Biological nitrogen fixation and nif H gene expression in dry beans ( Phaseolus vulgaris L.)

Akter, Z., Pageni, B. B., Lupwayi, N. Z. and Balasubramanian, P. M. 2014. Biological nitrogen fixation and nif H gene expression in dry beans ( Phaseolus vulgaris L.). Can. J. Plant Sci. 94: 203–212. Dry bean (Phaseolus vulgaris L.) is one of the least efficient nitrogen (N 2 ) fixers among legumes (approximately 30 to 50 kg N ha −1 ). To identify efficient N 2 fixers, 22 dry bean genotypes were screened for symbiotic N 2 -fixation in potted low-N soil in the greenhouse under four treatments: (1) uninoculated, (2) inoculated with commercial Rhizobium leguminosarum bv. phaseoli inoculant Nitrastik-D ® , (3) inoculated with commercial Rhizobium inoculant Nodulator ® , and (4) fertilizer N added. Expression of nitrogenase reductase (nifH) gene in selected uninoculated genotypes (nodulated by indigenous rhizobia) was quantified by reverse transcription polymerase chain reaction (RT-PCR) to know if expression of this gene was an indicator of N 2 -fixation. Nitrogen fixation potential varied among the genotypes and between the rhizobia inoculants. Genotypes differed in shoot (5.17 to 8.39 g plant −1 ) and root (1.71 to 6.44 g plant −1 ) dry weights, nodule number (0 to 155 nodules plant −1 ) and mass (0.055 to 0.250 mg plant −1 ), and N 2 -fixed (0 to 129 mg N plant −1 ). Genotypes GH–196, FR 266 and AC Redbond fixed the most N 2 (60 to 112 mg N plant −1 ) with either rhizobial inoculant, and Viva fixed the highest amount (129 mg N plant −1 ) when inoculated with Nitrastik-D. In contrast, AC Black Diamond, Island, Winchester, AC Polaris, LEF 2RB and SEA 5 had low N 2 -fixing potential (3 to 23 mg N plant −1 ) in inoculated treatments. The qRT-PCR analysis indicated that nifH gene expression was 5 fold higher in GH-196 compared to the non-nodulating mutant R99, which further confirmed the high N 2 -fixation capacity of GH-196. After confirmation in the field, dry bean genotypes with improved N 2 -fixation potential identified in this study may be used as parents in breeding programs in the development of future cultivars. The nifH gene expression may be used as a rapid test to select dry beans with high N 2 -fixation potential.