It insects with sucking mouth parts such as

It is often easy to overlook the roles of microbes in terms of wider ecosystems, with such keen
focus on their human medicinal aspects. Not only do microbes have pivotal roles
in the maintenance of stability in
ecosystems on a large scale, but the applications of their intricate molecular
functions can often induce outstanding biotechnological progress. With a
particular focus on the insect pathogenic
fungi and plant symbiont Metarhizium acridum, this essay will aim to investigate
the role of the microbe in the ecosystem both as a pathogen and as a
non-pathogenic symbiont, while applying its mechanisms and evolution to its
potential for advances in biotechnology.

 

I will first explore how the dual life cycles of this fungi as a pathogen
and an endophyte- a symbiont of a plant- are coupled by discussing their
mechanisms of action and
evolutionary history. The mechanism of action is complex both in M. acridum’s role as an insect pathogen and endophyte. As
a pathogen, the mode of infection begins with penetration of the insect
cuticle, by cuticular degradation using employment of enzymes such as proteases
and lipases(Pedrini et al.,
2013; Barelli et al., 2015). This technique is
non-specific to M. acridum but opens the opportunity for the general family of insect-infecting fungi to infect a wider range
of insects. The pathogens enter the insect’s body by the transgression of the cuticle rather than
ingestion, meaning that insects with sucking mouth
parts such as Aphids can also be affected (Chandler, 1997). The major proteases produced
by the Metarhizium genus and used in
this way are Pr1A, a cuticle degrading subtilisin-like protease, and Pr2, a trypsin-like serine proteinase (St. Leger, Joshi and Roberts, 1998). Not only do these proteases
play a key role in penetration, but they also contribute to necessary evasion
of host defences. This can be explained through research into how they
aided evasion of the host defence by degrading key antifungal proteins in the
insect (St Leger, Nelson and Screen, 1999).

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Following this, hydrophobic
conidial spores adhere to the cuticle (Small and Bidochka, 2005). These then germinate, forming
outgrowing germ tubes and appressoria (Deising, Werner and Wernitz, 2000; Holder et al., 2007), which are then capable of
further penetration. It is the structure of these appressoria that link the roles of insect pathogenic fungi with that
of the plant symbiont. Deising et al. (2000)
identified that the appressoria present in the insect fungi showed significant
similarity to those found in plant-infecting
fungi. From this, we can speculate that there may be a morphological link
between the two, coupling their roles. The genus Metarhizium utilises the proteins MAD1 and ssgA to
facilitate adherence of the conidia onto the insect cuticle surfaces (St. Leger, Staples and Roberts, 1992a; Wang and St.
Leger, 2007a).
A similar protein, MAD2 has been
found to be responsible for adherence to plant surfaces (Wang and St. Leger, 2007). Once again, the presence of
each of these genes suggests a dual role of the organism in the different
hosts.

Once
Metarhizium has finally entered the
main body cavity- the haemocoel, expression of a collagen-like protein MCLI evades the immune system of the
insect (Wang and St Leger, 2006), allowing it to produce toxins and absorb nutrients,
depleting them to fatal levels and eventually resulting in retreat of the
fungal hyphae and mummification of the host (Small and Bidochka, 2005; Schrank and Vainstein,
2010). Once inside the haemocoel, in
order to survive the osmotic pressure of the haemolymph in the host Metarhizium must adapt and it does this
through the expression of the
osmosensor-like protein Mos1 (Wang, Duan and St. Leger, 2008).

The mechanism of colonisation of M. acridum in plants as an endophyte is
not dissimilar to that of insect infection. Once again, the successful
association depends on adherence of the fungi to the plant surface, this time
facilitated by the protein MAD2 (Nicholson and Epstein, 1991; Wang and St. Leger,
2007).
Sequencing of the Pr1 subtilisin-like
protease indicated that this gene in the Metarhizium
was, in fact, homologous to the protease At1 from grass endophyte Acremonium
typhinum (Reddy, Lam and Belanger, 1996). This alternative fungal
protease At1 functions as a
facilitator of plant colonisation by cell wall degradation. The similarity of
these two proteases indicates a similar functional role of Metarhizium, enabling it to
successfully colonise as an endophyte. So why do the plant hosts’
defence pathways not result in expulsion or death of these invasions? It has been found that fungal endophytes such as M. acridum are capable of communicating
with the plant, indicating that they are in fact not pathogens. The molecule
responsible for this, mycorrhizal factor
(Myc) induces transcriptional and
morphological changes in the plant roots, such as activation of the symbiotic
signalling pathway and increased root hair growth to raise the likelihood of contact between the fungal hyphae
and the plant roots (Maillet et al.,
2011).
By doing this prior to and during root
colonisation, the fungi are able to trick
the desired host into withdrawing its defences
and is then able to successfully carry out
symbiosis.

 

Having looked in depth at the
mechanisms of the coupled functions of M.
acridum, it is obvious that the fungus provides a multitude of beneficial
services to its ecosystem. The primary benefits to the plant hosts are simply that they acquire insect-derived nitrogen in areas where soil
nitrogen may be limited (Behie, Zelisko and Bidochka, 2012)- they are therefore able to
regain nitrogen lost to insects through herbivory. In return from the plant, M. acridum is receiving access to simple
plant carbohydrates, those which are usually very difficult to access in the
soil due to being bound into complex carbohydrates such as cellulose and
lignin. This discovery was outlined in work
by Barelli et al. (2015), where they
introduced 13CO2 to plants colonised with Metarhizium, tracking the 13C.
Through doing this, they found that Metarhizium
mutants lacking raffinose transporter gene mrt showed a reduced
competency in the rhizosphere. This not
only leads us to the suggestion that carbon acquisition by fungi from plant
hosts is critical to their symbiotic relationship, but also that the mrt is
a possible uptake route of these plant-derived
carbohydrates. The acquisition of nitrogen, however, is not the only benefit of
M. acridum symbiosis for the plant.
Increased foliage biomass in corn seeds, greater plant height and root length,
and higher dry weights of shoots and roots are a few other proven benefits (Elena et al.,
2011; Liao et al., 2014). These would obviously
contribute strongly to crop yield in agriculture, but also in terms of the natural ecosystem, herbivores
would benefit from increased availability of food sources, leaving a cascading
abundance increase on their higher trophic levels.

Alongside the beneficial
ecosystem services to the plant symbionts, we cannot ignore the negative
impacts that exclusion of vital nutrients from the insects will ensue. As previously mentioned, eventual death and mummification
of the insects is the primary consequence of their pathogenesis, and on the
large scale this could impact food webs by removal of M. acridum specific hosts such as locusts, and the broader host
ranges of other species in the genus such as M. robertsii (Barelli et al.,
2015).  Reduction in herbivory
predation by depleted insect populations at the same time as the proliferation of plant abundance and yield with
M. acridum and other similar
symbioses may mean that imbalance to food webs could lead to increases in
abundance of primary consumers, and therefore declines in their other prey
which are not impacted by M. acridum.

 

The key biotechnological
applications of M. acridum lie in the
fields of agriculture and pharmaceuticals. The
potential for individuals from the genera Metarhizium
and Beauveria to be used in the
control of insect pests in agricultural ecosystems has been known for over 100
years, leading to the approval of many different formulations of these species
to be used in protection of crops (Madelin et
al., 1963; Faria and Wraight, 2007). Leading to the increased use and approval of
these methods in pest control is a 2007 study by Kabaluk and Ericsson, in which
they compared yields of corn from samples treated with only conventional
insecticide with those treated with Metarhizium
in addition to the conventional technique. Their
findings showed that the highest yield came undoubtedly from those which had been
treated with both treatments, not only showing progress from the previously
used methods but also providing significant evidence to that aforementioned
that the Metarhizium symbiosis and
its supply of nitrogen is the key factor implementing this increased health of
the crop, rather than the removal of the insect pests, which was effectively
carried out by the insecticide originally. A conversion from these
traditional insecticidal treatments to those involving a larger amount of
natural control through Metarhizium would
have positive climatic implications, reducing run-off of toxic chemicals from
the insecticides which have potential to accumulate in organisms and also cause
damaging eutrophication in aquatic systems.

Pharmaceutical advances are another area where
biotechnology of our example pathogen is
crucial. Genomic data of Metarhizium
species compared to other fungi show enrichment of secondary metabolite gene
clusters, with 52 core genes involved in the biosynthesis
of secondary metabolites in M.acridium.
Clusters of these important genes have been shown to have the potential for exploitation in biotransformation
and biocatalysis, as well as in novel drug discovery. A study from 2014
outlined the capability of endophytes such as M. acridum synthesising important metabolites with pharmaceutical
activity when associated with their plant hosts. An example of this they gave is
Taxol, an anti-cancer drug that has been found to be present in the association
of endophytic fungi with yew trees(Garyali, Kumar
and Reddy, 2014). The implication of this is that further areas
of research are outlined, and with such
high diversity of microbes, not only endophytic fungi, present in the soil
microbiome there seem limitless
exploitations in the case of secondary metabolites in novel drug synthesis. 

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