Application Box 52 C4 Pathways in C3 Plants

Rice, wheat, and other important food crops are C3 plants in which photo-synthetic rates decrease in hot, dry environments with high light intensities. As might be expected, scientists have attempted to use genetic engineering to introduce parts of the C4 pathway into these plants. Attempts also have been made to modify C3 enzymes.

Cartoon of rice-corn hybrid

Cartoon of rice-corn hybrid

Corn genes coding for phosphoenolpyruvate (PEP) carboxylase, pyruvate orthophosphate dikinase, and NADP+ malic enzyme have been cloned into rice. The transgenic rice plants can express high levels of these enzymes, but it is unclear whether the enzymes substantially increase photosynthesis or plant growth. Equivocal results are not surprising since the genetically modified C3 plants do not contain all the C4 enzymes and do not have the specialized leaf anatomy of C4 plants. Both issues will have to be addressed in future studies.

Attempts to modify C3 enzymes have met with some limited success. Many mutations have been introduced into the ribulose biphosphate car-boxylase (rubisco) gene but thus far, the attempts have not increased rubisco activity. Indeed, most mutations have decreased the catalytic activity of the enzyme. Overexpression of sedoheptulose bisphosphatase, one of the enzymes responsible for regenerating ribulose 1,5-bisphosphate (RuBP) from glyceraldehyde 3-phosphate (GAP), increases the photosynthetic rate in tobacco. Accordingly, sedoheptulose bisphosphatase and other enzymes in the regenerating system may be attractive targets for genetic engineering.

Scientists also have engineered plants to express ictB, a gene involved in bicarbonate (i.e., carbon dioxide) accumulation in cyanobacteria. Plants expressing this gene show faster photosynthetic rates in conditions of limiting carbon dioxide, presumably due to the increased availability of bicarbonate for fixation.

Genetic engineering that can increase carbon dioxide availability or carbon fixation in plants has the potential to increase crop yields. Such engineering will be critical to keep up with the increasing demand for food.

Oxaloacetate produced by carboxylation of PEP is transported into the chloroplast where it is reduced by electrons from NADPH to malate in a reaction catalyzed by NADPH-dependent malate dehydrogenase (Fig. 5.21). The malate is then transported through plasmodesmata to adjacent bundle sheath cells. In these cells, NADP+ malic enzyme catalyzes decarboxylation of malate, releasing carbon dioxide that can be fixed to RuBP by rubisco. The high concentration of carbon dioxide generated in bundle sheath cells by decarboxylation of malate decreases photorespiration and facilitates carbohydrate synthesis. Decarboxylation of malate also is accompanied by storage of electrons in NADPH—electrons needed for reduction of intermediates in the Calvin cycle. The net result is that C4 plants are more resistant than C3 plants to hot, dry environments with high light intensities.

The C4 pathway has some costs. Pyruvate produced in bundle sheath cells by decarboxylation of malate has to be converted back to PEP in mesophyll cells. Regeneration of PEP is catalyzed by pyruvate orthophosphate dikinase (PPDK), an enzyme unique to the C4 pathway. The reaction requires hydrolysis of ATP to AMP (Fig. 5.21), an energy loss which is the equivalent to conversion of two ATP to ADP. Because the C3 pathway avoids this energy requirement, C3 plants have an advantage in cooler, less arid, temperate regions (i.e., higher latitudes) where light intensity is lower.

5.5.4 CAM plants

Crassulacean acid metabolism (CAM) plants include cacti and a wide variety of succulent plants that are found in deserts and other arid regions. Like C4 plants, CAM plants avoid opening their stomata during the day by increasing the availability of carbon dioxide through carboxylation/decarboxylation reactions. However, unlike C4 plants, CAM plants do not partition carboxylation and decarboxylation reactions between two different types of cells. Instead, CAM plants run carboxy-lation and decarboxylation at different times in a single cell.

In CAM plants, carboxylation is carried out at night when cooler temperatures decrease water loss. Stomata open to allow entry of carbon dioxide, and PEP carboxylase catalyzes fixation of carbon dioxide to PEP, forming oxaloacetate and then malate. During the night, the malate is stored in the central vacuole to avoid decreasing the pH of the cytosol. During the day, the stomata close to prevent water loss, and malate moves into the cytosol from the vacuole. Decarboxylation of malate releases carbon dioxide which can be fixed to RuBP by rubisco. Photorespiration is low because the closed stomata prevent oxygen entry. Because CAM plants primarily open their stomata at night, they can attain water use efficiencies three- to sixfold higher than those of C4 and C3 plants.

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