C₄ photosynthesis is a remarkable example of convergent evolution, having evolved in at least 61 independent phylogenetic lineages over the last 60 million years. In C₄ species, Rubisco operates close to its maximal rate through the avoidance of the oxygenation reaction. This is accomplished via the establishment of a molecular CO₂ pump that delivers carbon in the form of C₄ acid intermediates to a spatially sequestered Rubisco. This carbon pump can be set up using a diverse array of complex biochemical and morphological modifications relative to the basal C₃ photosynthetic state. Most C₄ species use Kranz anatomy to create the carbon pump, wherein mesophyll cells surround highly developed bundle sheath cells that are concentrically arranged in a ring around the vasculature. In a small number of species, special organellar arrangements are used to achieve high carbon concentrations around Rubisco within a single cell. In addition, inducible forms of C₄ photosynthesis have evolved and there are many extant C₃-C₄ intermediates with varying degrees of C₄ anatomy and physiology.

The sheer number of times that C₄ photosynthesis has evolved, in addition to C₃-C₄ intermediate species, indicates that evolution from the ancestral C₃ photosynthetic state is both elastic and relatively straightforward. Correspondingly, a number of extant genes have been recruited to C₄ functions from the basal C₃ state through regulatory and/or enzymatic modifications. However, the genetics controlling the development of the C₄ syndrome is not yet well enough resolved that a C₃ plant can be converted to C₄ in the lab. Such a conversion applied to C₃ crops would be highly desirable because C₄ crops have higher yields and increased nitrogen and water use efficiency relative to C₃ crops. Replicating the C₄ process in C₃ crops such as rice would therefore help support a growing world population.

The aim of this Research Topic is to better understand the evolution and function of C₄ photosynthesis. We welcome Original Research, Reviews and Opinion articles focused on:

- Comparative approaches that employ distinct phylogenetic lineages of this convergent trait to expand our knowledge of the genetics underpinning C₄ anatomy, biochemistry, physiology and metabolism

- Comparative approaches that employ natural variation (C₃, C₃-C₄, C₄), mutant or hybrid populations to expand the known limits of C₄ photosynthesis and/or provide greater resolution for its genetic basis

- Regulatory mechanisms controlling the development or function of the C₄ syndrome

- Changes to enzymatic function or regulation necessary to operate C₄ photosynthesis

- Identification of candidate gene(s) and/or functional validation of those gene(s) in establishing C₄ biochemistry and/or morphology during leaf development or their involvement in C₄ metabolism of mature leaves

- Elucidation of the evolutionary steps necessary and/or sufficient for C₄

- Replicating the C₄ pathway and/or anatomy in C₃ species

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Frontiers in Plant Science

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