The use of nitrogen (N) fertilizers has contributed to a quadrupling of agricultural production. However, >50% of N fertilizers remain unabsorbed by crops and leak into the environment, demanding strategies to improve N use efficiency. To identify the regulatory mechanisms of N assimilation, we performed a genetic screen using a Chlamydomonas reinhardtii strain whose motility depends upon the transcription of nitrate reductase-encoding NIT1, which is upregulated in response to N starvation. A constantly swimming mutant, nitrogen repletion insensitive 1 (nri1), was isolated and found to express N starvation-induced genes in N-replete culture. NRI1, previously reported as NSG17 (nitrogen starvation-induced gene 17), encodes a basic helix-loop-helix (bHLH) family transcription factor homologous to BES-interacting myc-like (BIM) in plants. Transcriptome analysis of N-replete nri1 culture revealed a concerted upregulation of the genes involved in the uptake and assimilation of external N sources. Many transcription factor-encoding genes in the bHLH and SBP families were also upregulated in nri1 culture. The DNA-binding sites of bHLH and SBP proteins were enriched in the promoters of the upregulated genes in nri1, suggesting that NRI1 suppresses transcriptional activators to regulate N starvation-induced genes. Consistent with the ammonium-insensitive gene expression pattern, N-starved nri1 gametes could not dedifferentiate back to vegetative cells when incubated in ammonium-containing media. Taken together, we conclude that NRI1 represses N starvation-specific responses when sufficient N is supplied in C. reinhardtii. Further investigation of how cellular N conditions regulate NRI1 activity will unravel the elusive mechanisms by which plant cells sense and respond to N starvation.
Keywords: Chlamydomonas reinhardtii; bHLH transcription factor; nitrogen starvation responses; regulatory mechanisms of nitrogen assimilation.