Understanding the expression dynamics of symbiont rhizobial nifH and nitrogen assimilatory NR and GS genes in dry bean (Phaseolus vulgaris L.) genotypes at various growth stages

Abstract

Abstract Selection of dry bean (Phaseolus vulgaris L.) cultivars based on their symbiotic nitrogen fixation (SNF) could reduce the fertilizer N requirement. The nifH gene in the bacterial symbiont facilitates the organic N assimilation by N‐assimilatory enzymes, nitrate reductase (NR), and glutamine synthetase (GS). Fertilizer‐N recommendation of legumes could be adjusted with the relative potential of bacterial nifH gene expression. We hypothesized that quantification of bacterial and plant marker gene expression could predict the amount of total N‐assimilation. We studied the expression dynamics of dry bean NR and GS along with the Rhizobium phaseoli nifH gene in four dry bean commercial cultivars La Paz, Lariat, Windbreaker, and ND‐307 at the third trifoliate (V3) and late flowering (R2) stages using quantitative reverse transcription polymerase chain reaction. Association between gene expression and the amount of SNF using 15N stable isotope dilution was determined. At V3 stage, Lariat had a highest nifH expression, followed by ND‐307, La Paz, and Windbreaker. At R2 stage, ND‐307 had the highest nifH expression followed by Windbreaker, La Paz, and Lariat. At R2 stage, the nifH gene expression was upregulated, whereas NR and GS gene expressions were downregulated compared with the V3 stage, indicating dry bean might be benefitted from late‐season N application. Thus, cultivar dependent nifH, NR, and GS gene expression varied depending on the growth stage. The amount of SNF was significantly correlated (r2 = 0.82) with the relative normalized nifH gene expression in experimental cultivars, validating this molecular assay as a tool to screen SNF potential of dry bean cultivars.

Keywords

• glutamine synthetase (GS)
• nifH
• nitrogen assimilation
• nitrate reductase (NR)
• Rhizobium
• symbiotic nitrogen fixation