Catching up with the literature for C4 rice: what we know now and didn't then
2007
Mitchell, P.L., Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10, 2TN, UK
Developments in understanding C4 photosynthesis since 1999 are reviewed to provide a starting point for the conference on Supercharging the Rice Engine in 2006. It is now clear that all C3 plants have the enzymes for the C4 pathway and some plants employ them to capture carbon dioxide in particular tissues, although not as full C4 photosynthesis with reduced or absent photorespiration. Conversely, parts of C4 plants, mesophyll cells not close to bundle sheath cells, carry out C3 photosynthesis. Several types of single-cell C4 photosynthesis have now been studied; none seem to be highly productive but instead are adaptive for these plants in environments conducive to high rates of photorespiration. Introduction of one or a few genes for C4 enzymes into rice has become routine but full C4 photosynthesis or greatly increased growth has bot resulted. Kranz anatomy appears to be essential for productive C4 photosynthesis because there is no other way of confining carbon dioxide at high concentrations around ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco), which is the key to success. At least 45 independent origins of C4 photosynthesis across 19 angiosperm families are known. Study of C3-C4 intermediates has led to the definition of seven phases of evolution from c3 to c4. Given low atmospheric concentrations of carbon dioxide and high rates of photorespiration, C4 photosynthesis seems to evolve readily, although not in the subfamily of grasses containing rice. Nevertheless, these recent advances in knowledge are encouraging for those intending to produce C4 rice.
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