Compartmentation of photosynthesis in cells and tissues of C-4 plants

Critical to defining photosynthesis in C-4 plants is understanding the inte rcellular and intracellular compartmentation of enzymes between mesophyll a nd bundle sheath cells in the leaf. This includes enzymes of the C-4 cycle (including three subtypes), the Ca pathway and photorespiration. The curren t state of knowledge of this compartmentation is a consequence of the devel opment and application of different techniques over the past three decades. Initial studies led to some alternative hypotheses on the mechanism of C-4 photosynthesis, and some controversy over the compartmentation of enzymes. The development of methods for separating mesophyll and bundle sheath cell s provided convincing evidence on intercellular compartmentation of the key components of the C-4 pathway. Studies on the intracellular compartmentati on of enzymes between organelles and the cytosol were facilitated by the is olation of mesophyll and bundle sheath protoplasts, which can be fractionat ed gently while maintaining organelle integrity. Now, the ability to determ ine localization of photosynthetic enzymes conclusively, through ir, situ i mmunolocalization by confocal light microscopy and transmission electron mi croscopy, is providing further insight into the mechanism of C-4 photosynth esis and its evolution. Currently, immunological, ultrastructural and cytoc hemical studies are revealing relationships between anatomical arrangements and photosynthetic mechanisms which are probably related to environmental factors associated with evolution of these plants. This includes interestin g variations in the C-4 syndrome in leaves and cotyledons of species in the tribe Salsoleae of the family Chenopodiaceae, in relation to evolution and ecology. Thus, analysis of structure-function relationships using modern t echniques is a very powerful approach to understanding evolution and regula tion of the photosynthetic carbon reduction mechanisms.