Much like the guts of humans like you and me, plants harbor a variety of fungi and bacteria within their tissues. Fungi, bacteria, and other microorganisms in plants–called endophytes–have been found to improve plant health, both under normal and challenging environmental conditions.
Here we will discuss endophytes: what they are, what kinds of relationships they have with plants, and what are some known examples.
Endophyte definition
These endophytic organisms spend most-if not all-of their lives within their hosts, without causing visible indications of disease. The presence of endophytic microorganisms in plant tissue results in a microbiome called endosphere.
Some endophytes are unique to one tissue type, whereas others can be found throughout the plant. Endophyte populations in plants vary greatly depending on factors including host species, host developmental stage, colony density, and environmental conditions.
Endophytes isolated from coffee cultivated in Hawaii, Puerto Rico, Colombia, and Mexico have varying colonization patterns. While the majority of the endophytes were recovered from leaves in Hawaii and Columbia, a greater number were obtained from crown tissues in Puerto Rico and Mexico.
Yet in all four sites, fungal endophytes were recovered all over the plants including their roots, berries, and leaf stems.
To date, endophytes are known to occur in about 300,000 plant species on the planet. While this is the case, only several grass species out of extant plant species have had their whole endophyte complement investigated. As such, there are plenty of opportunities to discover new endophytes.
Plant-endophyte relationships
The relationship between plants and endophytes can be commensal or mutualistic:
A commensal relationship means the endophyte benefits, while the plant host is unharmed.
A mutualistic relationship means that both the endophyte and the plant host benefit from the presence of the endophyte.
Some of the benefits of the plant-endophyte relationship include gaining tolerance to salinity or heat or resistance to plant diseases, insects, and pests. They can also enhance the plant host’s ability to absorb nutrients.
Endophytes can also be further classified based on their relationship with plants. For example, endophytes can be obligate or facultative:
Obligate endophytes rely on plant metabolism for survival. They are transported across plants through the activities of several types of vectors or by vertical transmission (transmission from parent to offspring).
Facultative endophytes exist outside the host body at some point in their life cycle and typically interact with plants from the host body's surrounding soil and atmosphere.
Endophytic fungi in plants
Fungi that live inside a host plant are called endophytic fungi. The presence of endophytic fungi in plants depends on several factors including the host, the environment, the availability of nutrients, and the presence of other microorganisms.
It is believed that endophytic fungi can infect plant leaves when airborne spores make contact with the surface of a leaf. If the environmental conditions allow for spore penetration, spores are able to germinate and the fungal hyphae (filaments that allow for the flow of nutrients) can then enter the plant through the cuticle to inhabit the intercellular spaces of the leaf.
Over time, the whole leaf is inhabited by different endophytes.
Some grasses and other non-woody plants benefit from fungal endophytes because they produce toxins that discourage herbivores or increase the host plant’s tolerance to heat, drought, or heavy metals.
The majority of endophytic fungi that have been identified are ascomycetes--fungi that reproduce sexually by producing an ascus, a sac that contains spores.
Endophytic bacteria in plants
Bacteria that can be found in the tissues of a plant host are called endophytic bacteria. Over 200 genera from 16 phyla of bacteria are found to be endophytic, most of which belong to the phyla Proteobacteria, Actinobacteria, and Firmicutes.
Endophytic bacteria are regarded as a subtype of rhizospheric bacteria, generally referred to as plant growth-promoting rhizobacteria (PGPR), a specialized type of rhizobacteria that can invade their plant host. These bacteria live within the tissue of the plant host, whether below or above ground, and even in fruits, seeds, and tubers affecting the growth and development of the plant.
Rhizobacteria are bacteria that dwell near plant roots or in the rhizosphere, the soil that surrounds plant roots.
The colonization process usually begins with an interaction between the two at the plants’ roots: endophytic bacteria recognize certain compounds in root exudates and may allow the bacterium to enter the plant endosphere.
Plant exudates are fluids released by plants through their roots.
Endophytic bacteria typically enter through the plant's roots, but they can also use the aboveground parts such as stems, leaves, and flowers to enter. Once inside, endophytic bacteria can systematically infect nearby plant tissues.
Endophytic bacteria use the plant endosphere as a unique protective biological niche that provides a stable and constant habitat outside the influence of the ever-changing environmental conditions that could potentially harm them. Furthermore, the majority of endophytic bacteria have a biphasic life cycle in which they shift between plant and soil habitats.
Examples of endophyte associations
Here, we will discuss several examples of plant-endophyte relationships and how endophytes contribute to plant health.
Endophytic fungi and the cocoa tree
In 2003, Elizabeth Arnold and colleagues from the University of Arizona in Tucson investigated whether fungal endophytes help the cocoa tree (Theobroma cacao).
Note that in cacao, fungal endophytes tend to colonize leaves after germination so for the experiment, some cacao seedlings' leaves were treated with a specific combination of fungal endophytes, while others were not. The seedlings were then injected with a virulent disease, the protist Phytophthora.
It turned out that the pathogen destroyed fewer leaves in seedlings with fungal endophytes than in seedlings without endophytes. Moreover, among the surviving leaves, pathogens caused less damage to the leaf surface in seedlings with endophytes than in those without.
Arnold et al. concluded from this experiment that the presence of endophytes appeared to benefit cocoa trees by lowering leaf mortality and damage caused by Phytophthora.
Endophytic fungus Fusarium culmorum and the American dune grass
Puget Sound in western Washington is home to the American dune grass. Scientists Rodriguez and Redman found that Fusarium culmorum colonization promotes salt tolerance on the grass, so dune grass plants not colonized by F. culmorum are unable to survive in the salty environment of the location.
In addition to salt tolerance, it seems that the presence of endophytes can also help boost the plants’ pathogen resistance.
Endophytes and vegetable crops
Endophytes have been reported to reduce disease caused by a variety of pathogens in vegetable cropping systems. Eggplant seedlings treated with Penicillium then with a pathogenic isolate of Verticillium dahliae, showed no signs of wilt.
On the other hand, control plants showed substantial vascular discoloration and wilting.
In another study, cucumber seedlings were inoculated with endophytic bacteria called actinomycetes. Like fungi, actinomycetes possess mycelium and form spores. Such cucumber seedlings responded to the damping-off disease caused by the pathogen Pythium aphanidermatum (also known as water molds) just as they would to a successful metalaxyl fungicide treatment.
Medicinal importance of endophytes
In addition to being beneficial to plant hosts, endophytes are also found to be a valuable source of bioactive compounds that can be used to produce drugs against a wide variety of diseases.
These compounds include alkaloids, phenolic acids, quinones, steroids, saponins, tannins, and terpenoids, which have antibacterial, anti-insect, anticancer, and other properties. Furthermore, endophytes can be readily extracted from any microbial or plant growth media.
For example, fungal endophytes can be isolated and studied via surface sterilization, a process where plant parts like leaves and stems were washed under running water, then cut and surface sterilized by immersing into various solutions.
Endophytes - Key takeaways
- Endophytes are microorganisms–usually fungi and bacteria–that can be found in the leaves and/or roots of plant hosts.
- Endophytic organisms spend most-if not all-of their lives within their hosts, without causing visible indications of disease.
- Endophytic fungi can infect plant leaves when airborne spores make contact with the surface of a leaf. Once the spores germinate, the fungal hyphae enter the plant through the cuticle. They spread to the intercellular spaces of the leaf.
- Endophytic bacteria are regarded as a type of rhizobacteria that can invade their plant host. Endophytic bacteria typically enter through the plants’ roots, but they can also use the aboveground parts, then they can systematically colonize the rest of the plant.
- Endophytes are also a valuable source of bioactive compounds that can be used to produce drugs against a wide variety of diseases.
References
- Reece, Jane B., et al. Campbell Biology, Eleventh ed., 2016.
- Gouda, Sushanto, et al., Endophytes: A Treasure House of Bioactive Compounds of Medicinal Importance, Frontiers in Microbiology, Frontiers Media S.A., 29 Sept. 2016.
- Afzal, Imran, et al., Plant Beneficial Endophytic Bacteria: Mechanisms, Diversity, Host Range and Genetic Determinants, Microbiological Research, 4 Feb. 2019.
- Montana State University, What Is an Endophyte? What Is an Endophyte?, Plant Sciences and Plant Pathology, N.D.
- Tymon, Lydia, and Debra Inglis, What Is an Endophyte?, Apr. 2016.
- Khalil, Ahmed Mohamed Aly, et al, Isolation and Characterization of Fungal Endophytes Isolated from Medicinal Plant Ephedra Pachyclada as Plant Growth-Promoting, Biomolecules, 22 Jan. 2021.
Explanations 
Exams 
Magazine 
