Compartmentation in plant metabolism

Cell fractionation and immunohistochemical studies in the last 40 years have revealed the extensive compartmentation of plant metabolism. In recent years, new protein mass spectrometry and fluorescent-protein tagging technologies have accelerated the flow of information, especially for Arabidopsis thaliana, but the intracellular locations of the majority of proteins in the plant proteome are still not known. Prediction programs that search for targeting information within protein sequences can be applied to whole proteomes, but predictions from different programs often do not agree with each other or, indeed, with experimentally determined results. The compartmentation of most pathways of primary metabolism is generally covered in plant physiology textbooks, so the focus here is mainly on newly discovered metabolic pathways in plants or pathways that have recently been revised. Ultimately, all of the pathways of plant metabolism are interconnected, and a major challenge facing plant biochemists is to understand the regulation and control of metabolic networks. One of the best-characterized networks links sucrose synthesis in the cytosol with photosynthetic CO2 fixation and starch synthesis in the chloroplasts. One of the key features of this network is how the transport of pathway intermediates and signal metabolites across the chloroplast envelope conveys information between the two compartments, influencing the regulation of several enzymes to co-ordinate fluxes through the different pathways. It is widely accepted that chloroplasts and mitochondria originated from prokaryotic endosymbionts, and that new transporters and regulatory networks evolved to integrate metabolism in these organelles with the rest of the cell. Curiously, the present-day locations of many metabolic pathways within the cell often do not reflect their evolutionary origin, and there is evidence of extensive shuffling of enzymes and whole pathways between compartments during the evolution of plants.