Abstract
The two homologous genes, NIA1 and NIA2, encode nitrate reductases in Arabidopsis, which govern the reduction of nitrate to nitrite. This step is the rate-limiting step of the nitrate assimilation and utilization. Therefore, the regulation of NIA1 and NIA2 is important for plant development and growth. Although they are similar in sequence and structure, their regulations are different. Genetic analysis uncovers that NIA1, rather than NIA2, plays a predominant role in adopting to ABA stress. Although both long-term stress conditions can cause an improvement in NIA1 levels, a decrease in NIA1 levels under short-term treatments seems to be necessary for plants to switch from the growth status into the adopting status. Interestingly, the downregulation of the NR is distinct under different stress conditions. Under ABA treatment, the NR proteins are degraded via a 26S-proteasome dependent manner, while the transcriptional regulation is the main manner to rapidly reduce the NIA1 levels under nitrogen deficiency and NaCl stress conditions. These results indicate that under stress conditions, the regulation of NIA1 is complex, and it plays a key role in regulating the balance between growth and adaptation.
Highlights
Nitrogen is a major restricting factor for plant growth
The absorption of extracellular nitrate is through the nitrate transporter (NRT), and the nitrate is gradually reduced to nitrite by nitrate reductases (NRs) in the cytoplasm
NIA1 is responsible for 10% of the total nitrate reductase activity, while NIA2 takes up about 90% [10]
Summary
Nitrogen is a major restricting factor for plant growth. Most land plants obtain organic nitrogen and inorganic nitrogen from the soil [1]. The null-allelic mutant nia1-3nia is lethal, which can be rescued by the treatment of ammonium succinate, indicating that their functions of nitrite reductases are indispensable for normal plant growth and development (Figure S1). The single mutation of either NIA1 or NIA2 does not severely affect plant growth or development, indicating that the low level of nitrate reductase activity might be sufficient. The nitrate nuclear retention of NLP6/7 is responsible for the transcriptional regulation of NIA1 and NIA2, which plays an especially important role in plant growth and development. Our research indicated that NIA1 protein is involved in responding to the stress environment and maintaining the balance between plant growth and adaptation, while NIA2 protein is more about regulating plant growth and maintaining the nitrogen source required for normal plant growth
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