© Benaki Phytopathological Institute
Venieraki
et al.
60
Exploring endophytes for sustainable
and enhanced production of secondary
metabolites
The discovery of endophytic fungi ca-
pable of producing the same bioactive
compounds as their host medicinal plant
has raised the expectation that these com-
pounds could be produced in large scale
through fermentation processes, thus
meeting the growing demand of the mar-
ket, while relieving the dependence on
their respective endangered host plants
for the metabolites. However, this expecta-
tion remains hampered primarily by the low
yields as well as the attenuation of metabo-
lites production after sub-culturing of fungi
(Kusari
et al
., 2011; Kumara
et al
., 2014; El-Eli-
mat
et al
., 2014). The reasons for the attenua-
tion could be attributed to factors that stem
from loss of presumed signals provided by
the host or co-existing endophytes, result-
ing in the silencing of genes in axenic mon-
ocultures (Sachin
et al.
, 2013).
Passage of attenuated CPT-producing
endophytic fungi from the host plants re-
stored CPT production in the re-isolated en-
dophytic fungi (Vasanthakumari
et al
., 2015)
suggesting that a certain critical signaling
may be necessary for the fungus to main-
tain its endogenous production. Co-culti-
vation studies of taxol producing fungus
Paraconiothyrium
SSM001 with endophyt-
ic fungi isolated from
Taxus
tree revealed
an eightfold increase in fungal Taxol pro-
duction from SSM001 (Soliman and Raizada,
2013). Co-cultivation of the endophytic fun-
gus
Fusarium tricinctum
with the bacterium
Bacillus subtilis
, led to an up to 78-fold en-
hancement in the accumulation of the con-
stitutively present fungal metabolites (Ola
et al
., 2013). Co-cultivation (mixed fermenta-
tion) under optimized conditions of the two
CPT-producing fungal species
Colletotri-
chum fruticola
and
Corynespora cassiicola
isolated from the same host tree
N. nimmo-
niana
enhanced the yield of produced CPT
(Bhalkar
et al
., 2016).
Epigenetic modifications using chemical
inhibitors have also been found to be effec-
tive in stimulating the transcription of atten-
uated biosynthetic gene clusters of endo-
phytic fungi (Vasanthakumari
et al
., 2015;
Magotra
et al
., 2017), thereby resulting in
the enhancement of the production of de-
sired secondary metabolites. Bioprocess en-
gineering strategies such as manipulation
of media and culture conditions, co-culture
condition, epigenetic modulation, elicitor
and or chemical induction, mixed fermen-
tation, and fermentation technology, have
been proven promising in alleviating to
some extent these obstacles (Venugopalan
and Srivastava, 2015).
Upon availability of the endophytic fun-
gal genomes, the putative genes encoding
the enzymes involved in the biosynthesis
of bioactive compounds could be identi-
fied and their function could be verified
through transcriptomic, proteomic and me-
tabolomic, RNA interference, gene knock-
out, and gene over expression. Genome
editing technologies implemented for met-
abolic engineering of filamentous fungi may
be applied for triggering the biosynthesis
of metabolites. Alternatively, the identified
biosynthetic pathway of the correspond-
ing bioactive compounds can be assem-
bled, engineered and then introduced in
other genetically tractable microorganisms
to increase their yields (El-Sayed
et al
., 2017;
Wakai
et al
., 2017).
Medicinal plant endophytes in Greece
Greece is endowed with a rich biodiver-
sity of medicinal plant species with a long
tradition in herbal medicines, and their
complex endomicrobiome may be directly
and indirectly responsible for the produc-
tion of a wealth of explored and unexplored
bioactive compounds. Thus, it is expected
that many new or known products for medi-
cine may emerge through the exploration of
the endophytes of these medicinal plants.
We are currently isolating fungal and bac-
terial endophytes from indigenous medici-
nal plant species in the genera such as
Fritil-
laria
,
Hypericum
,
Teucrium
,
Calendula
,
Salvia
