3 Microbial Endophytes: Sustainable Approach …
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Recently, Adhikari and Pandey (2019) isolated endophytic fungi of genera Penicillium and Aspergillus from the roots of Taxus wallichiana. The endophytes solubilized
phosphate by utilizing the substrates, namely, calcium, aluminum, and iron phosphate
along with the production of phosphatase and phytase enzymes. Maximum phosphate solubilization and phytase activity were recorded in P. daleae, which produced
maximum calcium phytase. The phosphatase activity was higher in acidic conditions
in comparison to alkaline conditions due to release of different organic acids.
3.6 Beneficial Traits of Endophytic Microorganisms
Some of the phosphate solubilizing endophytes have also been found to possess
other beneficial traits, i.e., nitrogen fixation, hormone production, growth inhibition
of phytopathogenic fungi, stress amelioration by reduction of ethylene concentration
by the production of ACC deaminase, and solubilization of metals in the soil.
3.6.1 Nitrogen Fixation by Endophytes
The endophytic bacteria of the genera Gluconacetobacter diazotrophicus, Herbaspirillum seropedicae, H. rubrisubalbicans, Azotobacter, Beijeinckia, Methylobacterium,
and Azospirillum showed nitrogen fixation capacity in different cultures with nonleguminous plants and have been used as a prominent alternative of nitrogen fertilizers (Cavalcante et al. 2007). Endophytic bacteria increase the nitrogen availability
for their host plants. These bacteria can supply fixed atmospheric nitrogen to their
host plants by expressing nitrogenase activity (Montanez et al. 2012). Nitrogenfixing bacteria like Azoarcus sp. BH72, Azospirillum brasilense, Burkholderia spp.,
Gluconacetobacter diazotrophicus, and Herbaspirillum seropedicae have also been
reported to increase the host plant biomass by N 2 fixation under controlled conditions (Bhattacharjee et al. 2008). Strains of G. diazotrophicus have been identified living in symbiosis with sugarcane and pine plants (Carrell and Frank 2014).
Shabanamol et al. (2018) obtained nitrogen-fixing endophytic diazotrophic isolates
from surface-sterilized leaf, stem, and root samples of various rice cultivars using
Dobereiner’s semisolid N free bromothymol blue media. Associative nitrogen-fixing
endophytes perform better than rhizosphere microorganisms in enabling plants to
thrive in nitrogen-limited soil environments and promoted plant health and growth
(Hurek and Reinhold-Hurek 2003). Nitrogen-fixing endophyte Paenibacillus strain
P22 has been found in a poplar tree, and have been demonstrated to contribute
to the total nitrogen pool of the host plant (Scherling et al. 2009). Gupta et al.
(2013) reported that endophytic nitrogen-fixing bacteria may also enhance the rate
of nitrogen fixation and accumulation in plants residing in nitrogen-limited soils but
these are not as efficient as root nodule associated Rhizobium in nitrogen fixation
ability.
49
Recently, Adhikari and Pandey (2019) isolated endophytic fungi of genera Penicillium and Aspergillus from the roots of Taxus wallichiana. The endophytes solubilized
phosphate by utilizing the substrates, namely, calcium, aluminum, and iron phosphate
along with the production of phosphatase and phytase enzymes. Maximum phosphate solubilization and phytase activity were recorded in P. daleae, which produced
maximum calcium phytase. The phosphatase activity was higher in acidic conditions
in comparison to alkaline conditions due to release of different organic acids.
3.6 Beneficial Traits of Endophytic Microorganisms
Some of the phosphate solubilizing endophytes have also been found to possess
other beneficial traits, i.e., nitrogen fixation, hormone production, growth inhibition
of phytopathogenic fungi, stress amelioration by reduction of ethylene concentration
by the production of ACC deaminase, and solubilization of metals in the soil.
3.6.1 Nitrogen Fixation by Endophytes
The endophytic bacteria of the genera Gluconacetobacter diazotrophicus, Herbaspirillum seropedicae, H. rubrisubalbicans, Azotobacter, Beijeinckia, Methylobacterium,
and Azospirillum showed nitrogen fixation capacity in different cultures with nonleguminous plants and have been used as a prominent alternative of nitrogen fertilizers (Cavalcante et al. 2007). Endophytic bacteria increase the nitrogen availability
for their host plants. These bacteria can supply fixed atmospheric nitrogen to their
host plants by expressing nitrogenase activity (Montanez et al. 2012). Nitrogenfixing bacteria like Azoarcus sp. BH72, Azospirillum brasilense, Burkholderia spp.,
Gluconacetobacter diazotrophicus, and Herbaspirillum seropedicae have also been
reported to increase the host plant biomass by N 2 fixation under controlled conditions (Bhattacharjee et al. 2008). Strains of G. diazotrophicus have been identified living in symbiosis with sugarcane and pine plants (Carrell and Frank 2014).
Shabanamol et al. (2018) obtained nitrogen-fixing endophytic diazotrophic isolates
from surface-sterilized leaf, stem, and root samples of various rice cultivars using
Dobereiner’s semisolid N free bromothymol blue media. Associative nitrogen-fixing
endophytes perform better than rhizosphere microorganisms in enabling plants to
thrive in nitrogen-limited soil environments and promoted plant health and growth
(Hurek and Reinhold-Hurek 2003). Nitrogen-fixing endophyte Paenibacillus strain
P22 has been found in a poplar tree, and have been demonstrated to contribute
to the total nitrogen pool of the host plant (Scherling et al. 2009). Gupta et al.
(2013) reported that endophytic nitrogen-fixing bacteria may also enhance the rate
of nitrogen fixation and accumulation in plants residing in nitrogen-limited soils but
these are not as efficient as root nodule associated Rhizobium in nitrogen fixation
ability.