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

Abstract

SummaryPlants 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.