Mycorrhiza helper bacteria

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General association and effects of MHBs with mycorrhizal fungi. Impacts of arbuscular mycorrhizal fungi (AMF) and beneficial bacteria on plant performance and soil fertility.webp
General association and effects of MHBs with mycorrhizal fungi.

Mycorrhiza helper bacteria (MHB) are a group of organisms that form symbiotic associations with both ectomycorrhiza and arbuscular mycorrhiza. [1] MHBs are diverse and belong to a wide variety of bacterial phyla including both Gram-negative and Gram-positive bacteria. [1] Some of the most common MHBs observed in studies belong to the phylas Pseudomonas and Streptomyces . [1] MHBs have been seen to have extremely specific interactions with their fungal hosts at times, but this specificity is lost with plants. [2] MHBs enhance mycorrhizal function, growth, nutrient uptake to the fungus and plant, improve soil conductance, aid against certain pathogens, and help promote defense mechanisms. [1] These bacteria are naturally present in the soil, and form these complex interactions with fungi as plant root development starts to take shape. [2] The mechanisms through which these interactions take shape are not well-understood and needs further study. [1] [2] [3]

Contents

Taxonomy

MHBs consist of a diverse group of bacteria, often gram-negative and gram-positive bacteria. Most of the bacteria are associated with both ectomycorrhiza and arbuscular mycorrhiza, but some show specificity to a particular type of fungus. [1] The common phyla that MHB belong to will be addressed in the following sections, as well as common genera.

Pseudomonadota

The Pseudomonadota (formerly Proteobacteria) are a large and diverse group of gram-negative bacteria containing five classes. Pseudomonas is in the gammaproteobacteria class. Specific bacteria within this genus are strongly associated as being MHBs in the rhizosphere of both ectomycorrhiza and arbuscular mycorrhiza. [1] Pseudomonas fluorescens has been examined in several studies to understand how they work in benefiting the mycorrhiza and plant. [1] In one study, they found that the bacteria helped ectomycorrhizal fungi promote a symbiotic relationship with the plant by examining an increase in formation of mycorrhiza when Pseudomonas fluorescens was applied to the soil. [4] Some bacteria improve root colonization and plant growth when associated with arbuscular mycorrhiza. [5] It has been hypothesized that MHBs aid the plant in pathogenic defense by improving the nutrient uptake from the soil, allowing plants to allocate more resources to broad defense mechanisms. [6] However, the mechanism these species use to help both fungi is still unknown and needs to be further investigated. [5]

Actinomycetota

The branching shape of Streptomyces, a very common soil bacteria that often aids in the plant-mycorhiza relationship. Streptomyces sp 01.png
The branching shape of Streptomyces, a very common soil bacteria that often aids in the plant-mycorhiza relationship.

Actinomycetota are gram-positive bacteria and are naturally found in the soil. In this phylum, Streptomyces is the largest genus of bacteria, and are often associated with MHBs. [1] Streptomyces have been a model organism of study in biological research on MHBs. In one study, it has been reported that Streptomyces are responsible for increasing root colonization, plant biomass growth, mycorrhizal colonization, and fungal growth. [7] [8] However, there is not just a single mechanism that the MHBs participate in. [1] [7] [8] It has also been found that Streptomyces interact with ectomycorrhiza and arbuscular mycorrhiza. [1] While these interactions need further understanding, they seem to be extremely common in natural soil. [8]

Bacillota

Bacillota are gram-positive bacteria, many of which have a low GC content in their DNA. There are a few genera that act as MHBs, but one of the most common is Bacillius . [1] Bacillius belong to the class Bacilli, and are rod-shaped organisms that can be free-living or pathogenic. However, in the presence of mycorrhiza some species can be beneficial and are considered to be MHBs. [1] Since they are common, they can form a relationship with ectomycorrhiza and arbuscular mycorrhiza, similar to the previous genera. [1] Bacillius aids in the establishment and growth of mycorrhiza, and helps with the fixation of nitrogen in the rhizosphere. [9] [10] [11]

Impact

MHBs are known to have several functions when interacting with the roots of plants and growth of fungi. In several studies it has been reported that MHBs can help fungi by increasing mycelial growth and aid in nutrient intake. [3] The mycelial increase allows for fungi to absorb more nutrients, increasing its surface area. [9]

Growth promoted by nutrients

Some MHBs are known to help break down molecules to a more usable form. [1] MHBs can obtain both inorganic and organic nutrients in the soil through a direct process known as mineral-weathering which aids in the recycling of nutrients throughout the environment. [12] The process of mineral-weathering releases protons and iron into the soil. [12] This results in a lowering of the pH. [12] A diverse group of bacteria can participate in the mineral- weathering process, such as Pseudomonas , Burkholderia , and Collimonas . [12] The acidification of the soil by MHBs is hypothesized to be linked to their glucose metabolism. [12]

MHBs also help gather unavailable phosphorus from the surrounding soil. [13] Phosphate solubilizing rhizobacteria are the most common MHB that aids in phosphorus uptake. [13] The bacteria are involved in this process by releasing phosphate-degrading compounds in the soil to break down organic and inorganic phosphate. [14] As a result, the MHB create a pool of phosphate that the mycorrhiza then use. [14] [15] The bacteria work in phosphorus-limited conditions to help the mycorrhiza establish and grow. [13] Streptomyces can assist arbuscular mycorrhiza in phosphorus-limited conditions through a similar process. [8] [13]

MHBs in the rhizosphere often have the capability to acquire nitrogen that the plant can use. The MHBs are able to fix nitrogen in the soil, and create pools of available nitrogen. [16] However, MHBs do not cause plant modifications as legumes do, to help with nitrogen-fixation. [16] Nitrogen-fixation is done only in the surrounding soil in relation to the mycorrhiza. [16] In one study, researchers reported that a Bacillius MHB contributed to the nitrogen-fixation, and among other factors helped the plant grow when inoculated with a fungus. [11]

Plant growth hormones

It has been proposed (Kaska et al., 1994) that MHBs induce growth hormones in a plant, which helps the mycorrhiza interact with the lateral roots in soil. [17] An increase of root formation was also observed when Pseudomonas putida produced growth hormones, and was inoculated with the arbuscular mycorrhiza Gigaspora rosea on a cucumber plant. [18] The inoculation of both the MHB and the fungus allowed for an increase in root elongation and growth in the soil, similar to the previous study. [18] In another study, it was found that MHB can release gaseous compounds to attract and aid in the growth of fungi. [19] The introduction of growth hormones and gaseous compounds produced by MHBs was only discovered recently, and requires further study on how MHBs influence the mycorrhiza symbiotic relationship and root growth.

Alteration of fungal genes aiding in growth

An example of cell-signaling, a proposed method for communication between MHBs and their fungal hosts allowing for recognition and co-colonization of plants. Laura Kiessling research - glycopolymers and cell signaling.tif
An example of cell-signaling, a proposed method for communication between MHBs and their fungal hosts allowing for recognition and co-colonization of plants.

Researchers have reported that fungal genes can be altered in the presence of an MHB. [20] In one study, it was hypothesized that in the presence of a fungus, an MHB will promote an increase in the expression of a gene that helps to promote growth in the fungus. [20] The fungus changes its genes expression after the MHB has promoted growth of the fungus, thus the alteration of the gene is an indirect effect. [20] This is likely the cause of certain compounds or signals released by the MHBs, and further analysis is needed to better understand this communication. [20]

Interactions with specific fungi

Only certain bacteria are specific to mycorrhizal fungi groups. [15] Results have shown that the indigenous arbuscular mycorrhizal fungi of the clover plant could only grow in the presence Pseudomonas putida, but in fact, the plant could grow with the presence of multiple bacteria. [15] It has been hypothesized that rhizosphere helper bacteria, in the soil, have developed traits to aid them in competition for inoculating fungi in their environment. [8] Thus, it is plausible that MHBs select for certain fungi and developed some specificity towards a fungus that favors the bacteria. [8] [1]

Detoxifying soil

MHBs help mycorrhiza establish symbiotic associations in stressful environments such as those high in toxic metals. [21] In harsh environments, the bacteria assist in acquiring more nutrients such as nitrogen and phosphorus. [22] [23] MHBs help to prevent the uptake of toxic metals including lead, zinc, and cadmium. [22] [23] The bacteria decrease the amount of metals taken up by the plant through blockade mechanisms. [22] [23] The blockade of the toxic metals by the bacteria allows the fungus to form a stronger symbiotic association with the plant, and promotes the growth of both. [22] [23] Another proposed mechanism of MHBs in toxic environments is that the bacteria aid the mycorrhiza by compensating for the negative effects the toxic metal imposed. [24] The MHBs help by increasing the plant nutrition uptake, and creating a balance between the macronutrients and micronutrients. [23] [24] Thus, MHBs have mechanisms to help the plant tolerate harsh and otherwise unsuitable environments. This relationship makes them great candidates for bioremediation.

With pathogenic fungi

Pathogenic fungi, able to harm other fungi or plants. Rozlozek czarny.jpg
Pathogenic fungi, able to harm other fungi or plants.

In the presence of a pathogenic fungus, most studies show that MHBs aid in fighting off pathogens. [2] However, there have been a few cases where MHBs help to promote pathogenic effects of a fungus. [2]

Assisting pathogenic fungi

There have been a few studies that have found that MHBs aid pathogenic fungi. One study showed that MHBs aided in colonization of a type of fungal pathogen because the surrounding environment was unsuitable for the symbiotic mycorrhiza. [25] Thus the MHB became more harmful under certain conditions to increase their own fitness. [25] [2]   Researchers have also found that MHBs help the pathogenic fungus to colonize on the surface of the plant. [25] This has a negative effect on the plant, by increasing the deleterious effects of the fungus. Another proposed mechanism is that MHBs alter the defense mechanism of the plant, by shutting off degrading peroxidase enzymes, and allowing the pathogenic fungus to inoculate the plant. [26]

Defending against pathogenic fungi

In several studies, researchers have proposed numerous ways MHBs defend against pathogens. In one experiment researchers observed that MHBs produced acid in the surrounding environment, which helped to fight off various pathogens. [27] It has also been hypothesized that the defense mechanism against pathogens is from a combination of both fungi and plant. [27] Another study found that MHBs release antifungal metabolites into the soil. [28] The anti-fungal metabolites produce antagonistic effects towards the pathogenic fungi. [28] However, MHBs can help defend a pathogen depending on the nutrient availability and space in the rhizosphere. [1] [27] Further research is still necessary to understand the mechanism of how MHBs aid mycorrhiza in order to defeat pathogens, and if this role is symbiotic or more mutualistic in nature.

Related Research Articles

<span class="mw-page-title-main">Mycorrhiza</span> Fungus-plant symbiotic association

A mycorrhiza is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, soil biology, and soil chemistry.

<span class="mw-page-title-main">Endophyte</span>

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 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.

<span class="mw-page-title-main">Arbuscular mycorrhiza</span> Symbiotic penetrative association between a fungus and the roots of a vascular plant

An arbuscular mycorrhiza (AM) is a type of mycorrhiza in which the symbiont fungus penetrates the cortical cells of the roots of a vascular plant forming arbuscules.

<span class="mw-page-title-main">Myco-heterotrophy</span> Symbiotism between certain parasitic plants and fungi

Myco-heterotrophy is a symbiotic relationship between certain kinds of plants and fungi, in which the plant gets all or part of its food from parasitism upon fungi rather than from photosynthesis. A myco-heterotroph is the parasitic plant partner in this relationship. Myco-heterotrophy is considered a kind of cheating relationship and myco-heterotrophs are sometimes informally referred to as "mycorrhizal cheaters". This relationship is sometimes referred to as mycotrophy, though this term is also used for plants that engage in mutualistic mycorrhizal relationships.

<i>Rhizopogon</i> Genus of fungi

Rhizopogon is a genus of ectomycorrhizal basidiomycetes in the family Rhizopogonaceae. Species form hypogeous sporocarps commonly referred to as "false truffles". The general morphological characters of Rhizopogon sporocarps are a simplex or duplex peridium surrounding a loculate gleba that lacks a columnella. Basidiospores are produced upon basidia that are borne within the fungal hymenium that coats the interior surface of gleba locules. The peridium is often adorned with thick mycelial cords, also known as rhizomorphs, that attach the sporocarp to the surrounding substrate. The scientific name Rhizopogon is Greek for 'root' (Rhiz-) 'beard' (-pogon) and this name was given in reference to the rhizomorphs found on sporocarps of many species.

<span class="mw-page-title-main">Rhizobacteria</span> Group of bacteria affecting plant growth

Rhizobacteria are root-associated bacteria that can have a detrimental, neutral or beneficial effect on plant growth. The name comes from the Greek rhiza, meaning root. The term usually refers to bacteria that form symbiotic relationships with many plants (mutualism). Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature.

Microbial inoculants also known as soil inoculants or bioinoculants are agricultural amendments that use beneficial rhizosphericic or endophytic microbes to promote plant health. Many of the microbes involved form symbiotic relationships with the target crops where both parties benefit (mutualism). While microbial inoculants are applied to improve plant nutrition, they can also be used to promote plant growth by stimulating plant hormone production. Although bacterial and fungal inoculants are common, inoculation with archaea to promote plant growth is being increasingly studied.

<span class="mw-page-title-main">Fungivore</span> Organism that consumes fungi

Fungivory or mycophagy is the process of organisms consuming fungi. Many different organisms have been recorded to gain their energy from consuming fungi, including birds, mammals, insects, plants, amoebas, gastropods, nematodes, bacteria and other fungi. Some of these, which only eat fungi, are called fungivores whereas others eat fungi as only part of their diet, being omnivores.

The mycorrhizosphere is the region around a mycorrhizal fungus in which nutrients released from the fungus increase the microbial population and its activities. The roots of most terrestrial plants, including most crop plants and almost all woody plants, are colonized by mycorrhiza-forming symbiotic fungi. In this relationship, the plant roots are infected by a fungus, but the rest of the fungal mycelium continues to grow through the soil, digesting and absorbing nutrients and water and sharing these with its plant host. The fungus in turn benefits by receiving photosynthetic sugars from its host. The mycorrhizosphere consists of roots, hyphae of the directly connected mycorrhizal fungi, associated microorganisms, and the soil in their direct influence.

<span class="mw-page-title-main">Mycorrhizal network</span> Underground fungal networks that connect individual plants together

A mycorrhizal network is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects individual plants together. Mycorrhizal relationships are most commonly mutualistic, with both partners benefiting, but can be commensal or parasitic, and a single partnership may change between any of the three types of symbiosis at different times.

<span class="mw-page-title-main">Ectomycorrhiza</span> Non-penetrative symbiotic association between a fungus and the roots of a vascular plant

An ectomycorrhiza is a form of symbiotic relationship that occurs between a fungal symbiont, or mycobiont, and the roots of various plant species. The mycobiont is often from the phyla Basidiomycota and Ascomycota, and more rarely from the Zygomycota. Ectomycorrhizas form on the roots of around 2% of plant species, usually woody plants, including species from the birch, dipterocarp, myrtle, beech, willow, pine and rose families. Research on ectomycorrhizas is increasingly important in areas such as ecosystem management and restoration, forestry and agriculture.

<i>Rhizophagus irregularis</i> Species of arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture

Rhizophagus irregularis is an arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture. Rhizophagus irregularis is also commonly used in scientific studies of the effects of arbuscular mycorrhizal fungi on plant and soil improvement. Until 2001, the species was known and widely marketed as Glomus intraradices, but molecular analysis of ribosomal DNA led to the reclassification of all arbuscular fungi from Zygomycota phylum to the Glomeromycota phylum.

<span class="mw-page-title-main">Root microbiome</span>

The root microbiome is the dynamic community of microorganisms associated with plant roots. Because they are rich in a variety of carbon compounds, plant roots provide unique environments for a diverse assemblage of soil microorganisms, including bacteria, fungi and archaea. The microbial communities inside the root and in the rhizosphere are distinct from each other, and from the microbial communities of bulk soil, although there is some overlap in species composition.

Orchid mycorrhizae are endomycorrhizal fungi which develop symbiotic relationships with the roots and seeds of plants of the family Orchidaceae. Nearly all orchids are myco-heterotrophic at some point in their life cycle. Orchid mycorrhizae are critically important during orchid germination, as an orchid seed has virtually no energy reserve and obtains its carbon from the fungal symbiont.

Mycorrhizal amelioration of heavy metals or pollutants is a process by which mycorrhizal fungi in a mutualistic relationship with plants can sequester toxic compounds from the environment, as a form of bioremediation.

Dr. Mohamed Hijri is a biologist who studies arbuscular mycorrhizal fungi (AMF). He is a professor of biology and research at the Institut de recherche en biologie végétale at the University of Montreal.

Funneliformis mosseae is a species of fungus in the family Glomeraceae, which is an arbuscular mycorrhizal (AM) fungi that forms symbiotic relationships with plant roots. Funneliformis mosseae has a wide distribution worldwide, and can be found in North America, South America, Europe, Africa, Asia and Australia. Funneliformis are characterized by having an easily visible septum in the area of the spore base and are often cylindrical or funnel-shaped. Funneliformis mosseae similarly resembles Glomus caledonium, however the spore wall of Funneliformis mosseae contains three layers, whereas Gl. caledonium spore walls are composed of four layers. Funneliformis is an easily cultivated species which multiplies well in trap culture, along with its high distribution, F. mosseae is not considered endangered and is often used for experimental purposes when combined with another host.

Rhizophagus clarus is an arbuscular mycorrhizal fungus in the family Glomeraceae. The species has been shown to improve nutrient absorption and growth in several agricultural crops but is not typically applied commercially.

The International Collection of (Vesicular) Arbuscular Mycorrhizal Fungi (INVAM) is the largest collection of living arbuscular mycorrhizal fungi (AMF) and includes Glomeromycotan species from 6 continents. Curators of INVAM acquire, grow, identify, and elucidate the biology, taxonomy, and ecology of a diversity AMF with the mission to expand availability and knowledge of these symbiotic fungi. Culturing AMF presents difficulty as these fungi are obligate biotrophs that must complete their life cycle while in association with their plant hosts, while resting spores outside of the host are vulnerable to predation and degradation. Curators of INVAM have thus developed methods to overcome these challenges to increase the availability of AMF spores. The inception of this living collection of germplasm occurred in the 1980s and it takes the form of fungi growing in association with plant symbionts in the greenhouse, with spores preserved in cold storage within their associated rhizosphere. AMF spores acquired from INVAM have been used extensively in both basic and applied research projects in the fields of ecology, evolutionary biology, agroecology, and in restoration. INVAM is umbrellaed under the Kansas Biological Survey at The University of Kansas, an R1 Research Institution. The Kansas Biological Survey is also home to the well-known organization Monarch Watch. INVAM is currently located within the tallgrass prairie ecoregion, and many collaborators and researchers associated with INVAM study the role of AMF in the mediation of prairie biodiversity. James Bever and Peggy Schultz are the Curator and Director of Operation team, with Elizabeth Koziol and Terra Lubin as Associate Curators.

Ambispora granatensis is an arbuscular mycorrhizal fungal species in the genus Ambispora, family Ambisporaceae. It forms spores of the acaulosporois and glomoid morphs, thus the Ambispora classification. It was discovered in Granada Spain in 2010 and has unique spore characteristics, which distinguishes the species from the others in its genus.

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