Changes in concentration of leghemoglobin components in hypernodulation mutants of soybean

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Abstract
Leghemoglobin (Lb) plays a crucial role in N2 fixation of soybean (Glycine max. L. Merr.) nodules by facilitating O2 supply to the bacteroids. The concentration and composition of Lb components in hypernodulation mutant lines. NOD 1–3. NOD2-4. and NOD3-7 from cv. Williams, and En6500 from cv. Enrei, were compared in relation to their nodulation characteristics. Plants were hydroponically cultivated in N-free culture solution and the nodules were separated by size. Concentration and composition of Lb components in the same size nodules were analyzed by gel-electrophoresis or capillary electrophoresis. In all NOD mutants Lb concentration was about 70% of that in the parent Williams, irrespective of nodule size and growth stage. In the hypernodulating mutant En6500. the total Lb concentration was only 25% of that in the parent Enrei, irrespective of nodule size. In Enrei nodules, the relative concentration of Lba. Lbci, Lbc:. and Lbc.i was about 39, 26. 18. and 17%. respectively, and very stable irrespective of nodule size. En6500 had relatively equal amounts of each component (concentrations of Lba, Lbc1. Lbc2, and Lbc3 were 30, 22, 22. and 26%. respectively). The concentrations of Lbc forms in nodules were decreased by addition of nitrate to Enrei plants, but not to En6500 plants. The observation that Lb concentration was lower in hypernodulating mutant lines than in parents indicated that hypernodulating lines could not maintain maximum nodule growth and function. This appears to be due not only to excess nodulation which may result in a shortage of nutrients and photosynthates supply, but also due to some other inherent trait from mutation.