![Root-nodule01](https://upload.wikimedia.org/wikipedia/commons/a/ac/Root-nodule01.jpg)
An endophyte is an
endosymbiont
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
, often a
bacterium
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...
or
fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...
, that lives within a
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; however, most of the endophyte/plant relationships are not well understood. Some endophytes may enhance host growth, nutrient acquisition and improve the plant's ability to tolerate abiotic stresses, such as drought and decrease biotic stresses by enhancing plant resistance to insects, pathogens and
herbivores. Although endophytic bacteria and fungi are frequently studied, endophytic archaea are increasingly being considered for their role in plant growth promotion as part of the core microbiome of a plant.
History
Endophytes were first described by the German botanist
Johann Heinrich Friedrich Link in 1809. They were thought to be plant parasitic fungi and they were later termed as "microzymas" by the French scientist Béchamp. There was a belief that plants were healthy under sterile conditions and it was not until 1887 that
Victor Galippe discovered bacteria normally occurring inside plant tissues.
Though, most of the endophytic studies reports the mutualistic relationship of bacteria and fungus, Das et al., (2019) reported about endophytic virome and their probable function in plant defense mechanisms.
Transmission
Endophytes may be transmitted either vertically (directly from parent to offspring) or horizontally (among individuals). Vertically transmitted fungal endophytes are typically considered
clonal and transmit via
fungal
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
hyphae penetrating the embryo within the host's
seed
A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s, while reproduction of the fungi through asexual
conidia or sexual spores leads to horizontal transmission, where endophytes may spread between plants in a population or community.
Symbiosis
![Plant-endophytic bacteria interactions](https://upload.wikimedia.org/wikipedia/commons/c/c6/Plant-endophytic_bacteria_interactions.webp)
Most endophyte-plant relationships are still not well understood. However, recently it was shown that endophytes are transmitted from one generation to another via seeds, in a process called vertical transmission. Endophytes and plants often engage in mutualism, with endophytes primarily aiding in the health and survival of the host plant with issues such as pathogens and disease, water stress, heat stress, nutrient availability and poor soil quality, salinity, and herbivory.
In exchange the endophyte receives carbon for energy from the plant host. Plant-microbe interactions are not strictly
mutualistic, as endophytic fungi can potentially become
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s or
saprotrophs, usually when the plant is stressed. Endophytes may become active and reproduce under specific environmental conditions or when their host plants are stressed or begin to
senesce
Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence invol ...
, thereby limiting the amount of carbon provided to the endophyte.
Endophytes may benefit host plants by preventing other pathogenic or parasitic organisms from colonizing them. Endophytes can extensively colonize plant tissues and
competitively exclude other potential pathogens. Some fungal and bacterial endophytes have proven to increase plant growth and improve overall plant hardiness.
Studies have shown that endophytic fungi grow in a very intimate interaction with their host plant cells. Fungal
hyphae have been seen growing either flattened or wedged against plant cells. This growth pattern indicates that fungal hyphae are substantially attached to the plant host's cell wall, but do not invade plant cells.
Endophytic fungal hyphae appear to grow at the same rate as their host leaves, within the intercellular spaces of the plant tissue.
The presence of certain fungal endophytes in host
meristems
The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
, leaves and reproductive structures has been shown to dramatically enhance the survival of their hosts. This enhanced survivability is largely attributed to endophytic production of
secondary metabolite
Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norma ...
s which protect against herbivory as well as increased uptake of nutrients.
Studies have also shown that during experimental circumstances endophytes contribute significantly to plant growth and fitness under light-limited conditions, and plants appear to have increased reliance on their endophytic symbiont under these conditions.
There is evidence that plants and endophytes engage in communication with each other that can aid symbiosis. For example, plant chemical signals have been shown to activate gene expression in endophytes. One example of this plant-endosymbiont interaction occurs between
dicotyledonous
The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, t ...
plants in the
Convolvulaceae
Convolvulaceae (), commonly called the bindweeds or morning glories, is a family of about 60 genera and more than 1,650 species. These species are primarily herbaceous vines, but also include trees, shrubs and herbs. The tubers of several spe ...
and
clavicipitaceous fungi. When the fungus is in the plant it synthesizes
ergoline
Ergoline is a chemical compound whose structural skeleton is contained in a variety of alkaloids, referred to as ergoline derivatives or ergoline alkaloids. Ergoline alkaloids, one being ergine, were initially characterized in ergot. Some of thes ...
alkaloids at a higher rate, compared to when it is grown apart from the plant. This supports the hypothesis that plant signaling is required in order to induce expression of endophytic secondary metabolites.
Effects on plant behavior
There are various behaviors that have been studied that resulted from endophyte symbiosis with plants. Through association with fungal endophytes, the root and shoot structures of ''Pseudotsuga menziesii'' (
Douglas-fir
The Douglas fir (''Pseudotsuga menziesii'') is an evergreen conifer species in the pine family, Pinaceae. It is native to western North America and is also known as Douglas-fir, Douglas spruce, Oregon pine, and Columbian pine. There are three va ...
) saplings in low-nutrient conditions have been shown to be elongated, as well as undergo overall biomass increases.
The proposed mechanisms behind this include high inorganic phosphate solubilization ability by the fungi as well as organic phosphate mineralization, increased mycorrhizal associations through root colonization, and enhanced nitrogen and phosphorus uptake.
Specific endophyte species can also stimulate root growth by increasing the flux of
auxin to where the endophyte is.
Additionally, various reports on endophyte interactions have shown increased photosynthetic capacities of host plants as well as improved water relations.
Improvements in water use efficiency were observed in higher concentrations and a further increase was seen in water deficit conditions.
In addition, other various physiological pathways were activated upon endophytes interactions with host plants, enabling tighter water control and further water management, which are to be the main reasons behind improved water relations.
Specifically, evidence points to endophytes producing
ABA to affect stomatal conductance as well as microbial respiration and plants recycling .
However, the specific biochemical mechanisms behind these behavioral changes are still largely unknown and lower-level signal cascades have yet to be discovered. Furthermore, while the benefits of endophyte relations are well-studied, the costs of these relations are less well understood, such as the specific carbon costs, the system of endophyte governance, and the environmental conditions that facilitate a proper plant-endophyte relationship.
In an experiment investigating the interaction between ''
Miscanthus sinensis
''Miscanthus sinensis'', the eulalia or Chinese silver grass, is a species of flowering plant in the grass family Poaceae, native to eastern Asia throughout most of China, Japan, Taiwan and Korea.
Description
It is an herbaceous perennial gras ...
'' and the plant endophyte ''
Herbaspirillum frisingense'', a roughly 20% increase in fresh biomass was observed in ''M. sinensis'' following inoculation with ''H. frisingense''.
However, unique to this experiment was the mode by which this was thought to happen. Inoculation saw an upregulation in the genes relevant to
jasmonate
Jasmonate (JA) and its derivatives are lipid-based plant hormones that regulate a wide range of processes in plants, ranging from growth and photosynthesis to reproductive development. In particular, JAs are critical for plant defense against herb ...
and
ethylene production in the plant roots, although the mechanism to this is still unknown.
Specifically, ''H. frisingense'' was shown to upregulate ethylene receptors and repress ethylene response factors, overall leading to an increase in root growth.
Additionally, ''H. frisingense'' is known to produce
indoleacetic acid (IAA), and was also shown to manage IAA genes, indicating that there is an intricate balance maintained between ethylene and IAA by ''H. frisingense''.
Diversity
Endophytic species are very diverse; only a small minority of existing endophytes have been characterized. Many endophytes are in the phyla
Basidiomycota and
Ascomycota. Endophytic fungi may be from
Hypocreales and
Xylariales
The Xylariales are an order of fungi within the class Sordariomycetes (also known as Pyrenomycetes), subdivision Pezizomycotina, division Ascomycota. It is the only order of the subclass Xylariomycetidae. Xylariales was circumscribed in 1932 ...
of the
Sordariomycetes
Sordariomycetes is a class of fungi in the subdivision Pezizomycotina (Ascomycota), consisting of 28 orders, 90 families, 1344 genera. Sordariomycetes is from the Latin sordes (filth) because some species grow in animal feces, though growth habi ...
(Pyrenomycetes) class or from the class of Loculoascomycetes. One group of fungal endophytes are the
arbuscular mycorrhizal fungi
An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ...
involving biotrophic
Glomeromycota associated with various plant species. As often with other organisms associated with plants such as
mycorrhizal fungus
A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
, endophytes gain carbon from their association with the plant host. Bacterial endophytes are polyphyletic, belonging to broad range of taxa, including α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, Firmicutes, Actinobacteria.
One or more endophytic organisms are found in nearly every land plant.
It is suggested that areas of high plant diversity such as
tropical rainforest
Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equa ...
s may also contain the highest diversity of endophytic organisms that possess novel and diverse chemical metabolites.
It has been estimated that there could be approximately 1 million endophytic fungi that exist in the world.
A diazotrophic bacterium isolated in lodgepole pines (''
Pinus contorta
''Pinus contorta'', with the common names lodgepole pine and shore pine, and also known as twisted pine, and contorta pine, is a common tree in western North America. It is common near the ocean shore and in dry montane forests to the subalpine ...
'') in British Columbia, Canada, is ''
Paenibacillus polymyxa
''Paenibacillus polymyxa'', also known as ''Bacillus polymyxa'', is a Gram-positive bacterium capable of fixing nitrogen. It is found in soil, plant tissues, marine sediments and hot springs. It may have a role in forest ecosystems and potentia ...
'', which may help its host by fixing nitrogen.
Classification
Endophytes include a wide variety of microorganisms including fungi, bacteria and viruses. There are two different means of classifying endophytes.
Systemic and non-systemic
The first method divides endophytes into two categories: systemic (true) and nonsystemic (transient). These categories are based on the endophyte's genetics, biology, and mechanism of transmission from host to host.
Systemic endophytes are defined as organisms that live within plant tissues for the entirety of its life cycle and participate in a symbiotic relationship without causing disease or harm to the plant at any point. Additionally, systemic endophytes concentrations and diversity do not change in a host with changing environmental conditions.
Non-systemic or transient endophytes on the other hand vary in number and diversity within their plant hosts under changing environmental conditions. Non-systemic endophytes have also been shown to become
pathogenic to their host plants under stressful or resource limited growing conditions.
An example of this would be ''
Colletotrichum fioriniae
''Colletotrichum fioriniae'' is a fungal plant pathogen and endophyte of fruits and foliage of many broadleaved plants worldwide. It causes diseases on agriculturally important crops, including anthracnose of strawberry, ripe rot of grapes, bi ...
'', which is an endophyte of many
temperate
In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...
broadleaved trees and shrubs, but can also be a pathogen on many fruits and some leaves.
Clavicipitaceous and non-clavicipitaceous
The second method divides fungal endophytes into four groups based on taxonomy and six other criteria: host range, host tissues colonized, ''in planta'' colonization, ''in planta'' biodiversity, mode of transmission and fitness benefits.
These four groups are divided into
clavicipitaceous endophytes (Class 1) and non-clavicipitaceous endophytes (Class 2, 3, and 4).
Class 1 endophytes are all phylogenetically related and proliferate within cool and warm season grasses. They typically colonize plant shoots where they form a systemic intercellular infection. Class 1 endophytes are mainly transmitted from host to host by vertical transmission, in which maternal plants pass fungi on to their offspring through seeds. Class 1 endophytes can further be divided into Types I, II and III. Among these three types of clavicipitaceous endophytes are different interactions with their plant hosts. These interaction range from
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
ic to symbiotic and symptomatic to
asymptomatic. Type III clavicipitaceous endophytes grow within their plant host without manifesting symptoms of disease or harming their host. Class 1 endophytes typically confer benefits on their plant host such as improving plant biomass, increasing drought tolerance and increasing the production of chemicals that are toxic and unappetizing to animals, thereby decreasing herbivory. These benefits can vary depending on the host and environmental conditions.
Non-clavicipitaceous endophytes represent a
polyphyletic
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
group of organisms. Non-clavicipitaceous endophytes are typically
Ascomycota fungi. The ecological roles of these fungi are diverse and still poorly understood. These endophyte plant interactions are widespread and have been found in nearly all land plants and ecosystems.
Many non-clavicipitaceous endophytes have the ability to switch between endophytic behavior and free-living lifestyles. Non-clavicipitaceous endophytes are divided into class 2, 3 and 4. Class 2 endophytes can grow in plant tissues both above and below ground. This class of non-clavicipitaceous endophytes has been the most extensively researched and has been shown to enhance fitness benefits of their plant host as a result of habitat-specific stresses such as pH, temperature and salinity.
Class 3 endophytes are restricted to growth in above ground plant tissues and form in localized areas of plant tissue. Class 4 endophytes are restricted to plant tissues below ground and can colonize much more of the plant tissue. These classes of non-clavicipitaceous endophytes have not been as extensively studied to date.
Applications
Endophytes may have potential future applications in agriculture.
Use of endophytes might potentially increase
crop yields
In agriculture, the yield is a measurement of the amount of a crop grown, or product such as wool, meat or milk produced, per unit area of land. The seed ratio is another way of calculating yields.
Innovations, such as the use of fertilizer, the c ...
. Turfgrass seed of ''
Festuca
''Festuca'' (fescue) is a genus of flowering plants belonging to the grass family Poaceae (subfamily Pooideae). They are evergreen or herbaceous perennial tufted grasses with a height range of and a cosmopolitan distribution, occurring on ever ...
'' and ''
Lolium perenne
''Lolium perenne'', common name perennial ryegrass, English ryegrass, winter ryegrass, or ray grass, is a grass from the family Poaceae. It is native to Europe, Asia and northern Africa, but is widely cultivated and naturalised around the world.
...
'' infected with fungal
inoculants, ''
Acremonium coenophialum'' and ''
A. lolii'', is commercially available for use in growing lawns which might require less pesticide use -the grasses are poisonous to cattle and more resistant to some insect damage. As of 1999 this is only available in the afore-mentioned lawn grasses, which are sold as 'low maintenance'
cultivar
A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture ...
s. The fungi cause the grasses to contain toxic
alkaloid
Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar ...
s. The products provide high resistance to foliar lawn pests such as billbugs, chinch bugs, sod webworms, fall army-worms and Argentine stem weevils, but offer little protection to pests of grass roots such as grubs. The endophytes can survive most pesticides and are even resistant to some
fungicides, and are very suitable for use in
Integrated Pest Management
Integrated pest management (IPM), also known as integrated pest control (IPC) is a broad-based approach that integrates both chemical and non-chemical practices for economic control of pests. IPM aims to suppress pest populations below the econ ...
.
Biofuel
A 2008 experiment with an isolate of a fungus called NRRL 50072 found that this strain can produce a small amount of fuel-like hydrocarbon compounds which was promoted as "myco-diesel". It was hoped that perhaps in the future this might provide a possible source of
biofuel. It was first misidentified as the endophyte ''
Gliocladium roseum
''Clonostachys rosea f. rosea'', also known as ''Gliocladium roseum'', is a species of fungus in the family Bionectriaceae. It colonizes living plants as an endophyte, digests material in soil as a saprophyte and is also known as a parasite of ...
'', but later research showed that it was in fact the
saprophyte ''
Ascocoryne sarcoides
''Ascocoryne sarcoides'' is a species of fungus in the family Helotiaceae. The species name is derived from the Greek sarkodes (fleshy). Formerly known as ''Coryne sarcoides'', its taxonomical history has been complicated by the fact that it ma ...
''.
A strain of endophytic fungi which appeared to be closely related to ''
Nigrograna mackinnonii'' which was isolated from a stem of the plant ''
Guazuma ulmifolia
''Guazuma ulmifolia'', commonly known as West Indian elm or bay cedar, is a medium-sized tree normally found in pastures and disturbed forests. This flowering plant from the family Malvaceae grows up to 30m in height and 30–40cm in diameter. I ...
'' collected in Ecuador was found to produce a variety of
volatile organic compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s including
terpene
Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ...
s and odd chain
polyene
In organic chemistry, polyenes are poly- unsaturated, organic compounds that contain at least three alternating double () and single () carbon–carbon bonds. These carbon–carbon double bonds interact in a process known as conjugation, result ...
s. The polyenes isolated from the fungus have properties that are sought in
gasoline
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...
-surrogate biofuels.
Phytoremediation
Plants are potentially able to break down or sequester, or stimulate micro-organisms in the soil to break down or sequester, certain organic pollutants and inorganic pollutants such as
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
in degraded ecosystems, which is known as
phytoremediation
Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants. It is defined as "the use of green plants and the associated microorganisms, along with proper soil amendments and agronom ...
. In this endophytes may possibly assist plants in converting pollutants into less biologically harmful forms; in one of the few experiments performed a
plasmid called TOM from a strain of a bacterium in the ''
Burkholderia
''Burkholderia'' is a genus of Pseudomonadota whose pathogenic members include the ''Burkholderia cepacia'' complex, which attacks humans and ''Burkholderia mallei'', responsible for glanders, a disease that occurs mostly in horses and related ...
'' genus known as G4 which can break down
trichloroethylene
The chemical compound trichloroethylene is a halocarbon commonly used as an industrial solvent. It is a clear, colourless non-flammable liquid with a chloroform-like sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, w ...
(TCE) was transferred to endophytes of
popular trees; although it did not help the plants remove more of this chemical than non-inoculated plants, the plants transpired less TCE into the air. In another experiment ''
Burkholderia
''Burkholderia'' is a genus of Pseudomonadota whose pathogenic members include the ''Burkholderia cepacia'' complex, which attacks humans and ''Burkholderia mallei'', responsible for glanders, a disease that occurs mostly in horses and related ...
'' bacteria with both the TOM plasmid as well as nickel resistance genes was inoculated into
yellow lupine; this increased the root mass of the plants, but the amounts of TCE transpired was not
statistically significant. Despite these failures, such techniques might lead to some future improvements.
Two strains of the endophytic fungi ''
Pestalotiopsis microspora
''Pestalotiopsis microspora'' is a species of endophytic fungus capable of breaking down and digesting polyurethane. Originally identified in 1880 in fallen foliage of common ivy ('' Hedera helix'') in Buenos Aires, it also causes leaf spot i ...
'' isolated from stems of plants from the Ecuadorian rainforest were shown in laboratory experiments to be able to digest
polyurethane
Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from ...
plastic as the fungus's sole carbon source in
anaerobic conditions, although many other non-endophytic fungi have demonstrated this ability, and most isolates of endophytic fungi in this experiment could perform this to some degree.
Drug discovery
Endophytes produce a wide variety of
secondary metabolite
Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norma ...
s that might be useful as lead compounds in
drug discovery. Endophyte
bioprospecting
Bioprospecting (also known as biodiversity prospecting) is the exploration of natural sources for small molecules, macromolecules and biochemical and genetic information that could be developed into commercially valuable products for the agric ...
has already yielded compounds with
antibacterial
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...
,
antifungal,
antiviral
Antiviral drugs are a class of medication used for treating viral infections. Most antivirals target specific viruses, while a broad-spectrum antiviral is effective against a wide range of viruses. Unlike most antibiotics, antiviral drugs do no ...
,
antiparasitic
Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases, such as those caused by helminths, amoeba, ectoparasites, parasitic fungi, and protozoa, among others. Antiparasitics target the parasitic age ...
,
cytotoxic,
neuroprotective
Neuroprotection refers to the relative preservation of neuronal structure and/or function. In the case of an ongoing insult (a neurodegenerative insult) the relative preservation of neuronal integrity implies a reduction in the rate of neuronal los ...
,
antioxidant,
insulin-mimetic,
α-glucosidase
Glycoside hydrolases (also called glycosidases or glycosyl hydrolases) catalyze the hydrolysis of glycosidic bonds in complex sugars. They are extremely common enzymes with roles in nature including degradation of biomass such as cellulose (cel ...
inhibitory,
and
immunosuppressant
Immunosuppressive drugs, also known as immunosuppressive agents, immunosuppressants and antirejection medications, are drugs that inhibit or prevent activity of the immune system.
Classification
Immunosuppressive drugs can be classified in ...
properties. Manipulations of a plant's endosymbiots can affect plant development, growth and ultimately the quality and quantity of compounds harvested from the plant.
Studies have shown endophytic fungi are able to produce secondary metabolites previously thought to be manufactured by their plant hosts. The presence of these metabolites in plants could be attributable to endophyte production alone, or to combined endophyte and plant production following transfer of the corresponding genes from endophyte to plant or vice versa.
A well known example of the discovery of chemicals derived from endophytic fungi is from the fungus ''
Taxomyces andreanae'' isolated from the pacific yew ''
Taxus brevifolia
''Taxus brevifolia'', the Pacific yew or western yew, is a species of tree in the yew family Taxaceae native to the Pacific Northwest of North America. It is a small evergreen conifer, thriving in moisture and otherwise tending to take the form ...
''. ''T. andreanae'' produces
paclitaxel
Paclitaxel (PTX), sold under the brand name Taxol among others, is a chemotherapy medication used to treat a number of types of cancer. This includes ovarian cancer, esophageal cancer, breast cancer, lung cancer, Kaposi's sarcoma, cervical canc ...
, also known as taxol. This drug is important for the treatment of cancer. Other endophytes since have been discovered that also produce paclitaxel in other host species, but to date there has been no successful industrial source of paclitaxel created.
Endophytes have been discovered with various anti-tumor properties. Endophytic fungi produce many secondary compounds such as
alkaloid
Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar ...
s,
triterpene
Triterpenes are a class of chemical compounds composed of three terpene units with the molecular formula C30H48; they may also be thought of as consisting of six isoprene units. Animals, plants and fungi all produce triterpenes, including squa ...
s and
steroids which have been shown to have anti-tumor effects.
The alkaloid
beauvericin
Beauvericin is a depsipeptide with antibiotic and insecticidal effects belonging to the enniatin family. It was isolated from the fungus ''Beauveria bassiana'', but is also produced by several other fungi, including several ''Fusarium'' species; i ...
has been isolated from the fungus ''
Fusarium oxysporum
''Fusarium oxysporum'' (Schlecht as emended by Snyder and Hansen), an ascomycete fungus, comprises all the species, varieties and forms recognized by Wollenweber and Reinking within an infrageneric grouping called section Elegans. It is part of ...
'' and has shown
cytotoxicity against the tumor cells
PC3
PC3 (PC-3) is a human prostate cancer Cell culture#Isolation of cells, cell line used in prostate cancer research and drug development. PC3 cells are useful in investigating biochemical changes in advanced prostate cancer cells and in assessing t ...
,
PANC-1 PANC-1 is a human pancreatic cancer cell line isolated from a pancreatic carcinoma of ductal cell origin.
PANC-1 was derived from the tissue of a 56-year-old male. The cells can metastasize but have poor differentiation abilities. PANC-1 cells ...
, and
A549.
Two fusarubin derivatives: anhydrofusarubin and methyl ether of fusarubin were isolated from endophytic fungus ''
Cladosporium
''Cladosporium'' is a genus of fungi including some of the most common indoor and outdoor molds. Species produce olive-green to brown or black colonies, and have dark-pigmented conidia that are formed in simple or branching chains. Many species ...
sp.'' and have shown
cytotoxicity against human
leukemia
Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ...
(K-562).
Three triterpenes were found in the endophyte Xylarialean sp., all three of these compounds displayed mild cytotoxic effects on tumor cells.
Some of the antimicrobial compounds produced by endophytic fungi are of interest in their effectiveness against pathogens which have developed resistances to antibiotics. Different fractions of ''
Cladosporium
''Cladosporium'' is a genus of fungi including some of the most common indoor and outdoor molds. Species produce olive-green to brown or black colonies, and have dark-pigmented conidia that are formed in simple or branching chains. Many species ...
sp.'' including
secondary metabolite
Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norma ...
-methyl ether of fusarubin have shown antibacterial activity against ''
Staphylococcus aureus'', ''
E. coli'', ''
P. aeruginosa'', and ''
Bacillus megaterium
''Bacillus megaterium'' is a rod-like, Gram-positive, mainly aerobic spore forming bacterium found in widely diverse habitats.De Vos, P. ''et al.'' Bergey's Manual of Systematic Bacteriology: Volume 3: The Firmicutes. ''Springer'' (2009) It has ...
''.
Several isolates from the ascomycota ''
Pestalotiopsis'' sp. have been shown to have a broad range of antimicrobial effects,
even against
methicillin-resistant ''Staphylococcus aureus''. Also, compounds from the
marine fungus ''
Nigrospora'' sp. have activity against strains of multi drug-resistant ''
Mycobacterium tuberculosis''.
An endophytic fungus of the genus ''
Pseudomassaria'' has been found in the rainforest of the
Democratic Republic of the Congo
The Democratic Republic of the Congo (french: République démocratique du Congo (RDC), colloquially "La RDC" ), informally Congo-Kinshasa, DR Congo, the DRC, the DROC, or the Congo, and formerly and also colloquially Zaire, is a country in ...
. This fungus yields a metabolite that shows potential as an antidiabetic, also known as an insulin mimetic. This compound acts like insulin and has been shown to lower blood glucose levels in mouse model experiments.
Agriculture
Among the many promising applications of endophytic microbes are those intended to increase agricultural use of endophytes to produce crops that grow faster and are more resistant and hardier than crops lacking endophytes. ''
Epichloë
''Epichloë'' is a genus of ascomycete fungi forming an endophytic symbiosis with grasses. Grass choke disease is a symptom in grasses induced by some ''Epichloë'' species, which form spore-bearing mats ( stromata) on tillers and suppress t ...
'' endophytes are being widely used commercially in turf grasses to enhance the performance of the turf and its resistance to biotic and abiotic stresses. ''
Piriformospora indica'' is an interesting endophytic fungus of the order
Sebacinales
The Sebacinales are an order of fungi in the class Agaricomycetes. Taxa have a widespread distribution and are mostly terrestrial, many forming mycorrhizas with a wide variety of plants, including orchids
Orchids are plants that belong to ...
, the fungus is capable of colonising roots and forming symbiotic relationship with many plants.
Endophytes appear to enhance the growth of their plant host symbionts. Endophytes also provide their hosts with an increased resilience to both abiotic and biotic stressors such as drought, poor soils and herbivory. The increased growth and resilience is likely caused by the endophytes ability to improve plant nutrition or secondary metabolite production, as in the case of ''Phoma eupatorii''s inhibition of the phytopathogen ''Phytophthora infestans''.
Endophytes accomplish this by increasing the uptake of valuable land limited nutrients from the soil such as phosphorus and making other plant nutrients available to plants such as
rock phosphate
Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxi ...
and atmospheric nitrogen which are normally trapped in forms that are inaccessible to plants.
Many endophytes protect plants from herbivory from both insects and animals by producing secondary metabolites that are either unappetizing or toxic to the herbivore. Increasingly there has been great importance placed on endophytes that protect valuable crops from invasive insects. One example of an endophyte-plant-insect interaction is located in the New Zealand grasslands, where endophytes, known as AR1 and AR37 are utilized to protect valuable ryegrass from the
Argentine stem weevil but remain palatable to another important food source, livestock.
There are several endophytes that have been discovered that exhibit insecticidal properties. One such endophyte comes from the ''Nodulisporium sp''. which was first harvested from the plant ''
Bontia daphnoides
''Bontia daphnoides'', commonly known as wild olive or white alling, is the only species of the flowering plant genus ''Bontia'' in the family Scrophulariaceae. It is a shrub or small tree growing on many Caribbean islands both as a wild plant ...
''. Indole
diterpene
Diterpenes are a class of chemical compounds composed of four isoprene units, often with the molecular formula C20H32. They are biosynthesized by plants, animals and fungi via the HMG-CoA reductase pathway, with geranylgeranyl pyrophosphate being ...
s, known as nodulisporic acids, have been harvested from this endophyte which have effective insecticidal properties against the blowfly larvae.
There are many obstacles to successfully implementing the use of endophytes in agriculture. Despite the many known benefits that endophytes may confer to their plant hosts,
conventional agricultural practices continue to take priority. Current agriculture relies heavily on fungicides and high levels of chemical fertilizers. The use of fungicides has a negative effect on endophytic fungi and fertilizers reduce a plant's dependence on its endophytic symbiont.
Despite this, the interest and use of bio-insecticides and using endophytes to aid in plant growth is increasing as organic and
sustainable agriculture
Sustainable agriculture is farming in sustainable ways meeting society's present food and textile needs, without compromising the ability for current or future generations to meet their needs. It can be based on an understanding of ecosystem ser ...
is considered more important. As humans become more aware of the damage that synthetic insecticides cause to the environment and beneficial insects such as bees and butterflies biological insecticides may become more important to the agricultural industry.
See also
*
Biofertilizer
A biofertilizer is a substance which contains living micro-organisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of prim ...
*
List of endophytes
*
Plant use of endophytic fungi in defense
*
Arbuscular mycorrhiza
An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ( ...
*
Mycorrhiza
*
Rhizobia
References
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Microbial growth and nutrition
Fungus ecology
Bacteriology
Symbiosis