3 Microbial Endophytes: Sustainable Approach …
51
interesting to note that the same bacterium was able to prime two different pathways
to confer resistance to the different pathogens (Conn et al. 2008).
Mehta et al. (2014) reported that phosphate solubilizing endophytic bacteria
showed maximum antagonistic effect against S. rolfsii followed by F. oxysporum,
D. necatrix, P. aphanidermatum, and Phytophthora sp. by producing various antifungal compounds such chitinase, proteases, pectinase, and the antibiotic lipopeptides surfactin, fengycin, and iturin A. Shabanamol et al. (2018) reported that
three diazotrophic isolates, showed in vitro antifungal activities against rice sheath
blight pathogen R. solani. All the isolates produced IAA, GA, and cytokinins.
Morphological, physiological, biochemical characteristics, and 16S rDNA sequence
analysis identified the isolates as Lysinibacillus sphaericus, Klebsiella pneumonia,
and Bacillus cereus. Endophytic bacteria Pseudomonas stutzeri strain E25 and
Stenotrophomonas maltophilia strain CR71 produced volatile compound dimethyl
disulfide, which exhibited significant antifungal activity against Botrytis cinerea
(Rojas-Solis et al. 2018). Recently, Manganyi et al. (2019) determined the antimicrobial metabolites produced by endophytic fungi Sceletium tortuosum L. The
endophytic fungi produced secondary metabolites that displayed a narrow spectrum of activity against the bacterial strains. None of the fungal extracts inhibited
the growth of Enterococcus faecalis (ATCC S1299) and Enterococcus gallinarum
(ATCC 700425) while Bacillus cereus (ATCC 10876) was the most susceptible agent
against the fungal extracts. In general, Fusarium oxysporum (GG 008) displayed
significance because it was linked to high levels of 5-hydroxymethylfurfural (HMF)
and octadecanoic acid as revealed by GC-MS.
3.6.4 Endophytes in ACC Deaminase and Stress
Amelioration
As a consequence of the presence of an ACC deaminase-containing organisms, a plant
that has been exposed to either biotic or abiotic stress conditions may be partially
or even completely protected from the ethylene inhibition of plant growth (Glick
2014). Thus, ACC deaminase-containing microbes effectively protect against growth
inhibition by flooding, high salt, drought, against fungal and bacterial pathogens,
nematode damage, the presence of high levels of metals and organic contaminants,
as well as low-temperature stress condition (Khandelwal and Sindhu 2012; Glick
2015). Involvement of ACC deaminase in endophytic plant growth promotion was
reported in Burkholderia phytofirmans PsJN (Sessitsch et al. 2005). The mutants of
this bacterium lacking ACC deaminase activity were no longer able to promote canola
seedling root elongation (Sun et al. 2009). Besides, two of these ACC deaminaseproducing endophytic PGPB (compared to ACC deaminase minus mutants of the
same strains) have also been tested for the ability to promote tomato plant growth in
the presence of very high levels of salt and to delay the senescence of mini carnation cut flowers (Ali et al. 2012). ACC deaminase-producing endophytic PGPB were
51
interesting to note that the same bacterium was able to prime two different pathways
to confer resistance to the different pathogens (Conn et al. 2008).
Mehta et al. (2014) reported that phosphate solubilizing endophytic bacteria
showed maximum antagonistic effect against S. rolfsii followed by F. oxysporum,
D. necatrix, P. aphanidermatum, and Phytophthora sp. by producing various antifungal compounds such chitinase, proteases, pectinase, and the antibiotic lipopeptides surfactin, fengycin, and iturin A. Shabanamol et al. (2018) reported that
three diazotrophic isolates, showed in vitro antifungal activities against rice sheath
blight pathogen R. solani. All the isolates produced IAA, GA, and cytokinins.
Morphological, physiological, biochemical characteristics, and 16S rDNA sequence
analysis identified the isolates as Lysinibacillus sphaericus, Klebsiella pneumonia,
and Bacillus cereus. Endophytic bacteria Pseudomonas stutzeri strain E25 and
Stenotrophomonas maltophilia strain CR71 produced volatile compound dimethyl
disulfide, which exhibited significant antifungal activity against Botrytis cinerea
(Rojas-Solis et al. 2018). Recently, Manganyi et al. (2019) determined the antimicrobial metabolites produced by endophytic fungi Sceletium tortuosum L. The
endophytic fungi produced secondary metabolites that displayed a narrow spectrum of activity against the bacterial strains. None of the fungal extracts inhibited
the growth of Enterococcus faecalis (ATCC S1299) and Enterococcus gallinarum
(ATCC 700425) while Bacillus cereus (ATCC 10876) was the most susceptible agent
against the fungal extracts. In general, Fusarium oxysporum (GG 008) displayed
significance because it was linked to high levels of 5-hydroxymethylfurfural (HMF)
and octadecanoic acid as revealed by GC-MS.
3.6.4 Endophytes in ACC Deaminase and Stress
Amelioration
As a consequence of the presence of an ACC deaminase-containing organisms, a plant
that has been exposed to either biotic or abiotic stress conditions may be partially
or even completely protected from the ethylene inhibition of plant growth (Glick
2014). Thus, ACC deaminase-containing microbes effectively protect against growth
inhibition by flooding, high salt, drought, against fungal and bacterial pathogens,
nematode damage, the presence of high levels of metals and organic contaminants,
as well as low-temperature stress condition (Khandelwal and Sindhu 2012; Glick
2015). Involvement of ACC deaminase in endophytic plant growth promotion was
reported in Burkholderia phytofirmans PsJN (Sessitsch et al. 2005). The mutants of
this bacterium lacking ACC deaminase activity were no longer able to promote canola
seedling root elongation (Sun et al. 2009). Besides, two of these ACC deaminaseproducing endophytic PGPB (compared to ACC deaminase minus mutants of the
same strains) have also been tested for the ability to promote tomato plant growth in
the presence of very high levels of salt and to delay the senescence of mini carnation cut flowers (Ali et al. 2012). ACC deaminase-producing endophytic PGPB were