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

Akter, Z., Pageni, B. B., Lupwayi, N. Z. and Balasubramanian, P. M. 2014. Biological nitrogen fixation andnifH gene expression in dry beans (Phaseolus vulgarisL.). Can. J. Plant Sci. 94: 203–212. Dry bean (Phaseolus vulgaris L.) is one of the least efficient nitrogen (N₂) fixers among legumes (approximately 30 to 50 kg N ha⁻¹). To identify efficient N₂ fixers, 22 dry bean genotypes were screened for symbiotic N₂-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 N₂-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 N₂-fixed (0 to 129 mg N plant⁻¹). Genotypes GH–196, FR 266 and AC Redbond fixed the most N₂ (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 N₂-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 N₂-fixation capacity of GH-196. After confirmation in the field, dry bean genotypes with improved N₂-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 N₂-fixation potential.