Since the possible introduction of C4 traits into C, plants is a topic of this workshop, we will briefly comment on it here. In general, approaches could include incorporating some features of C4 photosynthesis into the mesophyll cells of rice (Oryza sativa), analogous to Hydrilla, the aquatic macrophyte, developing C,-C4 intermediate-type photosynthesis (dependent on mesophyll cells and BSC), or determining the genetic factors required for expression of Kranztype anatomy.
Currently, key steps of the C4 pathway are being incorporated into rice (see chapters by Matsuoka et al and Ku et al, this volume). Several considerations affect the possibility of success. A major question is whether the diffusive resistance between atmospheric CO, and Rubisco is too low, which could result in a high degree of overcycling with a minimal increase in CO, supply. As shown above, in the C4 model, a minimal resistance is required for the cycle to be effective in concentrating CO,. If the strategy is to recapture photorespired CO, as in C4 plants, then a Kranz-type anatomy will be required. Another consideration is whether the chloroplast will be able to generate ATP by cyclic or pseudocyclic photophosphorylation to support the C4 cycle, in that linear electron flow to NADP only generates sufficient ATP to support the C, cycle. One of the key steps for engineering C4 photosynthesis in rice is expression of the C4 isoform of PEP carboxylase in the cytosol, where it would fix atmospheric CO,. As noted earlier, in C4 plants, CA in the cytosol of MC is considered to function in generating bicarbonate for PEPC. Thus, a question in genetic engineering of C4 in rice is whether cytosolic CA will be limiting. To address these types of questions, models for incorporating C4 photosynthesis into rice that include the enzymatic steps, compartmentation of reactions, and diffusive resistances to CO, can be tested to aid genetic engineers in designing rice for improved photosynthesis.
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