Light and metabolic regulation of HAS1, HAS1.1 and HAS2, three asparagine synthetase genes in Helianthus annuus.

The role of light, carbon and nitrogen availability on the regulation of three asparagine synthetase (AS, EC 6.3.5.4)-coding genes, HAS1, HAS1.1 and HAS2, has been investigated in sunflower (Helianthus annuus). The response of each gene to different illumination conditions and to treatments that modify the carbon and nitrogen status of the plant was evaluated by Northern analysis with gene-specific probes. Light represses the expression of HAS1 and HAS1.1. Phytochrome and photosynthesis-derived carbohydrates mediate this repression. On the contrary, maintained HAS2 expression requires light and is positively affected by sucrose. HAS1 and HAS1.1 expression is dependent on nitrogen availability, while HAS2 transcripts are still found in N-starved plants. High ammonium level induces all three AS genes and partially reverts sucrose repression of HAS1 and HAS1.1. In summary, light, carbon and nitrogen availability control asparagine synthesis in sunflower by regulating three AS-coding genes. Illumination and carbon sufficiency maintain HAS2 active to supply asparagine that can be used for growth. Darkness and low C/N ratio conditions trigger the response of the specialized HAS1 and HAS1.1 genes which contribute to store the excess nitrogen as asparagine. Ammonium induces all three AS-genes which may favor its detoxification.