Plant Hemoglobins

Hbs are ubiquitous proteins in most organisms, including bacteria, protozoans, fungi, plants, and animals (Vinogradov et al., 1993; Bolognesi et al., 1997). The known and predicted roles of Hbs in each organism encompass functions that include the reversible binding of gaseous ligands and the ability to bind other cellular molecules. Hbs are widely distributed in higher plants, and by comparing sequences, expression patterns, and ligand-binding properties, it is evident that these fall into two broad groups (Fig.1; also see Andersson et al., 1996). The symbiotic-type Hbs are predominantly found in the infected cells of nitrogen-fixing nodules of legumes and nonlegumes and function to facilitate oxygen transport. The other group of plant Hbs, nonsymbiotic Hbs, appear to be ancestral to the symbiotic Hbs, are more widespread in the plant kingdom, and generally display a high affinity for oxygen. Their function in plants is the subject of much current inquiry (Appleby, 1992; Andersson et al., 1996; Arredondo-Peter et al., 1997a;Trevaskis et al., 1997; Hill, 1998). Some aspects of plant nonsymbiotic Hb function, particularly that of barley, have been succinctly reviewed by Hill (1998).