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Chapter 2 - Carbon dioxide assimilation and respiration


Tobacco plants (a) transformed with an antisense construct against Rubisco (anti-Rubisco) grow more slowly than wild types due to a 60% reduction in photosynthetic rate. Immunodetection of the large subunit polypeptide of Rubisco with an anti-Rubisco antiserum (b) shows that the anti—Rubisco transgenic plants contain less than 50% of the Rubisco detected in wild-type tobacco plants. Scale bar in (a) = 10 cm (Photograph courtesy Susanne von Caemmerer; original immunoblot courtesy Martha Ludwig)

Oula Ghannoum1, Susanne von Caemmerer2, Nicolas Taylor3 and A. Harvey Millar3
1Hawkesbury Institute of the Environment, University of Western Sydney
2Research School of Biology, Australian National University
3ARC Centre of Excellence inPlant Energy Biology, University of Western Australia

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant single protein on earth and is pivotal for CO2 assimilation by all plants. In higher plants, the holoenzyme consists of eight large subunits, each with a molecular mass of 50-55 kD and eight small subunits of molecular mass 12-18 kD. Large subunits are encoded by a single gene in the chloroplast genome while a family of nuclear genes encode the small subunits. Any loss of catalytic effectiveness or reduction in amount translates to slower photosynthesis and reduced growth.

Life on earth is sustained by photosynthetic use of sunlight energy to convert atmospheric CO2 into carbohydrates. Billions of years ago, photosynthetic cyanobacterium-like prokaryotes were engulfed by early heterotrophic eukaryotes to produce aquatic photosynthetic organisms harbouring chloroplasts with double membranes. These gave rise to vascular plants which in turn adapted to changing terrestrial environments via distinctive modes of photosynthesis.

Most terrestrial plants fix atmospheric CO2 into carbohydrates via the C3 photosynthetic pathway and its initial three-carbon fixation product (Section 2.1). Millions of years of evolution under conditions of water limitation, temperature variations and glacial CO2 concentrations have produced higher plants with significant biochemical variants for fixation of atmospheric CO2 into carbohydrate, namely C4 (initial four-carbon fixation product), CAM (crassulacean acid metabolism) and SAM (submerged aquatic macrophytes) (Section 2.2).

Photosynthesis in C3 plants is inhibited by oxygen, initiating a series of metabolic reactions termed photorespiration (Section 2.3). Mitochondrial respiration converts the carbon gained for generation of energy to sustain growth and nutrient upake, as well as providing carbon skeletons for a multitude of synthetic events (Section 2.4).