Mycorrhiza helper bacteria (MHB) are a group of organisms that form symbiotic associations with both

ectomycorrhiza An ectomycorrhiza (from Greek ἐκτός ', "outside", μύκης ', "fungus", and ῥίζα ', "root"; pl. ectomycorrhizas or ectomycorrhizae, abbreviated EcM) is a form of symbiotic relationship that occurs between a fungal symbiont, or mycob ...

and

arbuscular mycorrhiza An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. ( ...

. MHBs are diverse and belong to a wide variety of

bacterial phyla Bacterial phyla constitute the major lineages of the domain ''Bacteria''. While the exact definition of a bacterial phylum is debated, a popular definition is that a bacterial phylum is a monophyletic lineage of bacteria whose 16S rRNA genes s ...

including both

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

and

Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

. Some of the most common MHBs observed in studies belong to the phylas ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are able ...

'' and ''

Streptomyces ''Streptomyces'' is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positive, ...

''. MHBs have been seen to have extremely specific interactions with their fungal hosts at times, but this specificity is lost with plants. MHBs enhance mycorrhizal function, growth, nutrient uptake to the

fungus A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...

and

plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...

, improve

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

conductance, aid against certain

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...

s, and help promote

defense mechanisms In psychoanalytic theory, a defence mechanism (American English: defense mechanism), is an unconscious psychological operation that functions to protect a person from anxiety-producing thoughts and feelings related to internal conflicts and ...

. These bacteria are naturally present in the soil, and form these complex interactions with

fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...

as plant root development starts to take shape. The mechanisms through which these interactions take shape are not well-understood and needs further study.

Taxonomy

MHBs consist of a diverse group of

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

, often

gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

and

gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

. Most of the

are associated with both

and

, but some show specificity to a particular type of fungus. The common phyla that MHB belong to will be addressed in the following sections, as well as common

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...

Pseudomonadota

The

Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...

(formerly Proteobacteria) are a large and diverse group of

gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...

containing five classes. ''Pseudomonas'' is in the

gammaproteobacteria Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genera-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically imp ...

class. Specific bacteria within this genus are strongly associated as being MHBs in the

rhizosphere The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microor ...

of both ectomycorrhiza and arbuscular mycorrhiza. ''

Pseudomonas fluorescens ''Pseudomonas fluorescens'' is a common Gram-negative, rod-shaped bacterium. It belongs to the '' Pseudomonas'' genus; 16S rRNA analysis as well as phylogenomic analysis has placed ''P. fluorescens'' in the ''P. fluorescens'' group within the g ...

'' has been examined in several studies to understand how they work in benefiting the mycorrhiza and plant. 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. Some

improve root colonization and plant growth when associated with arbuscular mycorrhiza. 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. However, the mechanism these species use to help both fungi is still unknown and needs to be further investigated.

Actinomycetota

Actinomycetota The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to soi ...

are

and are naturally found in the soil. In this phylum, ''

'' is the largest genus of bacteria, and are often associated with MHBs. ''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. However, there is not just a single mechanism that the MHBs participate in. It has also been found that ''Streptomyces'' interact with ectomycorrhiza and arbuscular mycorrhiza. While these interactions need further understanding, they seem to be extremely common in natural soil.

Bacillota

Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the ea ...

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''. ''Bacillius'' belong to the class

Bacilli Bacilli is a taxonomic class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as ''Bacillus anthracis'' (the cause of anthrax). ''Bacilli'' are almost exclusively gram-posit ...

, 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. Since they are common, they can form a relationship with ectomycorrhiza and arbuscular mycorrhiza, similar to the previous genera. ''Bacillius'' aids in the establishment and growth of mycorrhiza, and helps with the fixation of nitrogen in the rhizosphere.

Impact

MHBs are known to have several functions when interacting with the

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

s 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. The mycelial increase allows for fungi to absorb more nutrients, increasing its surface area.

Growth promoted by nutrients

Some MHBs are known to help break down

molecules A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioc ...

to a more usable form. MHBs can obtain both

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemis ...

and organic nutrients in the soil through a direct process known as mineral-weathering which aids in the recycling of nutrients throughout the environment. The process of mineral-weathering releases protons and

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

into the soil. This results in a lowering of the pH. A diverse group of bacteria can participate in the mineral- weathering process, such as ''

'', ''

Burkholderia ''Burkholderia'' is a genus of Pseudomonadota whose pathogenic members include the ''Burkholderia cepacia'' complex, which attacks humans and '' Burkholderia mallei'', responsible for glanders, a disease that occurs mostly in horses and related ...

'', and '' Collimonas''. The acidification of the soil by MHBs is hypothesized to be linked to their

glucose metabolism Carbohydrate metabolism is the whole of the biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms. Carbohydrates are central to many essential metabolic pathways. Plan ...

. MHBs also help gather unavailable

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...

from the surrounding soil. Phosphate solubilizing

rhizobacteria Rhizobacteria are root-associated bacteria that can have a detrimental (parasitic varieties), neutral or beneficial effect on plant growth. The name comes from the Greek ''rhiza'', meaning root. The term usually refers to bacteria that form symbio ...

are the most common MHB that aids in phosphorus uptake. The bacteria are involved in this process by releasing phosphate-degrading compounds in the soil to break down organic and inorganic phosphate. As a result, the MHB create a pool of phosphate that the mycorrhiza then use. The bacteria work in phosphorus-limited conditions to help the mycorrhiza establish and grow. ''Streptomyces'' can assist arbuscular mycorrhiza in phosphorus-limited conditions through a similar process. MHBs in the rhizosphere often have the capability to acquire

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

that the plant can use. The MHBs are able to fix nitrogen in the soil, and create pools of available nitrogen. However, MHBs do not cause plant modifications as

legume A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock fo ...

s do, to help with

nitrogen-fixation Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atm ...

. Nitrogen-fixation is done only in the surrounding soil in relation to the mycorrhiza. 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.

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 root Lateral roots, emerging from the pericycle (meristematic tissue), extend horizontally from the primary root (radicle) and over time makeup the iconic branching pattern of root systems. They contribute to anchoring the plant securely into the soil, ...

s in soil. An increase of root formation was also observed when ''

Pseudomonas putida ''Pseudomonas putida'' is a Gram-negative, rod-shaped, saprotrophic soil bacterium. Based on 16S rRNA analysis, ''P. putida'' was taxonomically confirmed to be a '' Pseudomonas'' species (''sensu stricto'') and placed, along with several othe ...

'' produced growth hormones, and was inoculated with the arbuscular mycorrhiza ''Gigaspora rosea'' on a

cucumber Cucumber (''Cucumis sativus'') is a widely-cultivated creeping vine plant in the Cucurbitaceae family that bears usually cylindrical fruits, which are used as culinary vegetables. 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. In another study, it was found that MHB can release gaseous compounds to attract and aid in the growth of fungi. 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

Researchers have reported that fungal genes can be altered in the presence of an MHB. 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 In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

that helps to promote growth in the fungus. 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. This is likely the cause of certain compounds or signals released by the MHBs, and further analysis is needed to better understand this communication.

Interactions with specific fungi

Only certain bacteria are specific to mycorrhizal fungi groups. Results have shown that the indigenous arbuscular mycorrhizal fungi of the

clover Clover or trefoil are common names for plants of the genus ''Trifolium'' (from Latin ''tres'' 'three' + ''folium'' 'leaf'), consisting of about 300 species of flowering plants in the legume or pea family Fabaceae originating in Europe. The genus h ...

plant could only grow in the presence ''

,'' but in fact, the plant could grow with the presence of multiple bacteria. It has been hypothesized that

helper bacteria, in the soil, have developed traits to aid them in competition for inoculating fungi in their environment. Thus, it is plausible that MHBs select for certain fungi and developed some specificity towards a fungus that favors the bacteria.

Detoxifying soil

MHBs help mycorrhiza establish symbiotic associations in stressful environments such as those high in

toxic metal Metal toxicity or metal poisoning is the toxic effect of certain metals in certain forms and doses on life. Some metals are toxic when they form poisonous soluble compounds. Certain metals have no biological role, i.e. are not essential minerals, o ...

s. In harsh environments, the bacteria assist in acquiring more nutrients such as

and

. MHBs help to prevent the uptake of toxic metals including

lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...

, and

cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

. The bacteria decrease the amount of metals taken up by the plant through blockade mechanisms. 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. 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. The MHBs help by increasing the plant nutrition uptake, and creating a balance between the

macronutrients A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...

and

micronutrients Micronutrients are essential dietary elements required by organisms in varying quantities throughout life to orchestrate a range of physiological functions to maintain health. Micronutrient requirements differ between organisms; for example, huma ...

. Thus, MHBs have mechanisms to help the plant tolerate harsh and otherwise unsuitable environments. This relationship makes them great candidates for

bioremediation Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...

With pathogenic fungi

In the presence of a

pathogenic fungus Pathogenic fungi are fungi that cause disease in humans or other organisms. Approximately 300 fungi are known to be pathogenic to humans. Markedly more fungi are known to be pathogenic to plant life than those of the animal kingdom. The study of fu ...

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

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. Thus the MHB became more harmful under certain conditions to increase their own fitness. Researchers have also found that MHBs help the pathogenic fungus to colonize on the surface of the plant. 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 Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides. Functionality Peroxidases typically ca ...

enzymes, and allowing the pathogenic fungus to inoculate the plant.

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. It has also been hypothesized that the defense mechanism against pathogens is from a combination of both fungi and plant. Another study found that MHBs release antifungal metabolites into the soil. The anti-fungal metabolites produce antagonistic effects towards the pathogenic fungi. However, MHBs can help defend a pathogen depending on the nutrient availability and space in the rhizosphere. 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.

References

{{Reflist Bacteria Mutualism (biology)