Endophytes are also known to occur within algae. Many economically important kinds of grass carry fungal endophytes in genus Epichloe, some of which may enhance host growth and may improve the plant’s ability to tolerate abiotic stresses and enhance resistance to plant pathogens.
Endophytes are organisms (bacteria or fungi) that live within in a plant (for at least some part of their life cycle) within intercellular spaces, tissue cavities, or vascular bundles without causing apparent disease. They are ubiquitous and have been present in all species of plants.
Endophytes may benefit for host plants by preventing pathogenic or parasitic organisms from colonizing them. Colonization of the plant tissue by endophytes creates a “barrier effect”. Endophytes may also produce chemicals which inhance the growth of pathogenic organisms (competitors).
Fungal Endophytes
Fungal endophytes are micro-fungi (highly diverse) that live within the healthy tissues of plants but remain asymptomatic. They play an important role in growth promotion and resistance to disease in plants. They have been considered plant mutualists via production of mycotoxins such as alkaloids.
Some endophytes also act as insects pathogenic agents by infecting many kind of insects, which are great concern in agriculture. They have been present in every plant species and they are very important component of plant micro-ecosystem. According to an estimate over one million fungal endophytes existing in nature.
Classification of fungal endophytes
Fungal endophytes have been divided into two groups based on differences in ecological functions, taxonomy, host range and colonization patterns. These two groups are following
- Clavicipitaceous endophytes (class 1)
- Non-clavicipitaceous endophytes (class 2)
Clavicipitaceous Endophytes (class 1)
Clavicipitaceae (family of fungi) including free living and symbiotic species associated with insects and grasses. But some of its members are toxic to humans and animals because they produce alkaloids. Their effects on host plant listed below:
- Insects deterrence
- Mammalian herbivores deterrence
- Reduction of nematodes
- Increase resistance of host disease
- Enhance the ecophysiology of host plants
Non-Clavicipitaceous Cndophytes (Class 2)
Non-clavicipitaceous endophytes includes the fungi associated with leaves of tropical trees and above ground tissues of non vascular plants as well as conifers, vascular seedless plants and woody and herbaceous angiosperms in tropical forests. The effects of non-clavicipitaceous endophytes on host plant are following:
- Avoiding abiotic stress
- Increase of biomass
- Protection from fungal pathogens
Fungal Endophytes in Sustainable Agriculture
Fungal endophytes play an important role in agriculture. Endophytic fungi perform symbiotic associations with plants and enhance plant development by various direct and indirect mechanisms . They are essential components of sustainable agriculture by their ability to enhance plant growth ,yield and increase plant strength by providing biotic and abiotic stress tolerance. They release different secondary metabolites reducing the effect of pathogens and also promote host plant defenses against phytopathogens.
Production Phytohormone
Phytohormones help in the regulation of plant growth and plant responses toward the biotic stress. Fungal endophytes promote plant growth by producing various plant hormones. Among phytohormones, auxins constitute an important group influencing various cellular functions and maintaining plant growth. In response to light or gravity, they help in orientating the growth of root and shoot, differentiating vascular tissues, and initiating lateral and adventitious roots. Some of plant hormones that produced by fungal endophytes and their functions are following
Gibberellins are hormone produced by endophytic fungi that regulate many developmental processes such as plant growth , stem elongation, flower development and fruit senescence.
Jasmonic acid regulates plants growth response to biotic and abiotic stresses . It also promote plant growth.
Salicylic acid plays a role in the resistance to pathogens by production of pathogenesis related proteins.
Indoleacetic acid promotes the shoot and root development.
Biocontrol Agents
Pathogenic microorganisms that harmful for plant health and growth are a threat to crop production. Instead of using chemicals, biological control is an effective and eco-friendly alternative against pathogens. Both in medicine and agriculture, fungal infection is a serious problem. A considerable loss of post harvest food leads to fungal contamination during storage.
An eco-friendly way in agriculture can be achieved by beneficial management strategies involving endophytic symbiosis that can minimize excessive fungicide use.Some chemical compounds used in plant defenses produced by endophytic fungi are earlier thought to be produced by their host plant. Alkaloids are secondary metabolite produced by fungal endophytes, act as a highly toxic substance against phytophagous insects and mammalian herbivores.
Two compounds with anti fungal activity also have been identified as “Cryptocandin” and “Cryptocin.” Cryptocin has shown the potential activity against large numbers of other plant pathogenic fungi. Some other alkaloids compounds of fungal endophytes are perfumoid, joxysporidinone, phomoenamide, alantrypinene and alantryleunone.
Phosphate Solubilization
Phosphorus mineral is considered as an essential nutrient for plant growth. Phosphorus is important for the normal growth of plant and plant maturity. It plays role in cell division, photosynthesis, respiration, energy storage and transfer and many other processes in plant. Plants having lack mineral phosphorus shows stunted growth, immaturity, and reduced yield. In most soils P is not present in plant available (soluble) form due to high pH. pH and soil type effect the fixation and precipitation of P.
Fixation of P in acidic soil is done by oxides and hydroxides of aluminum and iron, while in the alkaline soils, it gets fixed by calcium. The availability of soil P to the plants is arbitrate by the endophytic microorganisms. Endophytic fungi like Aspergillus niger, Penicillium sclerotiorum, P chrysogenum, and Fusarium oxysporum showed phosphate solubilization, potassium solubilization, and zinc solubilization. Fusarium verticillioides and Humicola sp. have been shown to solubilize phosphates under salt stress. Trichoderma pseudokoningii exhibited plant growth-promoting activities of phosphate solubilization and synthesis of auxins, siderophores.
Production of Siderophores
Siderophores are iron-binding molecules having low molecular weight produced by microorganisms under low-iron conditions. Iron considered as an essential plant micro nutrient because Iron having redox activity act as a co-factor of many enzymes. Iron occurs in the form of ferric hydroxide Fe(OH)3 which is insoluble in nature. Siderophores help in iron uptake by binding Fe3+ with high affinity. Many mycorrhizal and endophytic fungi produce siderophores.
Plant Growth Promotion
Plant growth and yield are promoted directly or indirectly by plant growth promoting microorganisms. Fungi plays an important role in plant growth-promoting activities and plant growth promotion. Performance and health of plants enhance due to increased the availability of limiting nutrients. Growth of host plant has been improved by endophytic fungi (Piriformospora indica) through its root colonization.
Fungus Piriformospora indica that forms colonies around plant root, has shown to promote growth during its symbiotic relationship with a broad spectrum of plants. Plant growth promoting endophytes restrain plant tissues and facilitate nutrient exchange and enzymes activity. They improve plant health and growth by different mechanisms.