The Physiology of Flowering Plants - KHAM PHA MOI

THE EFFICIENCY OF ENERGY CONVERSION IN PHOTOSYNTHESIS 35C 4 plants can attain, in the field, rates of net photosynthesis averagingtwice those of C 3 plants (Table 2.1). Nearly all important cropplants of the world are C 3 species, but the C 4 crops maize and sugarcaneare very productive. Ever since the C 4 pathway was discovered,scientists have been mooting the idea of trying to convert C 3 crops toC 4 metabolism, with the hope of boosting net photosynthesis andharvest yield. Techniques of genetic engineering now permit us totransfer genes between species, but C 4 photosynthesis does notdepend on a single extra gene or even on a few defined extra genes.The enzymes of the C 4 cycle are in fact not unique to C 4 plants; nearlyall are present in all flowering plants, though amounts may be low.What is unique to C 4 plants is a combination of properties: the quantityin which the enzymes are found and the way in which they arepartitioned between the mesophyll and bundle sheath cells; themorphology of the leaves; and the transport and permeability propertiesof the mesophyll/bundle sheath interface. To transfer the entire‘C 4 syndrome’ is a pretty tall order. Many genes – including some as yetunidentified – must be involved. Any genetic engineering not onlyneeds to achieve transfer of genes, but must also ensure that particulargenes are expressed in the appropriate cells only. Nevertheless, progressin understanding genetic control of the cycle is being made. Evensingle-gene transfer has potential: the transfer of multiple copies ofthe maize PEP carboxylase gene into the C 3 cereal rice has resulted inhigh levels of the enzyme and a reduction in photorespiration (Ku et al.1999). But overexpression of C 4 enzyme genes has also led to metabolicdisturbances and stunted growth.It should also be remembered that C 4 photosynthesis is nota universal prescription for high photosynthetic rates, but confersan advantage only under appropriate environmental conditions,i.e. high temperature and high PFD, when the CO 2 concentrationbecomes limiting. Hence the value of converting temperate-zonecrop plants is doubtful, especially since now the atmospheric CO 2concentration is increasing and the CO 2 limitation of C 3 plantsshould become less marked (Section 2.7).The high net rates of photosynthesis of C 4 plants are in the lastanalysis largely the result of their lack of photorespiration. Perhapsthen one can improve the photosynthetic performance of C 3 plantsby the simpler expedient of suppressing photorespiration? Sincephosphoglycolate cannot be permitted to accumulate, suppressionmust be achieved at its source: one needs to eliminate the oxygenaseactivity of Rubisco, or at least to lower it substantially. Attemptsin this direction by modifying the protein’s structure are, however,being frustrated by the fact that the two gases react at thesame catalytic site. To date, modifications to the active site havechanged the reactivity of the enzyme more or less equally towardsboth substrates (e.g. Whitney et al. 1999, Spreitzer & Salvucci2002). There are naturally occurring variations in the carboxylase/oxygenase ratio of the enzyme, but a reduced oxygenase activity isassociated with an overall lowering of the catalytic rate. That is