A well-supported stepwise evolutionary model of C4 photosynthesis has been developed which places an essential role on the C2 photorespiratory glycine shuttling pathway in establishing the two-cell Kranz anatomy associated with C4 photosynthesis.1 However, the C2 pathway is comparatively much simpler, and it is not understood how and why the complex C4 metabolism and its associated tissue specificity arise. It has been suggested that some C4 enzymes play a role in supplying carbon skeletons to the vascular sheath cells to aid in reassimilation of ammonia that is released during photorespiration and restoring the mesophyll-sheath nitrogen balance.2 While a modest increase in C4 metabolism occurs in many C2 species, this nascent C4 biochemistry does not seem to function in supplying carbon to the Calvin cycle but rather to support the C2 pathway in some way.3,4 The function of C4 cycle enzymes prior to their integration into the C4 carbon pump and the manner in which this integration evolved represent the most important, fundamental open questions in C4 evolution.5,6 With comparative genomics, transcriptomics, and diverse phenotypic analyses from every known C4 lineage we aim to answer this central question of C4 evolution. Understanding how C4 metabolism evolved naturally will aid efforts to engineer C4 photosynthesis in non-C4 crops and allow humanity to better exploit existing and emerging C4 crops.

citations:

  1. Sage, R. F., Sage, T. L. & Kocacinar, F. Photorespiration and the Evolution of C4 Photosynthesis. Annu. Rev. Plant Biol. 63, 19–47 (2012).
  2. Mallmann, J. et al. The role of photorespiration during the evolution of C-4 photosynthesis in the genus Flaveria. Elife 3, e02478 (2014).
  3. Ku, M. et al. Photosynthetic Characteristics of C3-C4 Intermediate Flaveria Species .1. Leaf Anatomy, Photosynthetic Responses to O-2 and Co2, and Activities of Key Enzymes in the C-3 and C-4 Pathways. Plant Physiol. 71, 944–948 (1983).
  4. Chastain, C. J. & Chollet, R. Interspecific variation in assimilation of (14)CO 2 into C 4 acids by leaves of C 3, C 4 and C 3-C 4 intermediate Flaveria species near the CO 2 compensation concentration. Planta 179, 81–88 (1989).
  5. Monson, R. et al. Carbon-Isotope Discrimination by Leaves of Flaveria Species Exhibiting Different Amounts of C-3-Cycle and C4-Cycle Co-Function. Planta 174, 145–151 (1988).
  6. Stata, M., Sage, T. L. & Sage, R. F. Mind the gap: the evolutionary engagement of the C4 metabolic cycle in support of net carbon assimilation. Curr. Opin. Plant Biol. 49, 27–34 (2019).
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