Nitrogen fixation inside legume nodules requires a steady supply of oxygen to support high rates of bacterial respiration. However, the bacterial nitrogenase enzyme that fixes nitrogen is highly sensitive to oxygen. This contradiction, the oxygen paradox of biological nitrogen fixation, is addressed by small heme-containing proteins produced by the plant that can buffer cellular O2 levels.
In a study published in Science, Jeremy Murray's team and colleagues at CAS Center for Excellence in Molecular Plant Sciences (CEMPS) of the Chinese Academy of Sciences and the collaborators identified the molecular mechanism by which the NIN-like protein (NLP) family of transcription factors regulates the expression of leghemoglobin genes in root nodules.
NLPs are plant specific transcription factors that can bind the target gene promoters through special nitrate-responsive elements (NREs) to activate the downstream gene expression to participate in the process of plant nitrogen metabolism. Dr. Murray's team found that two members of the NLP family, NLP2 and NIN, have high expression levels in root nodules. When analyzing nodules of the NLP2 mutant, they unexpectedly found that they contained less leghemoglobin and fixed less N.
NIN and NLP2 were found to activate leghemoglobin gene expression in root nodules by directly binding to two overlapping NREs called double nitrate responsive elements (dNREs) conserved in legumes, balancing the oxygen microenvironment necessary for nitrogen fixation. Phylogenetic analysis suggested that dNREs and NLP2 evolved in legumes to help increase the expression levels of leghemoglobins in nodules. Leghemoglobins evolved from non-symbiotic hemoglobins which play an important role in survival under low oxygen (hypoxic) conditions that occur during flooding.
Besides, the researchers found that other NLPs activate expression of non-symbiotic hemoglobins through NREs. This suggested NLP-hemoglobin modules with roles in hypoxic survival were recruited to nodulation to address the O2 paradox of biological N-fixation.
As a source of nitrogen fertilizer, biological nitrogen fixation is of great significance to the development of sustainable agriculture. This study reveals a central regulatory mechanism in legume N-fixation, offers a chance to improve the nitrogen fixation in legumes, and provides a theoretical basis to transfer nitrogen fixation in non-legumes plants such as rice and maize.
