Endophyte

An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant 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 may improve the plant's ability to tolerate abiotic stresses, such as drought, and enhance resistance to insects, plant pathogens and herbivores. As often with other organisms associated with plants such as mycorrhizal fungus, endophytes gain carbon from their association with the plant host.

History

Endophytes were first described by the German botanist 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 when Victor Galippe discovered bacteria normally occurring in the inside of plant tissues that their role as beneficial to the plants was postulated.^[1]

Transmission

Endophytes may be transmitted either vertically (directly from parent to offspring) or horizontally (among individuals).^[2] Vertically transmitted fungal endophytes are typically considered clonal and transmit via fungal hyphae penetrating the embryo within the host’s seeds, 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.^[3]

Symbiosis

Most endophyte/plant relationships are not well understood.^[4]

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.^[5] Plant-microbe interactions are not strictly mutualistic, with endophytic fungi potentially becoming pathogens or saprotrophs that only become active and reproduce under specific environmental conditions or when their host plants are stressed or begin to senesce.^[6][7]

Endophytes may benefit host plants by preventing pathogenic or parasitic organisms from colonizing them, by extensively colonizing plant tissues and competitively excluding potential pathogens and herbivores.^[8][9]

Some fungal and bacterial endophytes have proven to increase plant growth and improve overall plant hardiness.^[10]

Diversity

Endophytic species are very diverse; only a small minority of existing endophytes have been characterized.^[11][12]

From within the phylums Basidiomycota and Ascomycota endophytic fungi may be from Hypocreales and Xylariales of the Sordariomycetes (Pyrenomycetes) class or from the class of Loculoascomycetes.^[13]

One group of fungal endophytes are the arbusbuclar mycorrhizal fungi involving biotrophic Glomeromycota associated with various plant species.^[14]

Bacterial endophytes are polyphyletic, belonging to broad range of taxa, including α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, Firmicutes, Actinobacteria.^[15]

Applications

Biofuel

The main quality selected for use in biofuel is high productivity. Through the above benefits use of endophytes can potentially increase productivity and allow production to occur on land otherwise unsuitable.^[16] Inoculating plants with certain endophytes may provide increased disease or parasite resistance ^[17] while others may possess metabolic processes that convert cellulose and other carbon sources into "myco-diesel" hydrocarbons and hydrocarbon derivatives.^[18] Common species used in biofuel in the US are Zea mays (corn), Salix species (poplars and willows), and sugarcane species.

Environmental remediation

In restoration ecology, endophytes can assist native species in outcompeting non-native invasive species and, colonizing barren land in secondary ecological succession, and restoring ecosystems degraded by pollutants. As with in biofuel production, in phytoremediation high productivity species are often used. Plants are able to contain, store, potentially break down, and stimulate microorganisms in the soil to break down certain pollutants.^[19] With phytoremediation the main challenge is the growth of plants in soil contaminated with organic pollutants and inorganic pollutants such as heavy metals. In this endophytes assist plants in converting pollutants into less biologically harmful forms such as breaking down TCE of PAHs in their metabolic pathways, and assist plants in tolerating higher levels of soil contamination with pollutants such as toluene.^[19]

The roots of plant communities to varying degrees help hold soil together by creating a network of roots that trap soil within. This in turn helps prevent soil erosion, stabilizes slopes and prevent landslides, helps prevent desertification in vulnerable areas, and controls pollution into waterways by acting as part of riparian buffers.^[20]

Drug discovery

Endophytes can produce a wide variety of compounds that might be useful as lead compounds in drug discovery.^[21][22] Certain fungal endophyte secondary metabolites have anti-fungal, anti-microbial, anti-viral, anti-oxidant, and anti-cancer properties; examples of this include taxol, torreyanic acid, exopolysaccharides, and solamargine.^[23] Manipulations of a plant's endosymbiots can also affect plant development, growth and ultimately the quality and quantity of compounds harvested from the plant.^[6]

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.^[24] Epichloë endophytes are being widely used commercially in turf grasses to enhance the performance of the turf and its resistance to biotic and abiotic stresses.^[25] Piriformospora indica is an interesting endophytic fungus of the order Sebacinales, the fungus is capable of colonising roots and forming symbiotic relationship with many plants.^[26]

See also

• Biofertilizer
• List of endophytes
• Plant use of endophytic fungi in defense
• Arbuscular mycorrhiza
• Mycorrhiza
• Rhizobia

References