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From nitrogen to amino acids: How do microorganisms help plants absorb nitrogen?
In nature, nitrogen is a vital element that is essential for plant growth and development. However, plants cannot directly utilize nitrogen in the form of nitrogen gas (N2). This makes microorganisms play a key role in the absorption and utilization of nitrogen. Microorganisms use a process called nitrogen fixation to convert nitrogen into forms that plants can absorb, such as ammonia (NH4+) and nitrate (NO3−). This process not only improves plant health, but also increases agricultural sustainability.
Nitrogen uptake is essential for plant growth, and microorganisms support this process.
Nitrogen transformation process
Plants absorb nitrogen from the soil primarily through their root systems, in the form of nitrates and ammonia. In healthy soils, nitrate is usually the major form of available nitrogen, while ammonia may predominate in grasslands and flooded anaerobic soils such as paddy fields.
Plant roots affect the abundance of different forms of nitrogen by changing the pH of the soil and secreting organic compounds or oxygen, thereby promoting the activity of microorganisms to promote the conversion of various nitrogen substances. Ammonium ions in the plant roots enter the plant through ammonia transport proteins, while nitrates are absorbed by a variety of nitrate transport proteins.
How plants process nitrogen
Nitrogen is transported primarily through the xylem, where it is delivered to other parts of the plant in the form of nitrates, dissolved ammonia, and amino acids. In most cases, nitrate reduction occurs mainly in the leaves of plants, while the roots only capture a small amount of nitrate for reduction. Regardless of the absorption or synthesis process, ammonia will eventually be incorporated into amino acids through the glutamate synthetase-glutamate synthetase (GS-GOGAT) pathway.
The transport and conversion of nitrogen is not only essential for plant growth, but also affects the health of the entire ecosystem.
pH and ion balance
Whenever nitrate is reduced to ammonia, the plant must maintain the pH balance of its surroundings, which often means emitting OH− ions or neutralizing them with organic acids. This makes the soil around the plants that absorb nitrates alkaline. In order to maintain ion balance, plants must absorb NO3− accompanied by the entry or excretion of negatively charged anions with positively charged cations. For example, plants such as tomatoes absorb metal ions such as potassium (K+), sodium (Na+), calcium (Ca2+) and magnesium (Mg2+) to balance the absorption of nitrate.
Nitrogen utilization efficiency
Nitrogen use efficiency (NUE) is the proportion of nitrogen that plants are able to absorb and use. Improving nitrogen use efficiency and fertilization efficiency is critical to sustainable agricultural development, reducing pollution (such as fertilizer runoff) and production costs while increasing yields. The NUE of crops worldwide is generally below 50%, prompting scientists to seek ways to improve nitrogen use efficiency through improved fertilizers, enhanced crop management, selective breeding and genetic engineering.
Improving nitrogen use efficiency and fertilizer efficiency is a major challenge in sustainable agriculture.
By understanding the important role of microorganisms in plant nitrogen uptake and the related physiological processes, can we find more innovative ways to improve agricultural production and sustainability?
