World unites to crack the cereals yield ceiling

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Key points

  • So efficient was the Green Revolution strategy for improving grain yields that gains from the key yield traits are now exhausted
  • International agricultural research centres are coordinating the investment of more than US$100 million to develop a new yield-improving trait
  • Australia, through the GRDC and Australian discoveries, is playing a key role in this international ‘yield consortium’ 
Image of farm equipment

Plant-analysing sensors developed in Australia and mounted on automated platforms are being used to analyse the photosynthetic efficiency of crop varieties in paddocks and potted plants.

PHOTO: CSIRO

A global push is on to re-engineer photosynthesis to deliver the next generation of yield gains for cereals

A global research investment of more than US$100 million (A$136.6 million) will underpin what some have dubbed the next Green Revolution to engineer plant photosynthesis and achieve the next breakthrough in grain yields.

This follows a consensus among researchers at all of the major international agricultural research centres that yield gains from the plant trait most closely associated with increased grain productivity over the past 50 years – the harvest index – have been largely exhausted.

New technology and breeding strategies are now needed to break through the yield ceiling imposed by current varieties and knowledge.

Included in this potentially historic push are two international consortia – the International Wheat Yield Partnership (IWYP) and the C4 Rice Consortium. These are seeking new physio-chemical efficiencies in the way crops use sunlight to fix carbon dioxide (CO2), since photosynthesis is currently the main limiting factor in biomass production and yield potential.

The new trait of interest is called radiation use efficiency (RUE) and builds on important discoveries made by Australian scientists, some starting in the 1960s. The task facing researchers is considered urgent given that a doubling of grain (and livestock) production is needed by 2050 to meet food demand from a global population of nine billion people.

The yield ceiling

Image of Professor Bob Furbank

Professor Bob Furbank pictured at the High Resolution Plant Phenomics Centre, CSIRO. He is leading an international push to ramp up plant photosynthesis to break through the yield ceiling.

PHOTO: Brad Collis

Taking part in these international research efforts is the Australian Research Council (ARC) Centre of Excellence for Translational Photosynthesis. At this centre Professor Robert Furbank – also a key figure in the creation of the C4 Rice Consortium – leads a collaborative research program between the Australian National University, the University of Queensland, CSIRO and the International Rice Research Institute (IRRI) that includes a CSIRO-based project on wheat funded by the GRDC.

Professor Furbank explains that past gains in yield potential – including famine-averting gains made during the Green Revolution – were primarily based on increasing harvest index (or the amount of carbon fixed by the plant that is partitioned into grain).

“However, starting about a decade ago, rice breeders at IRRI realised that annual gains in harvest index had been exhausted,” Professor Furbank says.

“The same now seems to be holding true for wheat. Since this has been our major breeding strategy in rice and wheat, such a stagnation in yield progress means we can’t currently meet the 2050 targets for increased cereal production.”

A crucial limiting factor has been identified. It is RUE or the efficiency with which sunlight is used to convert CO2 into biomass.

“Photosynthesis and RUE are currently the fundamental bottlenecks to raising productivity and RUE has barely changed in the modern breeding era, mainly because it is difficult to measure and breed for,” Professor Furbank says.

“However, theoretical considerations suggest that wheat yield potential, for example, could be increased by up to 50 per cent through the genetic improvement of RUE.”

The yield-related consortia were convened to develop a portfolio of activities through a network of global research organisations to increase RUE in ways that maximise benefits in growers’ fields.

C3 C4 explained

Graphic of photosynthesis

Figure 1 Differences in how plants photosynthesise – the C3 and C4 pathways.

Professor Furbank explains that RUE is not fixed by the laws of physics, but varies dramatically within and between plant species. The reason is that evolution revisited photosynthesis several times over the eons, tweaking the nature of the molecular engine that underlies light-driven sugar synthesis.

Wheat, beans and rice are C3 plants, which tend to thrive where sunlight intensity and temperature are moderate. From an evolutionary perspective they predate C4 plants and make up 95 per cent of the Earth’s plant biomass.

The more recently evolved C4 plants have a competitive advantage under drought and hot conditions, such as the tropics. They are able to concentrate CO2 within specialised chambers in the leaves so that carbon can be fixed more efficiently. This same mechanism also helps prevent water loss from the apertures (or ‘stomata’) that leaves open up whenever they need to draw in CO2 from the atmosphere.

As a result, C4 plants are responsible for 30 per cent of CO2 fixed by plants but constitute just five per cent of the Earth’s plant biomass. Present-day C4 plants include food crops such as maize, sugarcane, millet and sorghum, which have an average RUE up to 50 per cent greater than C3 plants.

The photosynthetic efficiency of C4 plants is especially attractive to scientists, both from a yield and water use efficiency (WUE) point of view.

When grown in the same environment (at 30°C), C3 grasses lose about 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277. The increased WUE of C4 grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.

“The C4 pathway was discovered in Australia in 1966 by Marshall Davidson Hatch and C. R. Slack and is sometimes called the Hatch–Slack pathway,” Professor Furbank says.

Australian excellence in photosynthesis research continues, with Australian science routinely producing among the most highly cited research papers in the world on this important subject, including work by Professor Furbank.

“As we approach the 50-year anniversary of the discovery of C4 photosynthesis, globally there is a major push to make C3 cereals more C4-like in terms of their RUE.”

Strategies include improving the performance of the key enzyme complex responsible for carbon fixation (RuBisCo), introduction of C4-like traits such as CO2-concentrating mechanisms, improvement of light interception, and improvement of photosynthesis at the spike and the whole canopy.

However, for these strategies to translate into agronomically viable extra yield, parallel improvements are needed in a crop’s structure and reproduction. For example, adequate partitioning among plant organs is needed to ensure a favourable harvest index and ensure plants with heavier grain have strong enough stems and roots to avoid lodging. These considerations are part of the RUE research initiatives.

Local know-how

Understandably, Australians are contributing (and benefiting) from the international RUE initiative, including in leadership and advisory roles.

The ARC Centre for Translational Photosynthesis – where A$26 million is being invested over seven years – is coordinating Australian research efforts. It has four major programs underway that draw on the expertise of many research organisations, including CSIRO, the High Resolution Plant Phenomics Centre, the Australian National University, the University of Sydney, the University of Western Sydney, the University of Queensland and IRRI.

At one extreme, the centre is involved in the C4 Rice Consortium, which is using genetic modification technology to mimic evolution and convert the molecular machinery in rice from a C3 to a C4 configuration. This is a massively ambitious, 15-year, US$25 million (A$34 million) project supported by the Bill & Melinda Gates Foundation.

At the other extreme, more incremental gains are being sought that can deliver benefits in the paddock much more quickly. Professor Furbank heads the centre’s activities in this domain to ‘mine’ natural genetic variation in RUE in rice, millet and sorghum, including a GRDC project at CSIRO targeting RUE improvements in wheat undertaken with the International Maize and Wheat Improvement Center in Mexico.

The power of phenomics

The approach draws on another area of Australian excellence – phenomics. This is the ability to rapidly analyse plant physiology across hundreds of lines by exploiting measurements that indicate performance for a key plant trait such as RUE. Phenomics involves innovation in robotics, imaging and computing technology.

“Australia built the world’s first publicly funded centre dedicated to plant phenomics – the Australian Plant Phenomics Facility,” Professor Furbank says. He established one of the facility’s two nodes: the High Resolution Plant Phenomics Centre in Canberra, where Australian expertise was mobilised to develop new analytical technology to screen germplasm collections.

“This unique phenomics capability was developed so that besides measuring potted plants in a glasshouse, it can also be deployed in field trial plots, paddocks or to the GRDC’s Managed Environment Facilities by mounting phenomics-measuring sensors – cameras and laser radar – on tractors, drones or manned flights. These can then relay data round the clock through the mobile phone network.”

For example, researchers are now able to rapidly analyse reflected sunlight and chlorophyll fluorescence to obtain information about photosynthesis rates, speeding up genetic screening more than 20-fold.

As a result, phenomics in Australia uses technology unavailable commercially anywhere else in the world and the analysis is often undertaken with a keen appreciation for the impact that agronomy and paddock conditions have on a crop’s growth characteristics.

The screening for RUE variation in wheat germplasm commenced two years ago. Initially more than 100 lines were screened in Australia and Mexico; however, the 2015 season will see much more material undergo analysis using new tools developed over the past two years.

“Included are the best of the elite varieties and also historic sets of lines from the 1960s,” Professor Furbank says. “The phenomic analysis means we can identify the best and worst-performing germplasm. This phenotypic difference can then be exploited to discover the underlying molecular basis for gains in RUE, followed by the development of molecular markers and the isolation of genes for use in fast, genome-based breeding programs.”

The phenomic data collated so far has created a sense of optimism, with lines detected that can photosynthesise with 30 per cent more efficiency than normal.

“The genetic diversity is there,” Professor Furbank says. “So the approach will work. From an Australian agribusiness perspective, we want to see a trait emerge that can deliver higher yields for the same inputs.”

The consortium approach

The International Wheat Yield Partnership (IWYP) represents a new consortium model to tackle global food security based on an unprecedented willingness of experts to share ideas and on new wheat research networks. For the first time, the fundamental bottleneck to yield potential in C3 crops – radiation use efficiency (RUE) – is being addressed in a breeding context.

Key people within the IWYP have published two strategy papers in the Journal of Experimental Botany. Among the authors were Australians Robert Furbank, David Bonnett, Tony Condon, Dean Price and Scott Chapman.

The papers explain how the International Maize and Wheat Improvement Center began consulting with crop experts worldwide about yield in 2009 and now coordinates a portfolio of research activities within three linked themes to:

  1. increase photosynthetic capacity and efficiency;
  2. optimise partitioning to grain yield while maintaining lodging resistance; and
  3. breed to accumulate yield potential traits and delivery of new germplasm.

Within each theme, a set of subprojects has been developed to capitalise on pre-existing knowledge and strengths of laboratories worldwide.

For the first theme, two main approaches are being taken to increase photosynthetic efficiency. The first is a phenomics approach with a five-year timeframe for delivery of a set of germplasm with defined variation in RuBisCo (the enzyme that fixes carbon) amount and tolerance to heat.

The second approach seeks to mimic the ability to concentrate carbon dioxide (CO2) in the leaf compartment
where RuBisCo is located by introducing a trait used by algae, namely the ability to pump bicarbonate across membranes as a means of concentrating CO2 to even higher levels than those in C4 plants.

More information:

Professor Robert Furbank,
0429 820 539,

robert.furbank@anu.edu.au;

Dr Tony Condon,

tony.condon@csiro.au See more on Ground Cover TV:
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