Light Regulation of Nitrate uptake, Assimilation and Metabolism

Evolving from a germinating seedling, dependent on a stored restricted energy supply, into an active photosynthetic plant capturing the energy from sunlight is obviously a vulnerable time period in the life of a plant. Expression of genes involved in nitrogen reduction and assimilation during this time span is strongly influenced by low intensity red and blue light acting through special light receptors, phytochromes and cryptochromes. After greening and establishing of the photosynthetic apparatus, light acts on nitrogen metabolism through photosynthesis and the products thereof, sugars and certain carbon compounds. The ratio between products of nitrogen and carbon metabolism regulates transcription of genes in the nitrogen pathway and acts posttranscriptionally on these gene products. Later in a plant’s life, day length, measured with the help of the red and blue-light absorbing receptors and circadian rhythms, are important for transition into different developmental stages, like reproduction or dormancy. Abundant nitrogen supply is known to sometimes counteract transition into flowering or dormancy, while on the other hand scarce nitrogen supply may induce flowering or dormancy. These interactions between nitrogen metabolism and development in the mature plant are well known but poorly understood and will not be covered in this review. The subject of this review is the influence of light on transcriptional and posttranscriptional regulation of genes involved in nitrate uptake, reduction and incorporation into organic compounds during greening and the vegetative stage.