Abstract
Haemoglobin is one of the best known and more completely characterized proteins in biochemistry. In mammals, its function is the bi-directional transport of oxygen from lungs to tissues and of carbon dioxide from tissues to lungs. Thus, haemoglobin travels in the circulatory system through veins and capillaries inside the erythrocytes in the blood stream. Haemoglobin was described in plants a long time ago in the legume root nodule, and its function was suggested to be associated with the oxygen supply to the symbiotic bacteroid. However, the question remains as to how plant haemoglobin can exert this function, being enclosed in the static plant cell system.In a recent article, K. Kawashima and colleagues 1xTwo types of pea leghemoglobin genes showing different O2-binding affinities and distinct patterns of spatial expression in nodules. Kawashima, K et al. Plant Physiol. 2001; 125: 641–651Crossref | PubMed | Scopus (19)See all References1 describe the existence of multiple symbiotic haemoglobins (leghemoglobins) in pea. These are grouped into two types, PsLbA and PsLbB, which differ in oxygen affinity as well as in cellular localization. PsLb5-10, the only protein representative of the PsLbA type, has a higher oxygen-binding capacity and the corresponding transcripts are detected throughout the central tissue of effective nodules. The transcripts for PsLb120-1, which is representative of the PsLbB type, are localized in a region from infection zone II to the distal part of nitrogen fixation zone.The presence of two types of leghemoglobins of different oxygen affinity could serve to create and maintain an oxygen gradient across the nodule tissue. This primary gradient might be enriched by the existence of multiple leghemoglobins of each type, but also the oxygen affinities of the leghemoglobins might be modulated in response to the metabolic reactions that occur inside the cells. For example, recently two carbonic anhidrases were described of different cellular expression across the nodule section in alfalfa (Medicago sativa). Carbonic anhidrases might modify the pH status of the cells, affecting in turn the oxygen-binding capacities of haemoglobins (the Bohr effect).Thus, the possibility exists that a coordinate action between multiple symbiotic haemoglobins and carbonic anhidrases could be important in the creation and maintenance of oxygen and carbon dioxide gradients required for nodule metabolism.
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