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Leghemoglobins in the genus phaseolus and their significance for nitrogen fixation

University of British Columbia

Leghemoglobins (Lbs) are important soluble proteins in the legume root nodule formed in response to infection by soil Rhizobium bacteria. With the exception of Phaseolus vulgaris, Lbs generally show structural and functional heterogeneity for their oxygen-binding function in the nodule. P. vulgar is seeds 'Contender' were mutagenized with gamma-rays or ethyl methane sulfonate. Among 1400 M2 offspring derived from the treated plants screened for Lb variability, no mutation in Lb was detected. These observations are consistent with a single active gene for Lb in Phaseolus. Fifty accessions from the genus Phaseolus were screened for Lb variation. P. acutifolius ssp. contained a single Lb component which had a different electrophoretic mobility compared to that in P. vulgar is. P. filiformis showed two Lb bands, both different from P. vulgaris, while P. lunatus was found to contain two major and probably three minor Lb components. These data represent the first report of intra-generic Lb variability in Phaseolus. Using embryo rescue technique, hybrids were produced from P. vulgar is x P. acutifolius ssp. and P. vulgar is x P. filiformis crosses. Rooted cuttings from each hybrid and its parents were used to estimate the nitrogen fixation (acetylene reduction) rate and to determine the total amount of nitrogen accumulated during the growth period. The P. vulgaris x P. filiformis hybrids had a significantly higher nitrogen fixation rate and accumulated more nitrogen than either parent. The P. filiformis parent performed better than the P. vulgar is parent but only in the acetylene reduction test. One of the P. vulgaris x P. acutifolius hybrids also fixed nitrogen at a higher rate than either parent. Leghemoglobin components from a P. filiformis hybrid and from P. lunatus 'Lima Hendersons’ were isolated by cellulose ion-exchange chromatography. The N-terminal amino acid sequence for peak II of the P. filiformis hybrid and peaks I and II from P. lunatus nodules was determined. The first 37 amino acids of each component were identical to the published sequence for P. vulgaris. Mobility and/or functional differences are likely to be found in the sequence nearer the location of the heme cavity. The data of this investigation support the contention that Lb heterogeneity is functional and constitutes an adaptation for more effective nitrogen fixation.

Text

2010-10-14

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http://hdl.handle.net/2429/29182

Plant Science

University of British Columbia

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