Bacteria and fungi team up to enhance plant growth and health.
One tablespoon of healthy soil can contain up to 50 billion busy microbes including arbuscular mycorrhizal fungi (AMF) and plant growth promoting rhizobacteria (PGPR). Plants obtain carbon dioxide from the air to produce photosynthetic compounds, some of which are released into the soil. Most microbes depend on as these released compounds as their food source. In exchange microbes such as AMF and PGPR protect the plant roots from parasites and pathogens, recycle and transport nutrients, and improve the surrounding soil structure to maximise plant health.
The interactions between microbes are important parts of a communication network that keeps microhabitats in balance, and here I would like to highlight a few synergies between PGPR and AMF. AMF symbiosis was found to cause remarkable changes in the physiology of the host plant. These changes alter the chemical composition of root exudates and, consequently, the microbial populations of both inoculated PGPR and indigenous bacteria. These bacteria in turn enhance AMF colonisation by releasing root and shoot growth promoting hormones to increase root surface area. This allows for more AMF association and a larger photosynthetic surface area to synthesise more carbon compounds.
Phosphorus and nitrogen are two of the most fundamental elements for plants. PGPR and AMF play crucial roles in the cycling and uptake of these elements. Some species of PGPR can release locked up phosphate (P) in the soil, to make it available for plants, through a process known as P-solubilisation. However the released P can be trapped again very quickly unless absorbed by a plant. Through their symbiotic relationship with plants, AMF can help plants take up this released P quickly. This is because the AMF growing on the root network increases the surface area. Furthermore, transport of absorbed P has been observed to be more efficient in plants with symbiotic AMF. This is due to increased expression of the genes responsible for P-transporters.
Most PGPR can fix atmospheric nitrogen gas into ammonium ions, which can be taken up effectively by AMF. The uptake of an ammonium ion, by either AMF or plants, releases one H+ ion into the soil; acidifying the soil. The acidifying effect drives inorganic P-solubilisation, thus AMF indirectly increase available P in the soil. In addition to P and N, AMF are able to improve the uptake of other nutrients, such as K, Ca, Zn, Cu or Fe, further boosting plant growth and health as well as benefitting the microbial community as a whole.
AMF and PGPR can work together to protect their associated plants from biotic and abiotic stresses. By virtue of residing on the plant roots, analogous to our gut flora, AMF and PGPR competitively exclude potential pathogens. AMF make plants more resilient to different abiotic stresses such as high heavy metal or salt concentrations, or drought. PGPR can induce systemic resistance in plants. Via the hyphal network of the AMF this effect can be extended to other plants. The continuity of the mutually beneficial relationships between plants, PGPR and AMF is fundamental to the distribution and movement of nutrients within the environment.