Partitioning of photosynthetic carbon: the role of nitrate activation of protein kinases

Abstract

Abstract Sucrose is the main product of photosynthesis in crop plants, but photosynthesis is also required for primary nitrogen (N) assimilation in leaves. The reductive assimilation of N0-to the level of glutamine (Gin) and glutamate (Glu) must be coordinated with photosynthesis, since the energy required for the assimilation process is derived from photosynthetic electron flow, and the carbon (C) skeletons needed for amino acid synthesis are provided by photosynthetic CO assimilation. In addition, it is evident that complex molecular strategies are employed to regulate the partitioning of photosynthetic C between sucrose and amino acid synthesis. Over large distances, for example between the shoots and roots, cycling of N and C compounds allows information on the C and N status to be conveyed from one tissue to another. This is used to modulate N-absorption in the roots to match the demand for N. In the leaf cytoplasm, a multifactorial network of regulatory processes is employed to modulate the relative fluxes of assimilated C through the two metabolic pathways. The levels of sucrose and amino acids in the foliar cytosol act as endogenous regulators at the molecular and metabolic levels. Sucrose-phosphate synthase (SPS), nitrate reductase (NR), and phosphoenolpyruvate carboxylase (PEPCase) are considered to be the gateways to the assimilatory pathways of sucrose synthesis and N assimilation and the anaplerotic pathway respectively. These cytosolic enzymes are modulated in response to the addition of N to the plants and their activities vary with the rate of photosynthesis. The regulation of PEPCase and NR gene expression by N is well characterized as is the regulation of NR gene expression by sucrose (Cheng et al. 1986; Galangau et al. 1988; Sugiharto and Sugiyama 1992). In addition, all these enzymes are subject to covalent modulation via protein phosphorylation (S.C. Huber et al. 1994). NR and SPS, for example, are inactivated rapidly by phosphorylation in response to changes in irradiance or CO2 supply.