Microbial inoculants also known as soil inoculants or bioinoculants are agricultural amendments that use beneficial rhizosphericic or

endophytic 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 dat ...

microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s to promote plant health. Many of the microbes involved form

symbiotic relationships Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...

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. Research into the benefits of inoculants in agriculture extends beyond their capacity as

biofertilizer A biofertilizer is a substance which contains living micro-organisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of prim ...

s. Microbial inoculants can induce

systemic acquired resistance Systemic acquired resistance (SAR) is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and although there are many shared aspects b ...

(SAR) of crop species to several common crop diseases (provides resistance against pathogens). So far SAR has been demonstrated for powdery mildew ( ''Blumeria graminis'' f. sp. ''hordei'', Heitefuss, 2001), take-all ( ''Gaeumannomyces graminis'' var. ''tritici'', Khaosaad ''et al.'', 2007), leaf spot (''

Pseudomonas syringae ''Pseudomonas syringae'' is a rod-shaped, Gram-negative bacterium with polar flagella. As a plant pathogen, it can infect a wide range of species, and exists as over 50 different pathovars, all of which are available to researchers from interna ...

'', Ramos Solano ''et al.'', 2008) and root rot (''

Fusarium culmorum ''Fusarium culmorum'' is a fungal plant pathogen and the causal agent of seedling blight, foot rot, ear blight, stalk rot, common root rot and other diseases of cereals, grasses, and a wide variety of monocots and dicots. In coastal dunegrass ( ...

'', Waller ''et al.'' 2005). However, it is increasingly recognized that microbial inoculants often modify the soil microbial community (Mawarda ''et al.'', 2020).

Bacterial

Rhizobacterial inoculants

The

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 ...

commonly applied as inoculants include nitrogen-fixers, phosphate-solubilisers and other root-associated beneficial bacteria which enhance the availability of the macronutrients nitrogen and phosphorus to the host plant. Such bacteria are commonly referred to as plant growth promoting rhizobacteria (PGPR).

Nitrogen-fixing bacteria

The most commonly applied rhizobacteria are ''

Rhizobium ''Rhizobium'' is a genus of Gram-negative soil bacteria that fix nitrogen. ''Rhizobium'' species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant cells ...

'' and closely related genera. ''Rhizobium'' are

nitrogen-fixing 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. At ...

bacteria that form symbiotic associations within nodules on the roots of

legumes 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 ...

. This increases host nitrogen nutrition and is important to the cultivation of soybeans, chickpeas and many other leguminous crops. For non-leguminous crops, '' Azospirillum'' has been demonstrated to be beneficial in some cases for nitrogen fixation and plant nutrition. For cereal crops, diazotrophic rhizobacteria have increased plant growth,Galal, Y. G. M., El-Ghandour, I. A., Osman, M. E. & Abdel Raouf, A. M. N. (2003), The e ffect of inoculation by mycorrhizae and rhizobium on the growth and yield of wheat in relation to nitrogen and phosphorus fertilization as assessed by 15n techniques, Symbiosis, 34(2), 171-183. grain yield (Caballero-Mellado ''et al.'', 1992), nitrogen and phosphorus uptake, and nitrogen (Caballero-Mellado ''et al.'', 1992), phosphorus (Caballero-Mellado ''et al.'', 1992; Belimov ''et al.'', 1995) and potassium content (Caballero-Mellado ''et al.'', 1992). Rhizobacteria live in root nodes, and are associated with legumes.

Phosphate-solubilising bacteria

To improve phosphorus nutrition, the use of phosphate-solubilising bacteria (PSB) such as '' Agrobacterium radiobacter'' has also received attention (Belimov ''et al.'', 1995a; 1995b; Singh & Kapoor, 1999). As the name suggests, PSB are free-living bacteria that break down inorganic soil phosphates to simpler forms that enable uptake by plants.

Fungal inoculants

Symbiotic relationships between fungi and plant roots is referred to as a Mycorrhiza association. This symbiotic relationships is present in nearly all land plants and give both the plant and fungi advantages to survival. The plant can give upwards of 5-30% of its energy production to the fungi in exchange for increasing the root absorptive area with

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

e which gives the plant access to nutrients it would otherwise not be able to attain. The two most common mycorrhizae are

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. ( ...

e and

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 ...

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 ...

e associations are most commonly found in woody-species, and have less implications for agricultural systems.

Arbuscular mycorrhiza

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. ( ...

(AM) has received attention as a potential agriculture amendment for its ability to access and provide the host plant phosphorus. Under a reduced fertilization greenhouse system that was inoculated with a mixture of AM fungi and

, tomato yields that were given from 100% fertility were attained at 70% fertility. This 30% reduction in fertilizer application can aid in the reduction of

nutrient pollution Nutrient pollution, a form of water pollution, refers to contamination by excessive inputs of nutrients. It is a primary cause of eutrophication of surface waters (lakes, rivers and coastal waters), in which excess nutrients, usually nitrogen or ...

, and help prolong finite mineral resources such as phosphorus (

Peak phosphorus Peak phosphorus is a concept to describe the point in time when humanity reaches the maximum global production rate of phosphorus as an industrial and commercial raw material. The term is used in an equivalent way to the better-known term peak o ...

). Other effects include increases in

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

tolerance, drought tolerance, and resistance to trace metal toxicity.

Fungal partners

Fungal inoculation alone can benefit host plants. Inoculation paired with other amendments can further improve conditions.

l inoculation combined with

compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting ...

is a common household amendment for personal gardens, agriculture, and nurseries. It has been observed that this pairing can also promote microbial functions in soils that have been affected by

mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ...

. Certain fungal partners do best in specific

ecotone An ecotone is a transition area between two biological communities, where two communities meet and integrate. It may be narrow or wide, and it may be local (the zone between a field and forest) or regional (the transition between forest and gras ...

s or with certain crops.

l inoculation paired with plant growth promoting bacteria resulted in a higher yield and quicker maturation in upland rice paddys. Maize growth improved after an amendment of

e and

biochar Biochar is the lightweight black residue, made of carbon and ashes, remaining after the pyrolysis of biomass. Biochar is defined by the International Biochar Initiative as "the solid material obtained from the thermochemical conversion of ...

. This amendment can also decrease

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 ...

uptake by crops.

Inoculant usage

Fungal inoculants can be used with or without additional amendments in private gardens, homesteads, agricultural production, native nurseries, and land restoration projects.

Composite inoculants

The combination of strains of Plant Growth Promoting Rhizobacteria (PGPR) has been shown to benefit rice and barley.Belimov, A. A., Kojemiakov, A. P. & Chuvarliyeva, C. V. (1995a) Interaction between barley and mixed cultures of nitrogen fixing and phosphate-solubilising bacteria. Plant and Soil, 173, 29-37. The main benefit from dual inoculation is increased plant nutrient uptake from both soil and fertilizer. Multiple strains of inoculant have also been demonstrated to increase total

nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only fam ...

activity compared to single strains of inoculants, even when only one strain is

diazotroph Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples of organisms that ...

ic. PGPR and

e in combination can be useful in increasing wheat growth in nutrient poor soil and improving nitrogen-extraction from fertilised soils.Galal, Y. G. M., El-Ghandour, I. A., Osman, M. E. & Abdel Raouf, A. M. N. (2003), The effect of inoculation by mycorrhizae and rhizobium on the growth and yield of wheat in relation to nitrogen and phosphorus fertilization as assessed by 15n techniques, Symbiosis, 34(2), 171-183.

References

Bibliography

{{more footnotes, date=February 2022 * Bashan, Y. & Holguin, G. (1997), Azospirillum-plant relationships: environmental and physiological advances (1990-1996), Canadian Journal of Microbiology 43, 103-121. * Bashan, Y., Holguin, G. & E., D.-B. L. (2004) Azospirillum-plant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003). Canadian Journal of Microbiology, 50, 521-577. * Belimov, A. A., Kunakova, A. M., Vasilyeva, N. D., Gruzdeva, E. V., Vorobiev, N. I., Kojemiakov, A. P., Khamova, O. F., Postavskaya, S. M. & Sokova, S. A. (1995b) Relationship between survival rates of associative nitrogen-fixers on roots and yield response of plants to inoculation. FEMS Microbiology Ecology, 17, 187-196. * Caballero-Mellado, J., Carcano-Montiel, M. G. & Mascarua-Esparza, M. A. (1992), Field inoculation of wheat (triticum aestivum) with azospirillum brasilense under temperate climate, Symbiosis, 13, 243-253. * Gutierrez Manero, F. J. (2008) Systemic disease protection elicited by plant growth promoting rhizobacteria strains: relationship between metabolic responses, systemic disease protection, and biotic elicitors. Phytopathology, 98 (4), 451-457. * Heitefuss, R. (2001) Defence reactions of plants to fungal pathogens: principles and perspectives, using powdery mildew on cereals as an example. Naturwissenschaften, 88, 273-283. * Khammas, K. M. & Kaiser, P. (1992) Pectin decomposition and associated nitrogen fixation by mixed cultures of Azospirillum and Bacillus species. Canadian Journal of Microbiology, 38, 794-797. * Khaosaad, T., Garcia-Garrido, J. M., Steinkellner, S. & Vierheilig, H. (2007) Take-all disease is systemically reduced in roots of mycorrhizal barley plants. Soil Biology and Biochemistry, 39, 727-734. * Lippi, D., Cacciari, I., Pietrosanti, T. & Pietrosanti, W. (1992) Interactions between Azospirillum and Arthrobacter in diazotrophic mixed culture. Symbiosis, 13, 107-114. * Mawarda, P.C., Le Roux, X., van Elsas, J.D. & Falcao Salles J. (2020) Deliberate introduction of invisible invaders: A critical appraisal of the impact of microbial inoculants on soil microbial communities. Soil Biology and Biochemistry, 148, 107874. * Nguyen, T. H., Kennedy, I. R. & Roughley, R. J. (2002) The response of field-grown rice to inoculation with a multi-strain biofertiliser in the Hanoi district, Vietnam. IN I. R. Kennedy & A. T. M. A. Choudhury (Eds.) Biofertilisers in Action. Barton, ACT, Rural Indrustries Research & Development Corporation. * Rabie, G. H. & Almadini, A. M. (2005) Role of bioinoculants in development of salt-tolerance of Vicia faba plants under salinity stress. African Journal of Biotechnology, 4 (3), 210-222. * Ramos Solano, R., Barriuso Maicas, J., Pereyra De La Iglesia, M. T., Domenech, J. & * Sullivan, P. (2001) Alternative soil amendments. Appropriate Technology Transfer for Rural Areas, National Center for Appropriate Technology. https://web.archive.org/web/20081011174000/http://www.attra.ncat.org/attra-pub/PDF/altsoil.pdf * Waller, F., Achatz, B., Baltruschat, H., Fodor, J., Becker, K., Fischer, M., Heier, T., Huckelhoven, R., Neumann, C., Von Wettstein, D., Franken, P. & Kogel, K.-H. (2005) The endophytic fungus Piriformis indica reprograms barley to salt-stress tolerance, disease resistance, and higher yield. Proceedings of the National Academy of Sciences, 102 (38), 13386-13391.

External links

* http://www.satavic.org/biofertilisers.htm * https://web.archive.org/web/20080509170441/http://mycorrhiza.ag.utk.edu/ Mycorrhiza Literature Exchange, Plant Sciences, University of Tennessee * http://www.soilfoodweb.com.au Soil Foodweb Institute Australia * http://attra.ncat.org ATTRA – National Sustainable Agriculture Information Service Bacteria Mycology Soil biology Soil improvers Symbiosis