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. Endophytes are also known to occur within lichens and algae. Many economically important grasses (e.g., Festuca spp. and Lolium spp.) carry fungal endophytes in genus Epichloë, some of which 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 mammalian herbivores.

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 hyphae penetrating the embryo within the host’s seeds (e.g., seed transmitting forms of Epichloë). Conversely, reproduction through asexual or sexual spores leads to horizontal transmission, where endophytes may spread between plants in a population or community. Some endophytes that frequently transmit vertically may also produce spores on plants that can be transmitted horizontally (e.g., Epichloë festucae). Some of the Epichloë endophytes have been found to produce a cryptic but infective conidial state on the surfaces of leaf blades. However, the extent to which endophytes rely on these cryptic conidia for horizontal transmission is still unknown. Some endophytic fungi are actually latent pathogens or saprotrophs that only become active and reproduce under specific environmental conditions or when their host plants are stressed or begin to senesce.

Endophyte-Host Interactions

Endophytes may benefit host plants by preventing pathogenic or parasitic organisms from colonizing them. Extensive colonization of the plant tissue by endophytes creates a "barrier effect", where the local endophytes outcompete and prevent pathogenic organisms from taking hold. Endophytes may also produce chemicals which inhibit the growth of competitors, including pathogenic organisms. Endophytes are also known to increase expression of defense-related genes in plants, making plants more resistant to many potential pathogens. Some fungal and bacterial endophytes have proven to increase plant growth and improve overall plant hardiness. The presence of fungal endophytes can cause higher rates of water loss in leaves. However, certain microbial endophytes may also help plants survive abiotic stresses, including salt, drought or heat stresses. Endophytes have also been shown to enhance plant development and increase nutrient (phosphorus and nitrogen) uptake into plants. Endophyte-related host benefits are common phenomena, and have been the focus of much research, particularly among the grass endophytes (see below). In spite of the many reports of beneficial effects of endophytes it has come to be understood that the relationship between endophytes and hosts may be considered a balanced antagonism with both positive and negative effects on hosts depending on the environmental conditions. Redman et al. advanced the hypothesis of 'habitat adapted symbiosis' where plants are proposed to associate with particular endophytes that increase tolerance or resistance to the predominant biotic or abiotic stresses of their habitats. Fungal and bacterial endophytes may comprise functional communities in plants that increase a plant's capacity to survive and thrive in its habitat.

Endophytes for Medicinal and Industrial Applications

The wide range of compounds produced by endophytes have been shown to combat pathogens and even cancers in animals including humans. One notable endophyte with medicinal benefits to humans was discovered by Gary Strobel: Pestalotiopsis microspora, an endophytic fungus of Taxus wallachiana (Himalayan Yew) was found to produce taxol. Endophytes are also being investigated for roles in biofuels production. Inoculating plants with certain endophytes may provide increased disease or parasite resistance while others may possess metabolic processes that convert cellulose and other carbon sources into "myco-diesel" hydrocarbons and hydrocarbon derivatives.

Endophytes for Agricultural Applications

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 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 fungus is capable of colonising roots and forming symbiotic relationship with many plants. P. indica symbiosis has been shown to increase crop yield for a variety of crops (barley, tomato, maize etc.) and provide a measure of protection against pathogens and abiotic stresses. Recent evidence suggests that communities of bacterial and fungal endophytes may work in functional consortia to promote growth and protect plants in natural populations; while plants in intensive cultivation may lose these defensive and growth promotional microbiome components. Some scientists propose that restoration of defensive and growth promotional endophytes in agricultural crops could result in reduction of agrochemical inputs to control pests and diseases and result in crops that would better tolerate droughts and other stresses. There is some evidence that some bacterial endophytes may establish symbiosis with both plants and animals. This raises the possibility that crops could one day be produced that carry probiotic endophytes to enhance human health.

The Search for Endophytes

It is speculated that there may be many thousands of endophytes useful to mankind but since there are few scientists working in this field, and since environmental contamination, deforestation and biodiversity loss are widespread, many endophytes might be permanently lost before their utility is explored.

Endophytic species are very diverse; only a small minority of existing endophytes have been characterized. A single plant organ (leaf, stem or root) of a plant can harbor many different species of endophytes, both bacterial and fungal. Additionally, some endophytic bacteria may live within endophytic fungi.

Endophytes can be identified in several ways, usually through amplifying and sequencing a small piece of DNA. Some endophytes can be cultured from a piece of their host plant in an appropriate growth medium. An important step in culturing endophytes is to surface disinfect plant tissues prior to placement on culture media. This kills epiphytic microbes, ensuring only growth of endophytic microbes. Not all endophytes can be cultured in this way, as shown by discovery of cryptic, unculturable endophyte species through DNA based analysis of leaf tissue. Some grass endophytes in genus Epichloë can be seen as intercellular sinuous strands of hyphae under the microscope following leaf sheath or culm tissue staining with aniline blue. Many endophytes do not sporulate when cultured. Since fungal identification by morphology is based primarily on spore-bearing structures, this fact makes visual identification of some endophytic cultures challenging.

Diversity of Fungal Endophytes

Fungal endophytes are generally from the phylum Ascomycota, though other phyla are represented. Some specific examples of which are found in orders Hypocreales and Xylariales of the Sordariomycetes (Pyrenomycetes) class. Additionally the class of Loculoascomycetes includes endophytes. Although endophytes may be diverse taxonomically, Rodriguez et al. classified fungal endophytes broadly into ecological categories or functional classes.

Diversity of Algal Endophytes

A number of endophytes are now known that grow within seaweeds and algae. One such example is Ulvella leptochaete, which has recently been discovered from host algae including Cladophora and Laurentia from India.

Diversity of Bacterial Endophytes

Bacterial endophytes may belong to a broad range of taxa, including α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, Firmicutes, Actinobacteria, etc... Bacterial endophytes have been found to become intracellular in root and shoot cells of many plants, with entry into cells in the meristems. In this intracellular form bacteria lose cell walls but continue to divide and metabolize. These wall-less intracellular forms of bacteria are called L-forms. Paungfoo-Lonhienne et al. observed the degradation of intracellular microbes within root cells and hypothesized that intracellular microbes may be a source of organic nutrients or vitamins for plants; they termed this process 'rhizophagy'.

See also

• Biofertilizer
• List of endophytes