One of the most important crops in the world is rice. And it is a crop of the world where people encounter substantial poverty and food shortage. Despite a lot of criticism, the “Green Revolution” in those areas in the second half of the 20th century played a significant role in encountering the hunger against the teeming millions and so many challenges like reduced per capita land and increased climate adversaries. But now the Green Revolution has already been faded away and the growth of rice productivity is swinging around one per cent which is not enough to satisfy the increasing demand every year. In other words, the yield potentiality of the conventional High Yielding Varieties (HYV) of rice has almost reached the ceiling level. Whatever the scientists are struggling is to develop varieties good for growing under stress environments to keep up the total production level higher than the previous year. The minimising of yield gap is a solution provided intensive agronomic management are assured. But that appears to be still difficult to achieve because of the socio-environmental context of the country Bangladesh. So there is no other way but to increase rice yield vertically to encounter the hunger in the near future. So the breaking of yield ceiling might be one of the options. But how? Hybrid rice is an option. The yield increase around 20% over the conventional HYV is not bad. We need more options to increase the rice yield. Getting any change to plant canopy architecture to utilize solar energy more efficiently might be the other way. Accordingly, IRRI had a project to develop New Plant Type (NPT) under the supervision of G S Khush during the latter part of the last millennium. The plants developed by the breeders were very near to their expectation. Unfortunately, their dream could not get success as the NPT advance lines could not adjust as expected under field conditions. Thereafter, scientists have considered that the inherent capacity of photosynthetic efficiency of rice plant should be increased than that of its existing level. Photosynthesis is the driving force of the crop productivity. Rice has the photosynthetic pathway to produce carbohydrate is called C3 pathway. There is another pathway called C4 observed in the crops like maize, sorghum grown relatively under adverse conditions. They are more efficient in terms of photosynthesis and their yield potential is significantly higher than that of rice.
In C3 cycle carbon dioxide (CO2) is fixed by Rubisco (Ribulose bi Phosphate Carboxylase/ oxygenase enzyme) into sugar to produce three carbon compound Phospho-Glyceric Acid (PGA). Rubisco can also react with oxygen to activate another metabolic process called photorespiration. Photorespiration is an energy wasting process occurs in the C3 plants (Rice) to cause a dramatic reduction in the amount of CO2 that is supposed to be converted into sugar. In contrast, C4 plants have a mechanism to bypass the wasting photorespiration process. In these plants, CO2 is first fixed by 3 carbon Phospho-Enol-Pyruvate (PEP) with the assistance of PEP carboxylase to produce 4 carbon Oxaloacetate first (so the process is called 4-carbon cycle) then converted to 4-carbon malate. These reactions occur in the mesophyll cells and malate is transferred to the bundle sheath (another type of cells having chloroplasts intimately arranged along with the vascular bundles known as Kranz anatomy). CO2 is released from malate to turn itself to PEP through Pyruvate and get back to the mesophyll cell to continue the cycle again. The released CO2 is fixed again by the Rubisco enzyme and continues the C3-cycle in the bundle sheath cells. The mechanism is something like a cooperative approach to utilize CO2 more efficiently. Thus, the C4 plant is not only efficient in photosynthesis, they are efficient in using nitrogen and water also. They are better adaptable to relatively dry and warm environment. Most of the highest yielding biomass crops (with few exceptions yield too) like sugarcane, maize sorghum are adaptable to hot and dry conditions belong to C4 group. That is why the scientists have an intention to the introduction of C4 mechanism into rice to increase its yield potential (50-60%) and adaptation ability as well to adverse conditions. So IRRI (International Rice Research Institute) coined a hypothesis like, “converting the photosynthetic system in rice to the more efficient supercharged one used by maize (C4 plant) would increase rice yields using scarce resources (land, water fertiliser) more effectively”.
Accordingly, IRRI initiated a project with a view to developing C4 rice. The project was the brainchild of John Sheehy, a Plant Physiologist and was the head of Applied Photosynthesis Group, IRRI from 1995 to 2009. He arranged a couple of workshops to pave the way towards the C4 rice. Bob Ziegler, the then Director General of IRRI was convinced of the importance of the project to embed in IRRI’s 2007-2015 strategic plan. The genome sequencing of rice in 2004 and the updated genetic engineering technologies might have a stimulating impact on the scientists to accept the challenge ahead to develop C4 rice.
However, the spirit of interest dwindled to some extent as many of the C4 experts associated with the project had to retire within the project tenure.
Therefore a rejuvenation of the project was needed with the newly recruited young experts. Anyway, this is a mega project and IRRI formed a consortium prior to the beginning of its journey with the experts from 12 leading laboratories of the world from eight countries. In 2008, Bill and Melinda Gates Foundation had the grant to support the project activities. The project was first led by John Sheehy and then by Paul Quick from IRRI. Large-scale genetic screening was done in the first and the second phases of the project. The ongoing third phase from 2015-2019 is concentrated with an emphasis on integrated “systemic” and “synthetic” approaches to plant biology (https://c4rice.com/the-project-2/our-history). The third phase was coordinated by Jane Lansdale of Oxford University.
Nearly two decades of exciting research since 1995, the scientists made an announcement in December 2014 to Kevin Bullis, an author of MIT-Technology Review that they had a breakthrough in the engineering of a rice plant with the maize GOLDEN2-LIKE gene to carry out photosynthesis in more efficient way. The author cited Thomas Brutnell, a member of the consortium in Danforth Institute, USA, “It is the first time we’ve seen evidence of the C4 cycle in rice, so it is very interesting”. In fact, that was still in the very preliminary stage. In spite of the new genetic makeup, still, the transgenic rice plants have to rely primarily on its original C3 cycle of photosynthesis. To have a complete triumph, the development of Kranz anatomy along with the vascular bundle of rice plant is a must. But still, a good number of genes involved in building up these tissues are to be explored. As per Peng et al., (2017) the engineering of a rice plant with the maize GOLDEN2-LIKE gene is good enough to express the anatomical features of a “proto-Kranz” species (not exactly the same as Kranz anatomy), a key step towards the transformation from C3 to C4 in rice plant. Professor Jane Lansdale and her colleagues of Oxford University are working for C4 rice consortium to identify the genes responsible to express the C4 pathway in the rice plant using bioinformatics and some other state of the art biotechnological approaches. They have already identified few hundred genes as candidate regulators of switching from C3 to C4. They are now in progress to understand the function of these candidate genes.
The project is in its right direction but it would take definitely a considerable time. The latest achievement of the project has aroused interest among the policymakers of the developing rice world like us. They would be happy to have a rice variety like this right now. So far I understand, whatever has been achieved by the C4-scientists is just a little ahead of the starting line. A lot of scientific mysteries are yet to be explored to develop even a mother line in those classical laboratories. Then IRRI would share its experience with the institutes of the rice growing countries to develop their own C4 rice lines/varieties.
So, a C4 rice variety to be cultivated in the farmers’ field is a matter of patience. More so, the C4 variety if we have one, that must be a GM crop. So we have to prepare ourselves accordingly to avoid the controversy also.
The writer is the Director General (Retired), Bangladesh Rice Research Institute