Over-Expression of the Class 1 Hemoglobin Gene Contributes to Symbiotic Nitrogen Fixation

Plant hemoglobins (Hbs), which were first reported as leghemoglobin in root nodules of soybean (Kubo, 1939), have been identified in various species of legumes, non-legumes, and actinorhizal plants, and are divided into three distinct types, class 1, class 2, and truncated Hb. The physiological functions of plant Hbs are still unclear except for that of leghemoglobins (Lb), which compose a subgroup of class 2 Hb. Lbs are crucial for nitrogen fixation in root nodules as an O₂ transporter and regulator, but not for general plant growth and development (Ott et al., 2005). Class 1 Hbs exhibit an extremely high affinity for O₂ (Arrendondo-Peter et al., 1997) and nitric oxide (NO) (Dordas et al., 2004). Class 1 Hbs may play significant roles by modulating NO levels in various physiological situations. During the Rhizobium-legume interaction, the produc-tion of NO was detected both at the early stage of host-symbiont recognition (Shimoda et al., 2005) and also in the nodules (Baudouin et al., 2006). NO can itself be an inducer of the class 1 Hb gene, so that the modulation of NO by class 1 Hb might be involved in establishment and maintenance of symbiotic nitrogen fixation.