Mechanism of Rapid Growth and Efficient Nitrogen Utilization of Duckweed under Nitrogen-free Conditions Revealed

Nitrogen is the most important nutrient for crop growth and an important resource. Since the green revolution in the mid-20th century, nitrogen fertilizers have been widely used to promote crop growth and increase yield. Up to now, the amount of nitrogen used in agricultural production has reached 110 million tons per year. The excessive application of nitrogen fertilizer not only increases the cost investment of crop production, but also directly leads to environmental problems such as water eutrophication and air pollution. Therefore, it is essential to optimize the amount of nitrogen fertilizer in sustainable agriculture. However, at present, the nitrogen utilization rate (NUE) of major cereal crops such as wheat, maize, and rice is low (21 kg/kg N), and the demand for nitrogen fertilizer is high. Finding new starchy crops with high NUE, thereby reducing nitrogen fertilizer application and improving environmental benefits, is an important direction for future agricultural development.     Duckweed is the fastest-growing higher plant in the world with low nutrient requirements and can even grow in the tap or purified water. Due to its characteristics of high biomass yield, high starch content, and efficient absorption of nitrogen and phosphorus growth in water bodies, it has attracted much attention in the academic community in recent years as a new biological resource with important application prospects in the fields of biomass energy development and environmental pollution biological treatment.   The results of Zhao Hai's team at Chengdu Institute of Biology, Chinese Academy of Sciences showed that the growth of duckweed cultured in the absence of nitrogen would not be inhibited, but the starch content could reach 52.37% of the dry weight of biomass within 10 days, and the mass production results of pilot-scale could also reach 46.98%. According to this estimate, the annual starch production of duckweed under nitrogen-free conditions can reach 14 tons per hectare, which is 1.8 ~ 3.6 times that of corn. Meanwhile, the NUE of duckweed reached 61.3 ± 1.8 kg/kg N, which was much higher than that of common crops (9 ~ 50 kg/kg N). Therefore, the nitrogen-free cultivation of duckweed can truly achieve efficient and environmentally friendly starchy biomass production, which is important for the development of sustainable agriculture.   In order to reveal the mechanism of rapid growth, rapid accumulation of starch and efficient nitrogen utilization of duckweed under nitrogen-free conditions, the research team conducted an in-depth and systematic study of carbon and nitrogen metabolism of duckweed under nitrogen-free conditions and made the following progress: (1) photosynthesis of duckweed was not significantly affected under nitrogen-free conditions, and the enzyme activities of key enzymes of photosynthesis such as Rubisco and PEPC were not inhibited, thus ensuring the effective accumulation of carbon in duckweed; while the enzyme activities of AGPase, SSS, and GBSS, key enzymes of starch synthesis, were significantly increased, and a large amount of carbon flowed to the starch synthesis pathway to achieve rapid accumulation of starch; at the transcriptional level, it was also demonstrated that the expression changes of related enzymes were highly consistent with the trend of enzyme activities. (2) the expression and enzyme activity of glutamine synthetase, which is the most critical for amino acid synthesis in duckweed, were significantly increased under nitrogen-free conditions, and this change was completely opposite to the trend of glutamine synthetase in wheat, maize, and Arabidopsis that was significantly decreased under nitrogen-deficient conditions. The main function of glutamine synthetase is nitrogen assimilation and redistribution, which directly determines the level of plant NUE to a large extent, so this finding may be the key factor causing the increase of duckweed NUE; at the same time, duckweed only depends on endogenous nitrogen to maintain growth and metabolism during the culture process, and its nitrogen source can only rely on the degradation of endogenous proteins, so most of the enzymes of ubiquitination and autophagy system related to protein degradation in duckweed are up-regulated; therefore, in the absence of exogenous nitrogen supply, duckweed can rapidly initiate endogenous protein degradation and realize the activation and redistribution of endogenous nitrogen, so as to ensure the most efficient use of endogenous nitrogen to continue to maintain its rapid growth.