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

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 (R), (3) inoculated with commercial Rhizobium inoculant Nodulator (R), 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.