Egener et al. (1999) noted that the presence of combined nitrogen such as ammonia has a strong impact on the expression of nif gene in most diazotrophs. Also, Vande Broek et al. (1993) estimated associative nifH expression
both qualitatively and quantitatively in A. brasilense on wheat roots through gusA fusion plasmid system. However,
as noted by Mantelin and Touraine (2004), there is no clear evidence that the expression of nif genes or active N2
fixation by PGPR will translate into measurable transfer of the fixed N to the plant.