ScienceWise - Nov/Dec 2009

Feed the World

Article Illustration
Professor Susanne von Caemmerer and technical officer Scott Kwasny in one of the greenhouses

Trying to Unlock the Potential of C4 Rice

The United Nations Food Agency recently announced that over the coming 40 years the world’s food production will need to rise by 70% in order to feed the growing population. Failure to achieve this is likely to result in widespread famine.  This in turn, may well lead to unrest that spreads well beyond the borders of the most affected nations, so in reality, it’s likely to become everyone’s problem.  The difficulty the world faces in addressing this is that most of the viable agricultural land is already used to capacity and production is limited by other factors such as water availability. The general consensus amongst scientists is that the only practical way to avert catastrophe is to enhance the photosynthetic yield per leaf area of food crops. In other words, to create more efficient plants.

Most plants, including many staple foods like rice, turn sunlight into sugar using what’s known as the C3 photosynthetic pathway. In this process gaseous CO2 is combined with an enzyme called RuBisCO to create sugar. However RuBisCO can and often does, combine with oxygen instead of CO2, leading to a loss of efficiency particularly at higher temperatures. 

Some more recently evolved plants have developed an alternate photosynthetic pathway called C4 that avoids this loss of efficiency by using some additional chemistry to saturate the RuBisCO enzyme with CO2 and starve it of oxygen. This avoids wasteful oxygen combinations and under most environmental conditions, leads to a higher sugar yield in the plant. Scientists believe that if they can introduce this C4 photosynthesis to rice, they may be able to create cultivars that produce more crop per area than existing rice without consuming more water or fertilizer.
One scientist studying the possibilities of C4 rice is Professor Susanne von Caemmerer of the Research School of Biology. Professor von Caemmerer is working on a project with the International Rice Research Institute, sponsored by a $10m grant from the Bill and Melinda Gates foundation. The ultimate aim of this work is to create C4 rice with a substantially better yield than existing plants, but this is a hugely complex task requiring multiple steps.

The basic idea is to look at millions of mutant seedlings of both C3 rice and C4 sorghum. Scientists expect the random mutations to cause some of the rice to move towards the C4 pathway and some of the sorghum to partially revert to the C3. If they can identify which specimens these transformations take place in, they can analyse their genomes and compare them to conventional rice and sorghum. Seeing both the C3-C4 and C4-C3 switch should help them to isolate the genes responsible for the two photosynthetic pathways.

It might sound fantastically unlikely that random mutations would produce a switch between photosynthetic pathways, but there are some good scientific reasons to think otherwise. “We know from studying various C4 plants that they have evolved from C3 plants on at least 40 separate occasions. So it seems highly probable that the jump isn’t a huge one in genetic terms.” Professor von Caemmerer explains. “We also know that the basic suite of enzymes involved in the biochemistry is essentially the same in C3 and C4 plants, so again we’re not talking about needing to introduce major changes to the plants genes.”

The plants will be grown at IRRI and at the High Resolution Plant Phenomics Centre co-located in Canberra at CSIRO Plant Industry and the Australian National University. This facility enables vast numbers of plants to be cultivated under highly specific conditions of atmosphere, water and nutrients.

But having grown a million seedlings, how do you pick out those with C3 or C4 pathways?

C4 plants can concentrate atmospheric CO2 within their leaf structures, so they can grow in low CO2 concentrations. C3 plants on the other hand, actually lose CO2 from their leaves under concentrations below about 50 parts per million, ultimately leading to the plant’s death. By growing seedlings under low CO2 concentrations, the scientists can pick the surviving rice, which is likely to be displaying at least some C4 characteristics. The can also rescue any dying sorghum, which is likely to have partially reverted to C3.

In both cases the plant’s anatomy will also give clues to the pathway it’s using because C3 plants have a different leaf structure to C4. In a C3 plant most of the chlorophyll lies in the tissue between the vascular bundles. In C4 plants it tends to cluster around the bundles themselves and in addition, the bundles are more numerous and more closely packed.

Once a subset of seedlings displaying a switch in photosynthetic characteristics have been identified, they will then be subjected to more advanced scrutiny such as isotopic analysis. RuBisCO discriminates strongly against the isotope carbon 13. In C3 plants this means that air flowing over the leaves tends to become carbon 13 enriched as carbon 12 is depleted. However, the CO2 concentration mechanism of C4 plants has the tendency to negate this effect by effectively feeding the enzyme with whatever carbon comes to hand, 12 or 13. As a result, air passing over C4 leaves has significantly less carbon 13 than that passing over C3 leaves. This gives scientists a very clear indicator of the different photosynthetic processes occurring in a particular plant.

“Ultimately, this is a gene discovery project. We’re hoping to isolate mutants that appear to switch photosynthetic pathways. What we can then do is look at the genome of those plants and try to identify which genes are responsible for C4. This would be a huge help to another arm of the project in which scientists would directly splice those genes into existing rice cultivars,” Professor von Caemmerer says.

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