The associative and
endophytic diazotrophic bacteria naturally colonize and contribute
with fixed N to several economically important plant
species, comprising a natural system to be explored. However,
the mechanisms regulating this particular type of plant–bacteria
association are still not clear; thus, a better understanding
of the mechanisms is necessary to allow improvement and
manipulation of this association, and possibly an extension
of it to non-natural hosts.
Quantitative analyses of BNF and plant growth promotion
demonstrated that plant and bacterial genotypes are important
factors in controlling the efficiency of the association
(Carvalho et al., 2011). In this context, one challenge in this
area is the determination of the best combination of diazotrophic
bacteria and plant varieties to obtain the maximum
benefit from this association in agriculture. A huge effort
should be made to understand the molecular and genetic factors
controlling all steps of the association: recognition, colonization,
N fixation, and plant growth promotion. Several
advances came from genomic approaches, and integrative
gene expression maps are being generated for some plant species
colonized with associative and endophytic diazotrophic
bacteria. Possible regulatory mechanisms involved were identified,
and functional analyses are now necessary. Also, it is
important to determine common regulatory pathways governing
a successful association with diazotrophic bacteria, as
well as those specific to particular plant–bacteria genotypes.
N status might act as a key signal regulating and integrating
various metabolic processes that occur during association
with diazotrophic bacteria. Besides directly providing ammonium
to plants, the associative and endophytic diazotrophic
bacteria enhance N uptake of inoculated plants, an effect
that could be important for enhancing NUE. Nevertheless,
high N levels inside plants seem to signal a feedback control,
negatively regulating BNF and bacterial colonization.
High N in soil could contribute to an increase in N levels in
plants, activating this negative feedback control. Therefore, a
clear understanding of the mechanisms in N regulation during
plant interaction with associative and endophytic diazotrophic
bacteria could provide tools to maximize the benefits
for crop production.