Mycorrhiza helper bacteria are a group of organisms that form symbiotic associations with both ectomycorrhiza and arbuscular mycorrhiza. MHBs are diverse and belong to different bacterial phyla including both gram-negative and gram-positive bacteria. Some of the most common types are Pseudomonas and Streptomyces. MHBs have specific interactions with fungi, but not with the plants. MHB enhance mycorrhizal function, increase mycorrhizal growth, provide nutrients to the fungus and plant, improve soil conductance, select to aid pathogens, and help promote defense mechanisms.
Taxonomy
MHBs consist of a diverse group of organisms, 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. The common phyla that MHB belong to will be addressed in the following sections, as well as common genera.
The 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. Pseudomonas fluorescens 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. Some bacteria improve root colonization and plant growth when associated with arbuscular mycorrhiza. The improvement of growth has been hypothesized that MHB aid the plant to defend against pathogens by improving the nutrient uptake. However, the mechanism these species use to help both of the fungi is still unknown and needs to be further investigated.
are gram-positive bacteria, and they are found in the soil. In this phylum, Streptomyces is the largest genus of bacteria, and are often associated with MHBs. Streptomyces are of the Actinobacteria class. Streptomyces have been an organism of study in biological research on MHB. One study, they reported that Streptomyces have increased root colonization, growth, mycorrhizal colonization, and fungal growth. However, there is not just a single mechanism that the MHB participates in. It has also been found that Streptomyces interact with ectomycorrhiza and arbuscular mycorrhiza.
Firmicutes
are gram-positive bacteria, many of which have a low GC content. There are a few genera that act as MHB, but one that is the most common is Bacillius. Bacillius belong to the class Bacilli, and is a rod-shaped organism 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 the establishment and growth of mycorrhiza, and helps with the fixation of nitrogen in the rhizosphere.
Impact
MHB are known to have several functions when interacting with the roots and growth of fungi. In several studies it has been reported that MHB can help fungi by increasing growth and aid with nutrient intake.
Growth promoted by nutrients
Some MHB are known to help break down molecules to a more usable form. MHBs can obtain inorganic and organic nutrients in the soil through a direct process known as mineral-weathering which aids in the recycling nutrients from the environment. The process of mineral-weathering releases protons and iron into the soil. This results in the lowering of the pH. A diverse group of bacteria can participate in the mineral- weathering process, such as Pseudomonas, Burkholderia, and Collimonas. The acidification of the soil by MHBs is hypothesized to be linked to their glucose metabolism. MHBs also help gather unavailable phosphorus from soil. Phosphate solubilizing rhizobacteria are the most common MHB that aid 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 could 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 that the plant can use. The MHBs are able to fix nitrogen in the soil, and create pools of available nitrogen. However, MHB do not cause plant modifications as legumes do, to help with nitrogen-fixation. 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.
It has been proposed by Kaska, Myllylä, and Cooper that MHB induce growth hormones in a plant, which helps the mycorrhiza interact with the lateral roots in soil. 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. 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 the growth of fungi. The introduction of growth hormones and gaseous compounds produced by MHB was only discovered recently, and requires further study on how MHB influence the mycorrhiza symbiotic relationship and root growth.
Alteration of fungal genes aiding in growth
Researchers have reported that the 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 gene expression of a gene 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.
Interactions with specific fungi
Only certain bacteria are specific to mycorrhizal fungi groups. Results have shown that only the indigenous arbuscular mycorrhizal fungi of the clover plant could grow in the presence Pseudomonas putida, but in fact, the plant could grow with the presence of multiple bacteria. 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. Thus, it is plausible that MHBs select for certain fungi and developed some specificity towards a fungus that favours the bacteria.
Detoxifying soil
MHB help mycorrhiza establish symbiotic associations in stressful environments such as those high in toxic metals. In harsh environments, the bacteria assist in acquiring more nutrients such as nitrogen and phosphorus. MHB helps to prevent the uptake of toxic metals including lead, zinc, and cadmium. 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 MHB in toxic environments is that the bacteria aid the mycorrhiza by compensating for the negative effects the toxic metal imposed. The MHB helps by increasing the plant nutrition uptake, and creating a balance between the macronutrients and micronutrients. Thus, MHB have mechanisms to help the plant tolerate harsh and otherwise unsuitable environments.
In the presence of a pathogenic fungus, most studies show that MHB aid in fighting off pathogens. However, there have been a few cases where MHB help to promote pathogenic effects of a fungus.
Assisting pathogenic fungi
There have been a few studies that have found that MHB aid pathogenic fungi. One study showed that MHB 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 MHB help the pathogenic fungus to colonize on the surface of the plant. This has a negative effect on the plant, but increasing the deleterious effects of the fungus. Another proposed mechanism is that MHB alters the defense mechanism of the plant, by shutting off degrading peroxidase enzymes, and allowing the pathogenic fungus to inoculate the plant.
Defending against pathogenic fungi
In several studies, researchers have proposed numerous ways MHB defends pathogens. In one experiment researchers observed that MHB produced acid in the surrounding environment, which helped to defeat off the pathogen. It has also been hypothesized that the defense mechanism against pathogens is from a combination of both fungus and plant. Another study found that MHB release antifungal metabolites into the soil. The anti-fungal metabolites produce antagonistic effects towards the pathogenic fungi. However, MHB can choose whether to 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 defeat pathogens.
