The above considerations may well induce an air of gloom and despondency. It is apparent that regulation is complex and in many instances appears to be specifically geared to the C4 syndrome. Are there too many structural and regulatory aspects to manipulate? C4 photosynthesis has evolved independently many times from C, photosynthesis, so this cannot involve too many steps or gross changes in the pattern of regulation and there seem to be several solutions to the same problem within C4 photosynthesis (exemplified by three biochemical subtypes and their variants). Evidence shows that the C, mechanism is inducible, either in single-cell systems, such as Hydrilla, or in its full glory in the amphibious leafless sedge, Eleocharis vivipara, which has C3 biochemical traits under submerged conditions, but develops C4 biochemical traits, as well as Kranz anatomy, under aerial conditions, a process regulated by abscisic acid (Ueno 1998). An additional observation is that C4 mutants of Amaranthus edulis, lacking either the C4 isozyme of PEP carboxylase (Dever et al 1995) or N AD-ME activity (Dever et al 1998), and therefore lacking the ability to concentrate CO,, grow normally (plants are virtually indistinguishable from the wild type) in 0.7% CO,. This demonstrates that C, photosynthesis and carbohydrate synthesis and export can function efficiently in a C4 structural and regulatory background. How this occurs has not been investigated and it is not necessarily true that the converse applies. However, it gives considerable cause for optimism that the introduction of limited C4 traits into a C, background will function effectively.
It is clear from a consideration of CO,-concentrating mechanisms in a variety of photosynthetic systems (Table 1) that single-cell CO,-concentrating mechanisms are effective in algae and cyanobacteria and that a single-cell C4-like system operates in Hydrilla verticillata. AC, system lacking Kranz anatomy also appears to operate in aquatic leaves of Orcuttia spp., which is a grass that can grow either terrestrially or in seasonal pools (Keeley 1998). Engineering single-cell C02-concentrating mechanisms into a leaf is a considerably less complicated task than the additional introduction of the Kranz anatomy found in terrestrial plants. The simplest theoretical system would be PEP carboxylation in the cytosol, followed by oxaloacetate transport into the chloroplast, decarboxylation by PEP carboxykinase in the chloroplast, and then transport of PEP back to the cytosol. This could also be achieved at the expense of 1 ATP per CO, transferred. Why no terrestrial plant has developed a single-cell CO,-concentrating mechanism is not clear—it may be that the system is unsuitable for the aerial environment. Thus, it may be necessary to have a compartment in which CO, leakage can be minimized.
Recent attempts to engineer the Hydrilla-typc system into potato suggest that even modest expression of PEP carboxylase and NADP-malic enzyme can influence photorespiratory characteristics. For example, a threefold overexpression of PEP carboxylase from Corynebacterium glutamicum in potato led to an increase in the rate of dark respiration and a decrease in the C02-compensation point measured in the absence of dark respiration (Hausler et al 1999). Double transformants with an additional three- to fivefold overexpression of Flaveria pringlei NADP-ME in the chloroplast showed a temperature-dependent decrease in the electron requirement for CO, assimilation, again suggesting the suppression of photorespiration (Lipka et al 1999).
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