6 Microbial Endophytes: New Direction to Natural Sources
141
6.10 Endophytic Bioactive Alkaloids
Endophytes are an active source of bioactive alkaloids that protect their host plants.
The positive effect of Epichloë endophytes on plant defense has been traditionally
attributed to fungal alkaloids. Alkaloids are nitrogen-rich compounds, four groups
of which have been detected in endophytes: ergot alkaloids (e.g., ergopeptine and
ergovaline), indole-diterpene (e.g., lolitrem B and epoxy-janthitrem), pyrrolizidine
(e.g., lolines), and pramine. All enzymes involved in their synthesis are encoded
in the fungal genome and all biosynthetic pathways are mainly defined. Alkaloid
profiles depend on the species and strain of endophyte and the amount of alkaloids
is related to plant phenological stage, plant tissue, and environmental conditions. In
addition, the effectiveness of an alkaloid defense against an invader depends on the
concentration and chemical type of the alkaloid produced by the endophyte (Bastias
et al. 2017).
Endophytic Bacillus cereus, Aranicola proteolyticus, Serratia liquefaciens,
Bacillus thuringiensis, and Bacillus licheniformis isolated from Pinellia ternata have
the ability to produce alkaloids (guanosine and inosine) in fermentation broth similar
to their host plant (Liu et al. 2015). The capsular endophyte Acinetobacter SB1B
in Opium poppy unregulated the expression of key genes for the benzylisoquinoline
alkaloid (BIA) biosynthesis except thebaine and codeine. In contrast, Marmoricola
sp. SM3B, another endophyte, could up-regulate the biosynthesis of both thebaine
and codeine. Acinetobacter and Marmoricola sp. as microbial inoculants modulated
the alkaloid producing genes in Opium poppy (Pandey et al. 2016). The impairment
of the cholinergic neurotransmission in the central nervous system is one of the bases
of memory deficit related to Alzheimer’s disease (AD), with the administration of
acetylcholinesterase (AChE) inhibitors representing the most acceptable strategy for
treating this illness. Huperzine A (Hup A) is an alkaloid, which was first isolated from
the Chinese Huperzia serrata (Thunb. ex Murray) Trev., and gained attention due to
its potent, reversible, and selective inhibition of AChE. Furthermore, the molecule
penetrates the hemato-encephalic barrier more effectively, has greater oral bioavailability and the AChE inhibitory action lasts longer than other commercial drugs
(donepezil and rivastigmine) (wang et al. 2006; Cruz-Miranda et al. 2020). For this
reason, in the last few years, there is an increased interest in the discovery of a microbial source with the potential to produce Hup A. Some reports have described fungal
endophytes that were isolated from different species of lycophytes producing Hup
A. The fungi Shiraira sp. SIf14, Cladosporium cladosporioides, and Colletotrichum
gloeosporioides ES026 were isolated from H. serrata (Shu et al. 2014), whereas the
endophytic fungi Ceriporialacerata and Hypoxylon investiens were isolated from
Phlegmariurus phlegmaria (Zhang et al. 2015; Cruz-Miranda et al. 2020). Some
endophytic fungi can produce the same bioactive compounds as their host plants,
e.g., camptothecin, hypericin, vinblastine, paclitaxel, podophyllotoxin, and diosgenin. For example, Taxomyces andreanae is an endophytic fungus isolated from
the Taxus brevifolia tree and has the potential to produce the anticancer compound
Taxol. In addition, endophytic fungi can produce the antidepressant hypericin and
141
6.10 Endophytic Bioactive Alkaloids
Endophytes are an active source of bioactive alkaloids that protect their host plants.
The positive effect of Epichloë endophytes on plant defense has been traditionally
attributed to fungal alkaloids. Alkaloids are nitrogen-rich compounds, four groups
of which have been detected in endophytes: ergot alkaloids (e.g., ergopeptine and
ergovaline), indole-diterpene (e.g., lolitrem B and epoxy-janthitrem), pyrrolizidine
(e.g., lolines), and pramine. All enzymes involved in their synthesis are encoded
in the fungal genome and all biosynthetic pathways are mainly defined. Alkaloid
profiles depend on the species and strain of endophyte and the amount of alkaloids
is related to plant phenological stage, plant tissue, and environmental conditions. In
addition, the effectiveness of an alkaloid defense against an invader depends on the
concentration and chemical type of the alkaloid produced by the endophyte (Bastias
et al. 2017).
Endophytic Bacillus cereus, Aranicola proteolyticus, Serratia liquefaciens,
Bacillus thuringiensis, and Bacillus licheniformis isolated from Pinellia ternata have
the ability to produce alkaloids (guanosine and inosine) in fermentation broth similar
to their host plant (Liu et al. 2015). The capsular endophyte Acinetobacter SB1B
in Opium poppy unregulated the expression of key genes for the benzylisoquinoline
alkaloid (BIA) biosynthesis except thebaine and codeine. In contrast, Marmoricola
sp. SM3B, another endophyte, could up-regulate the biosynthesis of both thebaine
and codeine. Acinetobacter and Marmoricola sp. as microbial inoculants modulated
the alkaloid producing genes in Opium poppy (Pandey et al. 2016). The impairment
of the cholinergic neurotransmission in the central nervous system is one of the bases
of memory deficit related to Alzheimer’s disease (AD), with the administration of
acetylcholinesterase (AChE) inhibitors representing the most acceptable strategy for
treating this illness. Huperzine A (Hup A) is an alkaloid, which was first isolated from
the Chinese Huperzia serrata (Thunb. ex Murray) Trev., and gained attention due to
its potent, reversible, and selective inhibition of AChE. Furthermore, the molecule
penetrates the hemato-encephalic barrier more effectively, has greater oral bioavailability and the AChE inhibitory action lasts longer than other commercial drugs
(donepezil and rivastigmine) (wang et al. 2006; Cruz-Miranda et al. 2020). For this
reason, in the last few years, there is an increased interest in the discovery of a microbial source with the potential to produce Hup A. Some reports have described fungal
endophytes that were isolated from different species of lycophytes producing Hup
A. The fungi Shiraira sp. SIf14, Cladosporium cladosporioides, and Colletotrichum
gloeosporioides ES026 were isolated from H. serrata (Shu et al. 2014), whereas the
endophytic fungi Ceriporialacerata and Hypoxylon investiens were isolated from
Phlegmariurus phlegmaria (Zhang et al. 2015; Cruz-Miranda et al. 2020). Some
endophytic fungi can produce the same bioactive compounds as their host plants,
e.g., camptothecin, hypericin, vinblastine, paclitaxel, podophyllotoxin, and diosgenin. For example, Taxomyces andreanae is an endophytic fungus isolated from
the Taxus brevifolia tree and has the potential to produce the anticancer compound
Taxol. In addition, endophytic fungi can produce the antidepressant hypericin and