian landraces and CSIRO germplasm – are being sown at the Indian sites where researchers intend to core the soil to a depth of two metres to physically measure the root system.
Dr Watt estimates that roots need to grow just 10 centimetres deeper by the time wheat is flowering to make an extra 10 millimetres of water available. “We have calculated that gain in root depth can contribute an extra half a tonne of grain per hectare,” Dr Watt says. “So this trait has very high water productivity – a high conversion of water into yield.”
While the project is relying on coring the soil to make progress, CSIRO intends to use the Indian sites to also test whether far simpler ways of phenotyping root traits are possible. On trial is technology such as thermal cameras to measure above-ground attributes, such as leaf temperature or stomatal conductance – traits that are affected by the root’s ability to make water available to leaves.
This kind of technology has the potential to radically simplify root improvement work, Dr Watt says. However, it is not the only way CSIRO is exploiting upper-plant traits to make genetic gain underground.
Dr Richards explains that CSIRO has developed three above-ground water productivity traits for use in Australian farming systems that are now suspected to also beneficially influence root architecture. The three traits involve:
• new dwarfing genes better suited to dryland farming;
• early shoot vigour, including longer coleoptiles; and
• reduced partitioning of carbohydrates to unproductive tillers through the use of the tiller inhibition (tin) gene.
“We found that the replacement dwarfing genes and high-vigour traits also promote below-ground vigour, while reduced tillering promotes more root branching in addition to larger ears and larger grains,” Dr Richards says.
“So rather than start from scratch with the root work, we can use the knowledge we have gained from improving above-ground water productivity and apply these traits strategically to influence root growth. All three traits are heading for India for extra testing.” 

Influencing root growth from above

While the Green Revolution dwarfing genes have been enormously important, Dr Richards says they were developed primarily for wheat grown under irrigation. The resulting varieties can be problematic in a lot of dry environments and were not adopted at all in Canada, North Africa and dry parts of India.
In Australia, the dwarfing genes were adopted primarily because they provided a yield advantage in favourable seasons. However, these genes cause the production of smaller cells and, as a result, smaller leaves and shorter coleoptiles. This can act like a brake on early growth, which compromises a seedling’s ability to establish in unfavourable dryland conditions.
“Older farmers who saw the transition noticed these changes and kept asking why we couldn’t go back to the old varieties growing in the 1950s and 1960s,” he says. “It is a regular comment from the old-timers.”
In a bid to capture the best of both worlds, CSIRO has identified a new class of dwarfing genes.
The CSIRO genes retain the benefits of short stature without reducing early vigour or coleoptile length. This allows wheat to establish more easily in dry conditions, even where there is stubble retention. Field trials to date indicate the new genes work well in good years and even better in poor seasons.
The team originally identified 10 such genes and the three most promising have been released to Australian breeding companies.
“In our experience, farmers really like this trait,” Dr Richards says. “Especially if they have problems with poor establishment, with seedlings not getting through stubble. Given that 97 per cent of wheat production in Australia is dryland, I think this trait could be important over the entire country and I hope to see the first commercial success within a few years.”
The replacement dwarfing genes are also needed to realise the potential of a second CSIRO trait, which targets the southern and western wheat-growing regions. This involves an early vigour trait that delivers a 50 per cent increase in coleoptile length (from about 7cm to 12cm) and a doubling in early leaf growth. This is a water-productivity trait that CSIRO found was being antagonised by the Green Revolution dwarfing genes and release of the trait to breeders was postponed until the genes could be replaced.
Dr Richards says there are numerous benefits to growers from early vigour.
Longer coleoptiles allow seed to be sown deeper and earlier, strategies that help crops avoid drought; they help improve establishment of crops sown with airseeders; and they make it easier for the shoot to penetrate through stubble. Early shoot vigour then results in bigger leaves, which provides ground cover that better conserves soil moisture as well as providing additional benefits for nutrient capture and improved competition against weeds. And below ground, CSIRO is finding that root growth too appears to occur with extra vigour, a finding that is being further tested in India.
A different strategy was required for the northern growing region, however, since early vigour can be a disadvantage if too much water is used up too quickly.
Instead, CSIRO spotted a valuable trait (the tin gene) in a North African wild landrace wheat that halves the number of tillers produced by a crop. Its use is allowing plants to redirect energy resources in the second half of the season from making unproductive tillers to growing more roots, larger ears and larger grain. It too, along with the other traits, is headed for India as part of the root-improvement project.
“The extra length of roots needed is amazingly little – just 10cm – so we don’t need a lot of genetic gain to make a big difference,” Dr Richards says. “It has proven elusive in the past but given the coordinated approach of this project and the interesting Australian and Indian germplasm being tested, the potential is there to make plants far more effective in terms of using available water to grow more grain.”
The good news for Australian growers is that germplasm with these traits is being delivered to all Australian wheat-breeding companies for the development of superior varieties.

GRDC Research Code CSP00057
More information: Dr Richard Richards, richard.richards@csiro.au

pbrlogo Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.

GRDC Project Code CSP00057