6 Microbial Endophytes: New Direction to Natural Sources
139
effect of phytopathogens, facilitating the accumulation of heavy metals (Glick 2010;
Guerrero-Zúñiga et al. 2020).
6.9 Antimicrobial Activity of Endophytes
Endophytes are recognized as valuable sources of biologically active secondary
metabolites and various structures. However, most of their gene clusters are still
silent, indicating a greater biosynthetic potential for the production of diverse metabolites. Several methods have been developed to activate biosynthetically silent gene
clusters in order to produce hidden natural products. One of them is co-cultivation
that is recognized as a powerful way to enhance chemical diversity. A reason why the
structural diversity of natural products is expanding is the interspecific interference
among microorganisms, especially those that reside in a similar ecological environment (Wang et al. 2019). Trait propagation, e.g., antibiotic production, through
activating biosynthetically silent gene clusters is typically associated with interspecific interference. Wang et al. (2019) isolated, detected, and biologically assessed
a new derivative of ergosterol (23R-hydroxy-(20Z,24R)-ergosta-4,6,8(14),20(22)tetraen3-one) and a biosynthetically known compound from the co-cultivation of
the endophytic fungus Pleosporales sp. F46 and the endophytic bacterium Bacillus
wiedmannii sp., both residing in the medicinal plant Mahonia fortune.
This is the first ergosterol derivative with a double Z20 bond in the side chain
and shows strong antibacterial activity. Peanut endophyte Bacillus velezensis LDO2
is highly capable of synthesizing various antimicrobial metabolites and shows
strong antagonistic activities against fungal and bacterial pathogens of peanut. The
gene clusters responsible for their antifungal metabolites (fengycin, surfactin, and
bacilysin) and antibacterial metabolites (butirosin, bacillaene, difficidin, macrolactin,
surfactin, bacilysin) were detected (Chen et al. 2019). Li et al. (2016) investigated
the antifungal activity of crude extracts of 93 endophytic fungi, isolated from five
kinds of tissues of Zanthoxylum bungeanum on F. sambucinum and P. zanthoxyli.
Another study on strawberry plants revealed that the soil application of endophyte
in the strawberry medium improved growth conditions and viability of the plants
significantly (Fig. 6.5).
Aboobaker et al. (2019) were the first to report Penicillium skrjabinii as an endophyte that synthesizes dibutyl phthalate. This compound contributes to endophytic–
host plant interactions and has an antimicrobial effect against S. aureus and E. coli.
Fructose and peptone are the best sources of carbon and nitrogen for the production
of most antimicrobial metabolites of PG159. Biological metabolites synthesized
by bacteria are generally regarded as bio-pesticides. Bacterial metabolites contain
bioactive compounds with antagonistic activities such as pyrrolnitrin, phenazine,
cepabactin, and other unknown compounds. Pyrrolnitrin is produced as a fungicide
against soil-borne fungal pathogens, such as Rhizoctonia solani that is the cause of
seedling death (Kim et al. 2019). Cui et al. (2020) reported that the healthy potatoes
can carry endophytic bacterium which have the antagonistic ability against potato
139
effect of phytopathogens, facilitating the accumulation of heavy metals (Glick 2010;
Guerrero-Zúñiga et al. 2020).
6.9 Antimicrobial Activity of Endophytes
Endophytes are recognized as valuable sources of biologically active secondary
metabolites and various structures. However, most of their gene clusters are still
silent, indicating a greater biosynthetic potential for the production of diverse metabolites. Several methods have been developed to activate biosynthetically silent gene
clusters in order to produce hidden natural products. One of them is co-cultivation
that is recognized as a powerful way to enhance chemical diversity. A reason why the
structural diversity of natural products is expanding is the interspecific interference
among microorganisms, especially those that reside in a similar ecological environment (Wang et al. 2019). Trait propagation, e.g., antibiotic production, through
activating biosynthetically silent gene clusters is typically associated with interspecific interference. Wang et al. (2019) isolated, detected, and biologically assessed
a new derivative of ergosterol (23R-hydroxy-(20Z,24R)-ergosta-4,6,8(14),20(22)tetraen3-one) and a biosynthetically known compound from the co-cultivation of
the endophytic fungus Pleosporales sp. F46 and the endophytic bacterium Bacillus
wiedmannii sp., both residing in the medicinal plant Mahonia fortune.
This is the first ergosterol derivative with a double Z20 bond in the side chain
and shows strong antibacterial activity. Peanut endophyte Bacillus velezensis LDO2
is highly capable of synthesizing various antimicrobial metabolites and shows
strong antagonistic activities against fungal and bacterial pathogens of peanut. The
gene clusters responsible for their antifungal metabolites (fengycin, surfactin, and
bacilysin) and antibacterial metabolites (butirosin, bacillaene, difficidin, macrolactin,
surfactin, bacilysin) were detected (Chen et al. 2019). Li et al. (2016) investigated
the antifungal activity of crude extracts of 93 endophytic fungi, isolated from five
kinds of tissues of Zanthoxylum bungeanum on F. sambucinum and P. zanthoxyli.
Another study on strawberry plants revealed that the soil application of endophyte
in the strawberry medium improved growth conditions and viability of the plants
significantly (Fig. 6.5).
Aboobaker et al. (2019) were the first to report Penicillium skrjabinii as an endophyte that synthesizes dibutyl phthalate. This compound contributes to endophytic–
host plant interactions and has an antimicrobial effect against S. aureus and E. coli.
Fructose and peptone are the best sources of carbon and nitrogen for the production
of most antimicrobial metabolites of PG159. Biological metabolites synthesized
by bacteria are generally regarded as bio-pesticides. Bacterial metabolites contain
bioactive compounds with antagonistic activities such as pyrrolnitrin, phenazine,
cepabactin, and other unknown compounds. Pyrrolnitrin is produced as a fungicide
against soil-borne fungal pathogens, such as Rhizoctonia solani that is the cause of
seedling death (Kim et al. 2019). Cui et al. (2020) reported that the healthy potatoes
can carry endophytic bacterium which have the antagonistic ability against potato