Hemoglobins are widespread throughout the biosphere (Wittenberg and Wittenberg, 1990. Annu Rev Biophys Biophys Chem 19:217-241). They are found in a broad range of organisms from bacteria, through unicellular eukaryotes, to plants and animals, suggesting that they predate divergence of life into plant and animal forms. Plant hemoglobins have been classified into symbiotic and nonsymbiotic types (Appleby. 1992, Sci Progress 70:365-398): symbiotic hemoglobins are found in plants that are capable of participating in microbial symbioses, where they function in regulating oxygen supply to nitrogen fixing bacteria; nonsymbiotic hemoglobins have only recently been discovered and are thought to be the evolutionary predecessors of the more specialized symbiotic leghemoglobins. The ubiquitous nature of nonsymbiotic hemoglobins is evidenced by their broad presence across the plant kingdom (Appleby, 1985, Nitrogen Fixation and CO2Metabolism, eds. Ludden and Bums, pp. 41-51) and the widespread presence and long evolutionary history of plant hemoglobins suggest a major role for them in the life of plants.
Specifically, plant hemoglobins have been known to exist in the root nodules of legumes for almost 60 years (Kubo, 1939, Acta Phitochem 11:19-200; Keilen and Wang, 1945, Nature 155:227-229). Over the years, hemoglobins have been positively identified in three non-leguminous dicotyledonous plants; Parasponia andersonil, Tream tomentosa, and Casuarina glauca (Appleby et al., 1983, Science 220:951-954; Bogusz et al., 1988, Nature 331:178-180; Kort et al., 1998, FEBS Lett 180:55-60). Recently, an Hb cDNA from badey was isolated and the gene was demonstrated to be expressed in seed and root issues under anaerobic conditions (Taylor et al., 1994. Plant Mol Biol 24:853-862), providing further evidence to support the contention that plant hemoglobins have a common origin (Landsmann et al., 1986. Nature 324:166-168). Since Hb has now been demonstrated to occur in two of the major divisions of the plant kingdom, it is likely that an Hb gene is present in the genome of all higher plants (Brown et al., 1984, J Mol Evol 21:19-32; Bogusz et al., 1988; Appleby, 1992, Sci Progress 76:365-398; Taylor et al., 1994; Andersson et al., 1996, Proc Natl Acad Sci USA 93:427-431; Hardison, 1996, Proc Natl Acad Sci USA 93:5675-5682).
Very little, however, is known about the function of Hb, although it has been proposed that nonsymbiotic hemoglobins may act either as oxygen carriers to facilitate oxygen diffusion, or oxygen sensors to regulate expression of anaerobic proteins during periods of low oxygen supply. The proteins from barley (Duff et al, 1997, J. Biol Chem 272: 16746-16752) and rice (Arrendondo-Peter et al, 1997, Plant Physiol 115:1259-1266) and AHB1 from Arabidopsis (Trevaskis et al, 1997, Proc Natl Acad Sci 94:12230-12234) have been shown to have high oxygen avidity, with dissociation constants for oxyhemoglobin of 2.86 nM, 0.55 nM and 1.6 nM respectively, resulting in conditions whereby the free protein will remain oxygenated at oxygen concentrations far below those at which anaerobic processes are activated. Thus, while roles for Hb in the facilitated diffusion and sensing of oxygen have been proposed (Appleby, 1992), it is unlikely that these hemoglobins would function as either facilitators of oxygen diffusion or sensors of oxygen, unless the oxygen avidity was modified by interaction with another component within the cell. Thus, while Hb or Hb related proteins are found in all divisions of living organisms, their function has not been well defined.
Herein, it is shown that nonsymbiotic hemoglobins function to maintain the energy status of cells exposed to low oxygen tensions and that this property may be a common feature throughout evolution, either during exposure to hypoxia or under high energy demand.