Carbon and energy sources for fatty acid biosynthesis in non-photosynthetic plastids of higher plants

Abstract

Non-photosynthetic plastids are the main sites of biosynthesis of a number of important plant products, including starch and oil. However, in contrast to chloroplasts, these plastids lack the ability to harvest light energy and to fix CO., via photosynthesis. Hence, they depend on continuous supply of energy (ATP), reducing power (NADH and NADPH) and carbon skeletons to sustain their biosynthetic activities. Studies conducted in the recent past have indicated these plastids to contain the full complement of glycolytic and oxidative pentose phosphate pathways, enabling them to metabolize carbon compounds to meet the above requirement. Furthermore, these plastids have also been shown to interact with extraplastidial compartments to maximize their rates of carbon metabolism. This interaction is achieved mainly through a large number of translocators present in the inner membrane of these plastids. These include triose phosphate translocator, hexose phosphate translocator, malate translocator, adenylate translocator and dicarboxylate translocator. Based on the nature of the substrates transported to plastids through translocators, a model has been proposed for the synthesis of fatty acids in leucoplasts of developing embryos of oil seed crops.