Chapter 4 C₄ Photosynthesis: Kranz Forms and Single-Cell C₄ in Terrestrial Plants

Summary

Plants identified as having C₄ photosynthesis have a C₄ metabolic cycle with phosphoenolpyruvate carboxylase as the initial catalyst for fixation of atmospheric CO₂, and a C₄ acid decarboxylase (NADP-malic enzyme, NAD-malic enzyme, or phosphoenolpyruvate carboxykinase), which releases CO₂ for fixation by the C₃ cycle. Effective donation of CO₂ to Rubisco minimizes competition by O₂ and photorespiration, and thus increases photosynthesis under conditions where CO₂ is limiting. To achieve this, fixation of atmospheric CO₂ in the cytosol by phosphoenolpyruvate carboxylase must be separated from the donation of CO₂ to Rubisco by the decarboxylation of C₄ acids. In most documented C₄ plants, this is accomplished through evolution of various forms of Kranz anatomy, with fixation of atmospheric CO₂ in mesophyll cells and donation of CO₂ from C₄ acids to Rubisco in bundle sheath cells. In the family Chenopodiaceae, two alternative means of accomplishing this spatial separation evolved within individual photosynthetic cells, whereby one cytoplasmic compartment specializes in fixation of atmospheric CO₂ in the carboxylation phase of the C₄ cycle, and the other cytoplasmic compartment specializes in donating CO₂ from C₄ acids to Rubisco. In this chapter, biochemical and structural variations of Kranz anatomy in three major C₄-containing families, Poaceae, Cyperaceae, and Chenopodiaceae, as well as other known forms for dicots, are summarized. Then, the phylogeny, biogeography, development, and structure-function relationships of the single-cell C₄ systems are discussed in comparison to Kranz type C₄ plants.