6 Microbial Endophytes: New Direction to Natural Sources
135
(2020) described Arthrobacter agilis UMCV2 and Bacillus methylotrophicus M496 as bacteria that stimulate plant growth in vitro. A. agilis UMCV2 behaves as
an endophytic bacterium of Medicago truncatula (Aviles-Garcia et al. 2016) and
promotes plant growth by emission of dimethylhexadecylamine, a volatile compound
that induces plant iron uptake mechanisms and systemic resistance (Raya-González
et al. 2017), and modulates plant morphogenesis (Castulo-Rubio et al. 2015; VázquezChimalhua et al. 2019). B. methylotrophicus M4-96 promotes growth in Arabidopsis
and strawberries by emission of the phytostimulant volatile compound acetoin, and
the production of the plant growth regulators (PGR), indole acetic acid and gibberellic
acid (Pérez-Flores et al. 2017; Vicente-Hernández et al. 2019). Because, endophytes
reside within plants and are continuously interacting with their hosts, it is conceivable
that plants would have a substantial influence on the in planta metabolic processes
of the endophytes. For example plants homoserine and asparagine act as host signals
to activate expression of a lethal gene in virulent strains of Nectria hematococca that
is only expressed in planta. Furthermore, expression of the gene cluster for lolitrem
biogenesis in endophytic Neotyphodium lolii resident in perennial ryegrass is high in
planta, but low to undetectable in fungal cultures grown in vitro, lending support to
the notion that plant signaling is required to induce expression (Young et al. 2006).
Another convincing example is that of the symbiotic association between dicotyledonous plants (Convolvulaceae) and clavicipitaceous fungi leading to synthesis of
ergoline alkaloids by the fungus, and question the origin of these compounds in plants
(Leistner and Steiner 2009). Recently, it was found that a camptothecin-producing
endophyte, F. solani isolated from C. acuminata, could indigenously produce the
precursors of camptothecin. However, a host plant enzyme absent in the fungus,
strictosidine synthase, was employed in planta for the key step in producing camptothecin. This was the main reason for substantial reduction of camptothecin production on subculturing under axenic conditions. Such plant-fungus interactions compel
reconsidering whether horizontal gene transfer (plant to endophyte genome or vice
versa) is the only mechanism by virtue of which endophytes produce associated plant
compounds (Kusari et al. 2012).
The production of natural products by endophytic fungi, once considered exclusive
to plants, also raises intriguing questions regarding the original source organism.
Actually, it is possible that various so-called ‘plant metabolites’ could in fact be the
biosynthetic products of their endophytes. An important example is the production of
the very potent antitumor maytansinoid ansamitocin, originally isolated from higher
plants, by the actinobacteria Actinosynnema pretiosum ssp. auranticum (Yu et al.
2002). This study substantiated the possibility that the true biosynthetic source of
the maytansinoid backbone could be a bacterial endophyte. Although, horizontal
gene transfer may explain the production of maytansinoids by plants, a more likely
scenario is the production of maytansinoids by symbionts. Because, the interaction
between endophytic fungi with the host plant and other endophytes remains versatile,
even slight variations in the in vitro cultivation conditions can impact the kind and
range of secondary metabolites they produce. It is well established that the metabolic
processes of microorganisms are critically dependent on the culture parameters. This
is especially exemplified by endophytes because their range of interactions is so
135
(2020) described Arthrobacter agilis UMCV2 and Bacillus methylotrophicus M496 as bacteria that stimulate plant growth in vitro. A. agilis UMCV2 behaves as
an endophytic bacterium of Medicago truncatula (Aviles-Garcia et al. 2016) and
promotes plant growth by emission of dimethylhexadecylamine, a volatile compound
that induces plant iron uptake mechanisms and systemic resistance (Raya-González
et al. 2017), and modulates plant morphogenesis (Castulo-Rubio et al. 2015; VázquezChimalhua et al. 2019). B. methylotrophicus M4-96 promotes growth in Arabidopsis
and strawberries by emission of the phytostimulant volatile compound acetoin, and
the production of the plant growth regulators (PGR), indole acetic acid and gibberellic
acid (Pérez-Flores et al. 2017; Vicente-Hernández et al. 2019). Because, endophytes
reside within plants and are continuously interacting with their hosts, it is conceivable
that plants would have a substantial influence on the in planta metabolic processes
of the endophytes. For example plants homoserine and asparagine act as host signals
to activate expression of a lethal gene in virulent strains of Nectria hematococca that
is only expressed in planta. Furthermore, expression of the gene cluster for lolitrem
biogenesis in endophytic Neotyphodium lolii resident in perennial ryegrass is high in
planta, but low to undetectable in fungal cultures grown in vitro, lending support to
the notion that plant signaling is required to induce expression (Young et al. 2006).
Another convincing example is that of the symbiotic association between dicotyledonous plants (Convolvulaceae) and clavicipitaceous fungi leading to synthesis of
ergoline alkaloids by the fungus, and question the origin of these compounds in plants
(Leistner and Steiner 2009). Recently, it was found that a camptothecin-producing
endophyte, F. solani isolated from C. acuminata, could indigenously produce the
precursors of camptothecin. However, a host plant enzyme absent in the fungus,
strictosidine synthase, was employed in planta for the key step in producing camptothecin. This was the main reason for substantial reduction of camptothecin production on subculturing under axenic conditions. Such plant-fungus interactions compel
reconsidering whether horizontal gene transfer (plant to endophyte genome or vice
versa) is the only mechanism by virtue of which endophytes produce associated plant
compounds (Kusari et al. 2012).
The production of natural products by endophytic fungi, once considered exclusive
to plants, also raises intriguing questions regarding the original source organism.
Actually, it is possible that various so-called ‘plant metabolites’ could in fact be the
biosynthetic products of their endophytes. An important example is the production of
the very potent antitumor maytansinoid ansamitocin, originally isolated from higher
plants, by the actinobacteria Actinosynnema pretiosum ssp. auranticum (Yu et al.
2002). This study substantiated the possibility that the true biosynthetic source of
the maytansinoid backbone could be a bacterial endophyte. Although, horizontal
gene transfer may explain the production of maytansinoids by plants, a more likely
scenario is the production of maytansinoids by symbionts. Because, the interaction
between endophytic fungi with the host plant and other endophytes remains versatile,
even slight variations in the in vitro cultivation conditions can impact the kind and
range of secondary metabolites they produce. It is well established that the metabolic
processes of microorganisms are critically dependent on the culture parameters. This
is especially exemplified by endophytes because their range of interactions is so