Compartmentation of photosynthesis in cells and tissues of C4 plants

Abstract

Critical to defining photosynthesis in C₄ plants is understanding the intercellular and intracellular compartmentation of enzymes between mesophyll and bundle sheath cells in the leaf. This includes enzymes of the C₄ cycle (including three subtypes), the C₃ pathway and photorespiration. The current state of knowledge of this compartmentation is a consequence of the development and application of different techniques over the past three decades. Initial studies led to some alternative hypotheses on the mechanism of C₄ photosynthesis, and some controversy over the compartmentation of enzymes. The development of methods for separating mesophyll and bundle sheath cells provided convincing evidence on intercellular compartmentation of the key components of the C₄ pathway. Studies on the intracellular compartmentation of enzymes between organelles and the cytosol were facilitated by the isolation of mesophyll and bundle sheath protoplasts, which can be fractionated gently while maintaining organelle integrity. Now, the ability to determine localization of photosynthetic enzymes conclusively, through in situ immunolocalization by confocal light microscopy and transmission electron microscopy, is providing further insight into the mechanism of C₄ photosynthesis and its evolution. Currently, immunological, ultrastructural and cytochemical 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 interesting variations in the C₄ 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 techniques is a very powerful approach to understanding evolution and regulation of the photosynthetic carbon reduction mechanisms.