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The protein content of seeds determines their nutritive value, downstream processing properties and market value. Up to 95% of seed protein is derived from amino acids that are exported to the seed after degradation of existing protein in leaves, but the pathways responsible for this nitrogen metabolism are poorly defined. The enzyme pyruvate,orthophosphate dikinase (PPDK) interconverts pyruvate and phosphoenolpyruvate, and is found in both plastids and the cytosol in plants. PPDK plays a cardinal role in C(4) photosynthesis, but its role in the leaves of C(3) species has remained unclear. We demonstrate that both the cytosolic and chloroplastic isoforms of PPDK are up-regulated in naturally senescing leaves. Cytosolic PPDK accumulates preferentially in the veins, while chloroplastic PPDK also accumulates in mesophyll cells. Analysis of microarrays and labelling patterns after feeding (13)C-labelled pyruvate indicated that PPDK functions in a pathway that generates the transport amino acid glutamine, which is then loaded into the phloem. In Arabidopsis thaliana, over-expression of PPDK during senescence can significantly accelerate nitrogen remobilization from leaves, and thereby increase rosette growth rate and the weight and nitrogen content of seeds. This indicates an important role for cytosolic PPDK in the leaves of C(3) plants, and allows us to propose a metabolic pathway that is responsible for production of transport amino acids during natural leaf senescence. Given that increased seed size and nitrogen content are desirable agronomic traits, and that efficient remobilization of nitrogen within the plant reduces the demand for fertiliser applications, PPDK and the pathway in which it operates are targets for crop improvement.
Taylor et al. (Wed,) studied this question.