Regulation of Photosynthesis in C3 and C4 Plants: A Molecular Approach.

Abstract

Most plants use the C3 pathway of photosynthesis, also called the photosynthetic carbon reduction cycle (PCR), shown in Figure 1A. C3 plants have a single chloroplast type that performs all of the reactions that convert light energy into the chemical energy that is used to fix COp and to synthesize the reduced carbon compounds upon which all life depends. Ribulose-i ,5-bisphosphate carboxylaseloxygenase (Rubisco) catalyzes primary carbon fixation, in which a fivecarbon sugar phosphate, ribulose-l,5-bisphosphate (RuBP), and COp are converted to two molecules of the threecarbon compound 3-phosphoglycerate (hence the name C3). Phosphoglycerate is then phosphorylated and reduced by the products of the light reactions of photosynthesis (ATP and NADPH) to produce triose phosphate (TP). TP can be exported from the chloroplast via the chloroplast envelope phosphate (Pi) transporter to the cytosol and used in the synthesis of sucrose, which is then translocated throughout the plant (see Sonnewald et al., 1994), or it can be retained within the chloroplast for starch synthesis or recycling to RuBP. Rubisco also catalyzes the fixation of Op in a process known as photorespiration, which competes directly with fixation of COp. At air levels of Coa, for every three COp molecules fixed by Rubisco to form 3-phosphoglycerate, approximately one O2 molecule is fixed, producing Sphosphoglycerate and 3-phosphoglycolate (Figure 1A). Because 3-phosphoglycolate cannot be used in the PCR cycle, it must be recycled to phosphoglycerate via the photorespiratory pathway, expending ATP and NADPH. This competition between 0 2 and COp and the energy costs associated with recycling phosphoglycolate largely determine the efficiency of C3 photosynthesis in air (Hatch, 1988; Woodrow and Berry, 1988). The C4 pathway is a complexadaptation of the C3 pathway that overcomes the limitation of photorespiration and is found in a diverse collection of species, many of which grow in hot climates with sporadic rainfall. The C4 pathway effectively suppresses photorespiration by elevating the C02 concentration at the site of Rubisco using a biochemical C02 pump. C4 plants have two chloroplast types, each found in a specialized cell type. Leaves of C4 plants show extensive vascularization,