https://orcid.org/0000-0001-6912-0156
Extract
Research on C4 photosynthesis has seen a revival in recent years, leading to a Nature Scientific Reports article announcing the discovery of C4 photosynthesis in grains of the C3 plant wheat (Triticum aestivum; Rangan et al., 2016). The authors indicate the presence of C4 cycle enzymes, which they interpret as evidence for the existence of a C4-type carbon-concentrating mechanism in the wheat pericarp. “Extraordinary claims require extraordinary evidence,” however, and the evidence presented in this case is based entirely on transcriptome data of the entire wheat grain, with a notable absence of biochemical or physiological support, or the exact location of expression. One needs to consider the wider context of C4 photosynthesis, and this challenges the interpretation provided by Rangan et al. (2016).
It is a common misconception that in C4 photosynthesis C3’s primary CO2-fixing enzyme Rubisco is simply replaced by phosphoenolpyruvate carboxylase (PEPC). PEPC indeed initially incorporates CO2 as into the C4 acid oxaloacetate, but this molecule is difficult to chemically reduce beyond the level of malate to that required for sugar, cellulose, and other carbohydrates, while still recycling the substrate PEP. So the CO2 is later released to increase the CO2 concentration in a specialized compartment, such as the bundle sheath (Leegood, 2002). There, the photosynthetic assimilation of CO2 into carbohydrates occurs, via the same pathway as in C3 plants, by Rubisco, ATP, and NADPH. This C4 carbon-concentrating mechanism thus requires a PEPC:Rubisco activity ratio of roughly 1:1; a larger ratio means that ATP is spent on running a futile C4 cycle. Without Rubisco, PEPC is only able to fix CO2 via anaplerotic reactions. Two observations contradict the conclusions of Rangan et al. (2016) in this regard.
