The regulation of starch biosynthesis and intermediary metabolism in plants

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Starch is an insoluble polymer that plays a critical role in the carbon economy of most plants on earth. It is composed of two glucose polymers: linear amylose, whose synthesis requires just a single enzyme; and the more highly branched amylopectin which requires the coordinated activity of the products of at least 10 genes. Starch is the major determinant of yield in domesticated cereals such as maize, wheat, and rice, whose high caloric content is critical for both human and animal consumption. It is also an important commodity in non-food industries where its functionality is dependent on the organisation of glucose residues within a tightly defined, semi-crystalline structure, the assembly of which remains poorly understood. Metabolism of starch underpins the interplay between genetic and metabolic decision-making in plants, dictating the timing and initiation of developmental processes including flowering and seed production. Its mobilisation is critical for the growth of plants, and we are beginning to understand that affecting its synthesis and structure in source tissues can have significant effects on productivity. My research is focussed on understanding: 1) the biochemical and molecular mechanisms which are responsible for starch synthesis and architecture 2) how plants allocate resource to support growth and development. ******We have shown that several enzymes involved in starch biosynthesis form heteromeric complexes, working together physically and synergistically to produce the major component of starch, amylopectin. Assembly of these complexes is controlled post-translationally by protein kinases located within either amyloplasts of developing seeds or chloroplasts of leaves where starch is made. Recently we discovered that by changing the particular isoforms of starch branching enzymes involved in amylopectin synthesis in the model plant, Arabidopsis thaliana, not only was it possible to modify the structure of amylopectin, but there were profound, positive changes in biomass and seed production offering new opportunities to enhance the yield of oilseed crops. We have also identified a non-catalytic subunit of a global regulator of metabolism, SnRK1, which has significant effects on the utilisation of carbohydrate and integrates the metabolism of organic acids, lipids and the uptake of nitrate. Thus the main goals of this proposal are to 1) understand how post-translational mechanisms affect both the amount and structure of starch produced 2) identify the molecular, physiological and biochemical determinants which underpin the effective utilisation of starch for growth and development 3) investigate the role of global regulators of metabolism in controlling allocation of resources and nutrients to growth. We are well-poised to deliver such knowledge, providing opportunities for crop bioengineering to meet the challenges of a burgeoning global population.