Markers and signals associated with nitrogen assimilation in higher plants

A key concept underpinning current understanding of the carbon/nitrogen (C/N) interaction in plants is that the capacity for N assimilation is aligned to nutrient availability and requirements by the integrated perception of signals from hormones, nitrate, sugars, organic acids, and amino acids. Studies on the nature and integration of these signals over the last ten years has revealed a complex network of controls brokered by an interplay of C and N signals. These controls not only act to orchestrate the relative rates of C and N assimilation and carbohydrate and amino acid production, but they also have a significant influence on plant development. Amino acids are the hub around which the processes of N assimilation, associated C metabolism, photorespiration, export of organic N from the leaf, and the synthesis of nitrogenous end-products revolve. Since specific major amino acids or their relative ratios are modulated differentially by photorespiration and N assimilation, even though these processes are tightly intermeshed, they are potentially powerful markers for metabolite profiling and metabolomics approaches to the study of plant biology. Moreover, while minor amino acids show marked diurnal rhythms, their contents fluctuate in a co-ordinated manner. It is probable that factors associated with early events and processes in C and N assimilation influence the relative composition of minor amino acids.