To boost food production for a rapidly growing global population, crop yields must significantly increase. One of the avenues being recently explored is the improvement of photosynthetic capacity by installing the C4 photosynthetic pathway into C3 crops like rice to drastically increase their yield. Crops with an enhanced photosynthetic mechanism would better utilize the solar radiation that can be translated into yield. This subsequently will help in producing more grain yield, reduce water loss and increase nitrogen use efficiency especially in hot and dry environments. This review provides a summary of the factors that need to be modified in rice so that the C4 pathway can be introduced successfully. It also discusses the differences between the C3 and C4 photosynthetic pathways in terms of anatomy, biochemistry and genetics.Electronic supplementary materialThe online version of this article (doi:10.1186/1939-8433-6-28) contains supplementary material, which is available to authorized users.

These authors contributed equally to this work. SUMMARYThe specification of vascular patterning in plants has interested plant biologists for many years. In the last decade a new context has emerged for this interest. Specifically, recent proposals to engineer C 4 traits into C 3 plants such as rice require an understanding of how the distinctive venation pattern in the leaves of C 4 plants is determined. High vein density with Kranz anatomy, whereby photosynthetic cells are arranged in encircling layers around vascular bundles, is one of the major traits that differentiate C 4 species from C 3 species. To identify genetic factors that specify C 4 leaf anatomy, we generated ethyl methanesulfonate-and cray-mutagenized populations of the C 4 species sorghum (Sorghum bicolor), and screened for lines with reduced vein density. Two mutations were identified that conferred low vein density. Both mutations segregated in backcrossed F 2 populations as homozygous recessive alleles. Bulk segregant analysis using nextgeneration sequencing revealed that, in both cases, the mutant phenotype was associated with mutations in the CYP90D2 gene, which encodes an enzyme in the brassinosteroid biosynthesis pathway. Lack of complementation in allelism tests confirmed this result. These data indicate that the brassinosteroid pathway promotes high vein density in the sorghum leaf, and suggest that differences between C 4 and C 3 leaf anatomy may arise in part through differential activity of this pathway in the two leaf types.

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All grass leaves are strap-shaped with a series of parallel veins running from base to tip, but the distance between each pair of veins, and the cell-types that develop between them, differs depending on whether the plant performs C3 or C4 photosynthesis. As part of a multinational effort to introduce C4 traits into rice to boost crop yield, candidate regulators of C4 leaf anatomy were previously identified through an analysis of maize leaf transcriptomes. Here we tested the potential of 60 of those candidate genes to alter leaf anatomy in rice. In each case, transgenic rice lines were generated in which the maize gene was constitutively expressed. Lines grouped into three phenotypic classes: (1) indistinguishable from wild-type; (2) aberrant shoot and/or root growth indicating possible perturbations to hormone homeostasis; and (3) altered secondary cell wall formation. One of the genes in class 3 defines a novel monocot-specific family. None of the genes were individually sufficient to induce C4-like vein patterning or cell-type differentiation in rice. A better understanding of gene function in C4 plants is now needed to inform more sophisticated engineering attempts to alter leaf anatomy in C3 plants.