Nitrogen is one of the most crucial macronutrients, playing a vital role in the normal growth, development, and yield determination of plants. In agricultural production, the nitrogen use efficiency of rice is often low, and excessive use of nitrogen fertilizers poses multiple environmental challenges such as soil acidification, eutrophication, and greenhouse gas emissions. Thus, improving nitrogen use efficiency is of utmost urgency.
The absorption of nitrate in plants mainly occurs through nitrate transporters (NRTs) located in the roots, including low-affinity nitrate transporters (LATs) and high-affinity nitrate transporters (HATs). NO is an important intermediate in nitrogen metabolism, participating in various biotic and abiotic stresses in plants and multiple physiological processes including nutrient absorption. Moreover, NO plays a significant role in the transcriptional regulation of various target genes. Nitrogen nutrition, especially nitrate nutrition, regulates NO production, yet the role of NO in the induction of HAT-related genes remains unclear. Additionally, Phytoglobin 1 (Pgb 1), which scavenges NO in plants, is directly related to nitrogen metabolism. Studies have shown that intracellular NO levels and related NO metabolism and balance pathways can regulate nitrogen assimilation, but the specific mechanism is still unknown.
Recently, the Kapuganti Jagadis Gupta team from the National Institute of Plant Genome Research in India published a research paper titled “Overexpression of Phytoglobin1 in Rice Leads to Enhanced Nitrogen Use Efficiency via Modulation of Nitric Oxide” in the renowned international journal Plant, Cell & Environment. The study revealed that Phytoglobin 1 regulates the transcriptional control of nitrate transporters by NO, thereby enhancing the nitrogen use efficiency of plants and is of great significance for elucidating the role of NO in plant nitrogen nutrition.
Firstly, the researchers investigated the regulatory effect of NO on nitrate transporters. The results showed that compared with the optimal nitrogen condition, plant growth was significantly inhibited under low nitrogen conditions, and the expression of HAT was enhanced while the expression level of LAT was significantly reduced. Meanwhile, the researchers used two methods, DIF-FM and ECO-FM, to detect NO levels, and the results indicated that the NO released by plants under low nitrogen conditions was reduced (Figure 1). Thus, it was preliminarily hypothesized that low levels of NO could enhance the expression of HAT.
The study also found that the overexpression of Phytoglobin1 could enhance the expression of high-affinity nitrogen transporters (HATs). Compared with WT, the Arabidopsis Pgb 1 OE lines exhibited stronger growth under low nitrogen conditions, and the expression of HAT was significantly enhanced compared with WT.
The researchers designed a cPTIO priming experiment and found that after switching to low nitrogen conditions, the rice primed with cPTIO grew better than the unprimed rice, and the nitrate content, ATP level, and HAT gene expression were significantly enhanced.
To verify the regulatory role of reduced NO under low nitrogen conditions on the nitrogen use efficiency of rice, the researchers generated overexpressing phytoglobin lines (Pgb-OE) using pMDC 99 and used NH-FM assay to detect NO production in WT and Pgb-OE lines. The results showed that compared with WT, the NO production in Pgb-OE lines was significantly reduced. Moreover, the growth, HAT expression, and nitrogen content of Pgb OE under low nitrogen conditions were significantly higher than those of WT (Figure 5). This indicates that under low nitrogen conditions, Phytoglobin1 enhances the growth performance and nitrogen accumulation of rice under low nitrogen conditions by regulating nitric oxide (NO) levels and enhancing HATs gene expression, thereby enhancing the nitrogen fertilizer use efficiency.
Finally, the researchers detected the changes in the concentration of amino acids that maintain cellular nitrogen balance in Pgb-OE lines. GC-MS analysis was performed on WT and Pgb 1 OE lines grown for 10 days under optimal and low nitrogen conditions. The results showed that Pgb1 OE lines accumulated more asparagine under both optimal and low nitrogen conditions. In addition, glutamine and α-ketoglutarate also increased significantly under low nitrogen conditions. The results suggest that the overexpression of Phytoglobin1 gene can increase the concentration of amino acids related to nitrogen balance and thus increase the nitrogen use efficiency of plants.
The above research results demonstrate that under low nitrate nitrogen conditions, the overexpression of Phytoglobin 1 in rice can regulate NO levels, cause an increase in HAT expression, thereby promoting the growth and nitrogen accumulation of rice and showing enhanced amino acid accumulation. The research results confirm that Phytoglobin 1 plays an important role in the absorption and assimilation of nitrogen.