Perspective

The Development of C4 Rice: Current Progress and Future Challenges

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Science  29 Jun 2012:
Vol. 336, Issue 6089, pp. 1671-1672
DOI: 10.1126/science.1220177

Abstract

Another “green revolution” is needed for crop yields to meet demands for food. The international C4 Rice Consortium is working toward introducing a higher-capacity photosynthetic mechanism—the C4 pathway—into rice to increase yield. The goal is to identify the genes necessary to install C4 photosynthesis in rice through different approaches, including genomic and transcriptional sequence comparisons and mutant screening.

As the world population races toward 10 billion, agricultural scientists are realizing that another “green revolution” is needed for crop yields to meet demands for food. In rice, yield potential is limited by the photosynthetic capacity of leaves that, as carbohydrate factories, are unable to fill the larger number of florets of modern rice plants. One potential solution is to introduce a higher-capacity photosynthetic mechanism—the C4 pathway—into rice. This is the goal of researchers in the international C4 Rice Consortium: to identify and engineer the genes necessary to install C4 photosynthesis in rice (1).

Rubisco, the primary CO2-fixing enzyme in rice, is a poor catalyst of CO2 at current atmospheric conditions. It has a tendency of confusing its substrate CO2 with the more abundant O2 as well as being a very slow catalyst of CO2, turning over only once or twice per second. Rubisco’s oxygenase activity requires the recycling of phosphoglycolate in the photorespiratory pathway, resulting in an energy cost and loss of previously fixed CO2. Many photosynthetic organisms, including cyanobacteria, algae, and land plants, have developed active CO2-concentrating mechanisms to overcome Rubisco’s inefficiencies (2). Among land plants, this led to the development of C4 photosynthesis, a biochemical CO2-concentrating mechanism. C4 photosynthesis arose multiple times in the past 60 million years in warm semi-arid regions, with early occurrences coinciding with low atmospheric CO2 in the late Oligocene (3). During C4 photosynthesis, CO2 is fixed within specialized leaf tissues known as mesophyll cells to produce C4 acids, which diffuse to and are decarboxylated in another type of specialized tissue, the bundle sheath cells. This process elevates the CO2 concentration in the bundle sheath and inhibits Rubisco oxygenase activity, allowing Rubisco to operate close to its maximal rate (Fig. 1). In comparison with C3 crops such as rice, C4 crops (such as maize and sorghum) have higher yields and increased water- and nitrogen-use efficiency (1, 4).

Fig. 1

(A) C3 photosynthesis fixes atmospheric CO2 into C3 acids with Rubisco in single cells. (C) Two-cell C4 photosynthesis requires spatial separation of fixation of atmospheric CO2 into C4 acids and the donation of CO2 from these C4 acids to Rubisco. Also shown are light microscopy images of transverse sections of leaves of (B) rice, a C3 plant, and (D) sorghum, a C4 plant. The rice section shows vascular bundles with few chloroplasts and large numbers of mesophyll cells between the vascular bundles typical for C3 species. The sorghum leaf section shows chloroplasts in bundle sheath and only two or three mesophyll cells in between the vascular tissue typical of a C4 species.

Building the C4 Machinery

In an evolutionary context, the transition from C3 to C4 photosynthesis has occurred independently in more than 60 different plant taxa (3). Genomic and transcriptional sequence comparisons of cell-specific and leaf-developmental gradient transcription profiles between closely related C3 and C4 species are being used to identify C4-specific regulatory genes (4). Combining this information in parallel with screens of mutagenized C4 Sorghum bicolor and Setaria viridis along with activation-tagged rice populations hopefully will reveal candidate genes in the C3-to-C4 switch that can be tested in transgenic rice and S. viridis (5). Because C4 plants can carry out net CO2 assimilation at very low CO2 levels whereas C3 plants cannot (Fig. 2), we can use growth screens to identify gain of function in activation-tagged rice mutants and loss of function in S. viridis mutants (Fig. 2). We are also using the fact that C4 photosynthesis imparts a distinct carbon isotope signature on dry matter (6) in a loss-of-function screen for C4 mutants.

Fig. 2

(A) Modeled changes in CO2 assimilation rate in response to changes in leaf intercellular CO2 partial pressure for C3 and C4 photosynthesis and for a hypothetical C4 rice. Curves 1, 2, and 4 have Rubisco levels typically found in a C4 leaf (10 μmol m−2 catalytic Rubisco sites). Curve 3 shows a typical response for C3 leaves with three times the Rubisco level of C4 leaves. Curve 1 shows the response of a C4 leaf with C4 Rubisco kinetic properties. Curve 2 models how a C4 leaf with C3 Rubisco kinetic properties would respond (a hypothetical C4 rice with C3 Rubisco kinetics). The comparison of these two curves shows the increase in CO2 assimilation rate achieved with C4 compared with C3 Rubisco kinetic properties within a functional C4 mechanism. Arrows to curves 1 and 3 show intercellular CO2 partial pressures typical at current ambient CO2 partial pressures for C4 and C3 photosynthesis. To generate the curves, model equations were taken from (11) and comparative Rubisco kinetic constants from (12). (B) Growth of 21-day-old rice and S. viridis seedlings at different ambient CO2 concentrations ranging from 30 to 800 parts per million.

A subset of genes required for the major biochemical components and metabolite transporters involved in the C4 pathway have been cloned and coupled to suitable promoters to give cell-specific expression in rice (7). Attempts to install C4 photosynthesis in plants lacking the appropriate anatomy show that a biochemical approach alone will not be enough (8). Bundle sheath cells in rice are smaller than in C4 plants and have less chloroplasts, and there are a large number of mesophyll cells between vascular bundles (Fig. 1) (4). Promising mutants have been identified in rice that show reduced vein spacing. Combined with studies of sorghum, we are optimistic that we will be able to identify the genes controlling this aspect of anatomy (4, 7).

Lessons Learned and Future Challenges

Although C4 leaves have close veins and high rates of photosynthesis, C4 photosynthesis is also naturally supported around widely spaced veins in maize husk tissue, albeit at lower rates (6). Thus, a prototype C4 rice may be achievable with a subset of C4 genes, but a “good” C4 rice will require substantial fine tuning of biochemistry and anatomy. Particularly intriguing is the need for additional metabolite transport across membranes of organelles in C4 photosynthesis (4). A functional C4-concentrating mechanism in rice would allow for an approximately two-thirds reduction in Rubisco levels, relative to wild-type rice, but Rubisco would be sequestered in bundle sheath cells and ideally have a greater catalytic turnover rate (Fig. 2) (2). Antisense gene suppression of key photosynthetic enzymes has illuminated C4 metabolism and engineering strategies, including the surprising find that phosphorylation of phosphoenolpyruvate (PEP) carboxylase by the regulatory enzyme PEP carboxylase phosphokinase is not needed for C4 function (9). With the adoption of the C4 model plant S. viridis—with its short life cycle, small stature, and genome size—along with advances in efficient transformation, we anticipate that much more will soon be learned (5). We expect to have a C4 rice prototype within 3 years. However, we estimate that another 15 years of research are required for optimization of the phenotype and field testing for C4 rice to become ready for cultivation in farmers’ fields.

Norman Borlaug’s green revolution was based on just a handful of genes (10). However, the need for even greater food plant production looms. The promise of C4 rice has resulted in one of the largest consortia of plant biologists pursuing a common goal. We optimistically take on this challenge, anticipating that advances in our understanding of plant metabolism, and C3 and C4 photosynthesis in particular, will better serve humanity in years to come.

References and Notes

  1. Acknowledgments: This work was supported by the Bill and Melinda Gates Foundation. We are thankful for the scientific contributions of all the members of the C4 Rice Consortium.
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