270
B. N. Aloo et al.
Endophytic N 2 -fixing rhizobacteria are now emerging as one of the most efficient
and environmentally sustainable approaches for increasing N acquisition for crops
(Suman et al. 2016; Defez et al. 2017). Their potential has been illustrated in many
studies, examples of which are provided in Table 12.1. It is proposed that endophytic
N 2- fixers can enable plants to survive under N-limiting conditions better than their
external rhizobacterial counterparts (Gupta et al. 2013). For instance, the N 2 fixation
process requires energy to reduce the bonds in the N 2 molecules and the endophytic
N 2- fixers can obtain the required energy from plant host tissues (Olivares et al. 2013).
Similarly, their internal plant habitats offer favorable micro-aerobic environments
that are more conducive to the nitrogenase enzyme complex that catalyzes the N 2
fixation process (Doty et al. 2016).
Although all diazotrophs are important for providing N to plants and enhancing
their growth (Kumar et al. 2017), endophytic rhizobacteria not only provide the fixed
N to their plant hosts more directly but also more efficiently (Suman et al. 2016; Lata
et al. 2019). This is because the BNF process is largely mediated by the nif and fix
genes whose transcriptions are primarily induced under low-oxygen conditions as
in the interior plant tissues parts that host the endophytes (Bhagya and Rajkumar
2017). Literature suggests that the fixed N 2 is converted to NH 4
+ in the bacterial
cytoplasm and subsequently excreted into the host cytoplasm (Mia and Shamsuddin
2010), where it is assimilated into glutamate and transported in the xylem from the
plant roots to their shoots as the major source of organic N (Nawaz et al. 2017). Thus
the endophytic diazotrophs can release NH 4 easily and directly into the plant host cell
cytoplasm. Although some N 2 -fixers can assimilate the produced NH 4 into organic
compounds, most N 2 -fixing strains have unique regulatory mechanisms to secrete
the NH 4 outside their cells by diffusion instead of assimilating it (Day et al. 2001).
This has a significant implication on the utilization of rhizobacteria as biofertilizers
since the absence of this negative feedback mechanism can allow the nitrogenase
enzyme complex to produce NH 3 continuously for plant uptake.
The symbiotic N 2 -fixing rhizobia inhabiting in the cortial tissues of roots have
been researched for several decades (Santoyo et al. 2016). The inoculation of crops
and agricultural fields with such PGPR can help to maintain the N levels (Daman
et al. 2016). For instance, about 1–2 kg N ha
−1 day
−1 can be obtained for all legumes
by rhizobial N 2 fixation alone (Lesueur et al. 2016). Apart from legumes, rhizobia
have also been found living endophytically with rice, sweet corn, cotton, maize, bean,
barley, and wheat among others as outlined in the review by Bhagya and Rajkumar
(2017). This shows that there is a great possibility that several rhizobial interactions
can similarly enhance N acquisition with non-leguminous crops. For instance, the
discovery of N 2 -fixing endophytic rhizobacteria in sugarcane (Ohyama et al. 2014;
Mus et al. 2016) and cereals (Annapurna et al. 2004; Suman et al. 2016) especially
sparked a substantial interest. Rhizobia have also been found to infect Brassica
campestris and enhance its growth by increasing its N content (Chandra et al. 2007).
Gluconacetobacter diazotrophicus which is the main endophytic diazotroph in sugarcane can fix up to 150 kg N ha
−1 year
−1 (Muthukumarasamy et al. 2005), and previous
in vivo studies on this species also showed that it can promote the growth, germination, height, and nutrient uptake of sugarcane (Suman et al. 2008). Recently, a study
B. N. Aloo et al.
Endophytic N 2 -fixing rhizobacteria are now emerging as one of the most efficient
and environmentally sustainable approaches for increasing N acquisition for crops
(Suman et al. 2016; Defez et al. 2017). Their potential has been illustrated in many
studies, examples of which are provided in Table 12.1. It is proposed that endophytic
N 2- fixers can enable plants to survive under N-limiting conditions better than their
external rhizobacterial counterparts (Gupta et al. 2013). For instance, the N 2 fixation
process requires energy to reduce the bonds in the N 2 molecules and the endophytic
N 2- fixers can obtain the required energy from plant host tissues (Olivares et al. 2013).
Similarly, their internal plant habitats offer favorable micro-aerobic environments
that are more conducive to the nitrogenase enzyme complex that catalyzes the N 2
fixation process (Doty et al. 2016).
Although all diazotrophs are important for providing N to plants and enhancing
their growth (Kumar et al. 2017), endophytic rhizobacteria not only provide the fixed
N to their plant hosts more directly but also more efficiently (Suman et al. 2016; Lata
et al. 2019). This is because the BNF process is largely mediated by the nif and fix
genes whose transcriptions are primarily induced under low-oxygen conditions as
in the interior plant tissues parts that host the endophytes (Bhagya and Rajkumar
2017). Literature suggests that the fixed N 2 is converted to NH 4
+ in the bacterial
cytoplasm and subsequently excreted into the host cytoplasm (Mia and Shamsuddin
2010), where it is assimilated into glutamate and transported in the xylem from the
plant roots to their shoots as the major source of organic N (Nawaz et al. 2017). Thus
the endophytic diazotrophs can release NH 4 easily and directly into the plant host cell
cytoplasm. Although some N 2 -fixers can assimilate the produced NH 4 into organic
compounds, most N 2 -fixing strains have unique regulatory mechanisms to secrete
the NH 4 outside their cells by diffusion instead of assimilating it (Day et al. 2001).
This has a significant implication on the utilization of rhizobacteria as biofertilizers
since the absence of this negative feedback mechanism can allow the nitrogenase
enzyme complex to produce NH 3 continuously for plant uptake.
The symbiotic N 2 -fixing rhizobia inhabiting in the cortial tissues of roots have
been researched for several decades (Santoyo et al. 2016). The inoculation of crops
and agricultural fields with such PGPR can help to maintain the N levels (Daman
et al. 2016). For instance, about 1–2 kg N ha
−1 day
−1 can be obtained for all legumes
by rhizobial N 2 fixation alone (Lesueur et al. 2016). Apart from legumes, rhizobia
have also been found living endophytically with rice, sweet corn, cotton, maize, bean,
barley, and wheat among others as outlined in the review by Bhagya and Rajkumar
(2017). This shows that there is a great possibility that several rhizobial interactions
can similarly enhance N acquisition with non-leguminous crops. For instance, the
discovery of N 2 -fixing endophytic rhizobacteria in sugarcane (Ohyama et al. 2014;
Mus et al. 2016) and cereals (Annapurna et al. 2004; Suman et al. 2016) especially
sparked a substantial interest. Rhizobia have also been found to infect Brassica
campestris and enhance its growth by increasing its N content (Chandra et al. 2007).
Gluconacetobacter diazotrophicus which is the main endophytic diazotroph in sugarcane can fix up to 150 kg N ha
−1 year
−1 (Muthukumarasamy et al. 2005), and previous
in vivo studies on this species also showed that it can promote the growth, germination, height, and nutrient uptake of sugarcane (Suman et al. 2008). Recently, a study