Legume biological nitrogen fixation "oxygen paradox" solved

Jeremy introduces the research results.
Photo by Qin Zhiwei

Nodule is called the "nitrogen-fixing factory" of legumes, reflecting the symbiotic relationship between legumes and nitrogen-fixing rhizobia
.

On October 29th, Jeremy Dale Murray's team from the Center for Excellence in Molecular Plant Science of the Chinese Academy of Sciences (hereinafter referred to as the Center for Excellence in Molecular Plant) successfully solved the nitrogen-fixing "oxygen paradox".
They discovered for the first time that the transcription factor NLP family regulates the expression of leghemoglobin genes in root nodules.
Mechanism
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The "oxygen paradox" is unresolved

In nature, in addition to external nitrogen fertilizer, plant growth is also "self-sufficient"
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The "oxygen paradox" starts from the root nodule of the "nitrogen fixation plant"
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However, the nitrogen fixation reaction process consumes a lot of energy
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More importantly, "nitrogenase is highly sensitive to oxygen and needs to work in a low-oxygen environment, but the host cells and rhizobia themselves require a lot of oxygen for respiration
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"Leghemoglobin is similar to hemoglobin in human blood, containing heme and protein
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It is worth mentioning that the hemoglobin bean root nodules appear pink, "which explains why legume root nodules is pink"
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Studies have shown that the content and components of leghemoglobin directly affect the activity of nitrogenase in root nodules, and make it play a key role in the biological nitrogen fixation of legumes
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In fact, the research of biological nitrogen fixation has a history of hundreds of years, but so far the regulation mechanism of leghemoglobin gene expression in nodules is still unclear
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Finding a "comfortable home" for rhizobia

The Jeremy team targeted NLP
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The NLP family is a type of plant-specific transcription factors.
It can bind to a special "element" in the target gene promoter, that is, the nitrate response element (NRE), and then activate the expression of downstream genes and participate in the regulation of plant nitrogen metabolism
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They found that the two members of the NLP family, NLP2 and NIN, have a "superior" expression in nodules
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"When analyzing the NLP2 mutant nodules, it was unexpectedly found that when the plant lacks NLP2, the expression of leghemoglobin is also affected, and it has a lighter pink than the wild type
.
" Jiang Suyu said
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Jeremy further explained that the leghemoglobin and heme levels in the NLP2 mutant nodules were significantly reduced, which explains why the mutant nodules are lighter in pink
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"Because the mutation occurs in the transcription factor, this is a protein that can initiate the expression of other factors
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" So they speculated that this gene might activate the expression of leghemoglobin
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Then, the research team analyzed the leghemoglobin genes of different types of legumes and found that a DNA sequence exists in all leghemoglobin gene promoters, and they are called dual nitrate response elements (dNRE)
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NLP2 "recognizes" dNRE and regulates the expression of leghemoglobin to balance the oxygen microenvironment necessary for nitrogen fixation, which means that it finds a "comfortable home" for rhizobia
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Jeremy believes that dNRE and NLP2 are only highly conserved in legumes, implying that their evolution helps to increase the expression of leghemoglobin in root nodules
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Non-symbiotic hemoglobin plays an important role in removing oxygen from the plant body and helps plants survive in a low-oxygen environment, which is helpful to the study of non-legumes such as rice and corn to achieve autonomous nitrogen fixation
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Jeremy successfully solved the "oxygen paradox" of nitrogen fixation by legumes, making it possible for biological nitrogen fixation to become a new source of nitrogen fertilizer, which is of great significance for saving agricultural production costs and ecological environmental protection
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Photo courtesy of Jeremy at the Laboratory of  Molecular Plant Excellence Center

Researcher Jeremy joined the Molecular Plant Center of Excellence full-time in 2017
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The team is also affiliated to the International Joint Unit Plant and Microbial Science Joint Research Center jointly established by the Chinese Academy of Sciences and the John Innes Center in the United Kingdom
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The research was funded by the Young Scientists Project of Basic Research of the Chinese Academy of Sciences, the National Natural Science Foundation of China, the National Key Research and Development Project, the Leading Science and Technology Special Project of the Chinese Academy of Sciences, and the State Key Laboratory
.
(Source: China Science News Qin Zhiwei and Huang Xin)

Related paper information: https://doi.
org/10.
1126/science.
abg5945