Hemoglobins, key components of our blood, are ancient proteins with well-known roles in oxygen transport and respiration in animals. Hemoglobins are also present in plants and bacteria, but until now the physiological role of plant hemoglobins has been unclear. A group of researchers reveal this week that one such mysterious plant hemoglobin serves to assist in the fixation of nitrogen in the root nodules of legumes through a process that is conceptually not unlike that undertaken by mammalian hemoglobins in facilitating oxygen transport and exchange in the blood.
The most conspicuous plant hemoglobins are the symbiotic hemoglobins of legumes; these hemoglobins accumulate in root nodules and give these specialized organs their distinctive red color. Legume root nodules accommodate bacteria, called rhizobia, that reduce atmospheric nitrogen to ammonia, which is subsequently used by the plant for growth and colonization of nitrogen-poor soils. Symbiotic nitrogen fixation is important for sustainable agriculture and contributes millions of tons of reduced nitrogen to crops and pastures each year.
As reported this week, researchers led by Dr. Michael Udvardi at the Max Planck Institute of Molecular Plant Physiology have succeeded in eliminating the production of symbiotic hemoglobins in nodules of the model legume Lotus japonicus, enabling researchers to assess for the first time the role and importance of these proteins in plants. The results of the study indicate that symbiotic hemoglobins are important for oxygen transport and energy metabolism in plant root nodules. Furthermore, these proteins help to maintain free-oxygen concentrations in nodules at levels low enough to avoid damage of oxygen-sensitive nitrogenase, the bacterial enzyme complex responsible for symbiotic nitrogen fixation. Thus, plant hemoglobins fulfill roles analogous to those of animal hemoglobins, as well as novel roles that are apparently unique to symbiotic nitrogen fixation.
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