Pseudomonas is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 validly described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a wide range of niches. Their ease of culture in vitro and availability of an increasing number of Pseudomonas strain genome sequences has made the genus an excellent focus for scientific research; the best studied species include P. aeruginosa in its role as an opportunistic human pathogen, the plant pathogen P. syringae, the soil bacterium P. putida, and the plant growth-promoting P. fluorescens, P. lini, P. migulae, and P. graminis.
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, salinity and decrease biotic stresses by enhancing plant resistance to insects, pathogens and herbivores.
Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia.
Paenibacillus is a genus of facultative anaerobic, endospore-forming bacteria, originally included within the genus Bacillus and then reclassified as a separate genus in 1993. Bacteria belonging to this genus have been detected in a variety of environments, such as: soil, water, rhizosphere, vegetable matter, forage and insect larvae, as well as clinical samples. The name reflects: Latin paene means almost, so the paenibacilli are literally "almost bacilli". The genus includes P. larvae, which causes American foulbrood in honeybees, P. polymyxa, which is capable of fixing nitrogen, so is used in agriculture and horticulture, the Paenibacillus sp. JDR-2 which is a rich source of chemical agents for biotechnology applications, and pattern-forming strains such as P. vortex and P. dendritiformis discovered in the early 90s, which develop complex colonies with intricate architectures as shown in the pictures:
Paenibacillus polymyxa, also known as Bacillus polymyxa, is a Gram-positive bacterium capable of fixing nitrogen. It is found in soil, plant tissues, marine sediments and hot springs. It may have a role in forest ecosystems and potential future applications as a biofertilizer and biocontrol agent in agriculture.
Pseudomonas citronellolis is a Gram-negative, bacillus bacterium that is used to study the mechanisms of pyruvate carboxylase. It was first isolated from forest soil, under pine trees, in northern Virginia, United States.
Pseudomonas migulae is a fluorescent, Gram-negative, rod-shaped bacterium isolated from natural mineral waters in France. This bacterium has also been isolated from endophytic tissues of lodgepole pine trees growing on gravel mining sites with potential to perform biological nitrogen fixation and plant growth promotion. Based on 16S rRNA analysis, P. migulae has been placed in the P. fluorescens group.
Pseudomonas stutzeri is a Gram-negative soil bacterium that is motile, has a single polar flagellum, and is classified as bacillus, or rod-shaped. While this bacterium was first isolated from human spinal fluid, it has since been found in many different environments due to its various characteristics and metabolic capabilities. P. stutzeri is an opportunistic pathogen in clinical settings, although infections are rare. Based on 16S rRNA analysis, this bacterium has been placed in the P. stutzeri group, to which it lends its name.
Paraburkholderia phytofirmans is a species of proteobacteria. They have been reported to colonize endophytic tissues of hybrid spruce and lodgepole pine with a strong potential to perform biological nitrogen fixation and plant growth promotion.
Caballeronia sordidicola is a species of proteobacteria which has been reported to perform biological nitrogen fixation and promote plant growth
Rhizobacteria are root-associated bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). They are an important group of microorganisms used in biofertilizer. Biofertilization accounts for about 65% of the nitrogen supply to crops worldwide. 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. PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and endophytic. Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming root nodules. The dominant species found in the rhizosphere is a microbe from the genus Azospirillum. Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space. PGPRs provide benefits to the plants via Direct mechanisms and Indirect mechanisms. PGPRs are sustainable and eco-friendly substitute of chemical fertilizers and pesticides.
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 primary nutrients to the host plant. Biofertilizers add nutrients through the natural processes of nitrogen fixation, solubilizing phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances. The microorganisms in biofertilizers restore the soil's natural nutrient cycle and build soil organic matter. Through the use of biofertilizers, healthy plants can be grown, while enhancing the sustainability and the health of the soil. Biofertilizers can be expected to reduce the use of synthetic fertilizers and pesticides, but they are not yet able to replace their use. Since they play several roles, a preferred scientific term for such beneficial bacteria is "plant-growth promoting rhizobacteria" (PGPR).
Azospirillum brasilense is a well studied, nitrogen-fixing (diazotroph), genetically tractable, Gram-negative, alpha-proteobacterium bacterium, first described in Brazil by the group of Johanna Döbereiner and then receiving the name "brasilense". A. brasilense is able to fix nitrogen in the presence of low oxygen levels, making it a microaerobic diazotroph. An isolate from the genus Azospirillum was isolated from nitrogen poor soils in the Netherlands in 1925, however the species A. brasilense was first described in 1978 in Brazil, since this genus is widely found in the rhizospheres of grasses around the world where it confers plant growth promotion. Whether growth promotion occurs through direct nitrogen flux from the bacteria to the plant or through hormone regulation is debated. The two most commonly studied strains are Sp7 and Sp245, both are Brazilian isolates isolated from Tropical grasses from Seropedica, Brazil.
Bradyrhizobium betae is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacterium first isolated from the roots of Beta vulgaris, hence its name. It is slow-growing an endophytic. The type strain is PL7HG1T.
Caballeronia udeis is a bacterium from the genus Caballeronia and family Burkholderiaceae which has been reported to perform biological nitrogen fixation and promote plant growth
Microbacterium hydrocarbonoxydans is a Gram-positive and Crude oil-degrading bacterium from the genus of Microbacterium which has been isolated from oil contaminated soil in Germany.
Paraburkholderia is a genus of Proteobacteria that are gram negative, slightly curved rods that are motile by means of flagella. They have been reported to colonize endophytic tissues of hybrid spruce and lodgepole pine with a strong potential to perform biological nitrogen fixation and plant growth promotion. Unlike Burkholderia species, Paraburkholderia members are not commonly associated with human infection. Paraburkholderia members form a monophyletic clade within the Burkholderiaceae family, which is what prompted their distinction as a genus independent from Burkholderia species, in combination with the finding of robust conserved signature indels which are unique to Paraburkholderia species, and are lacking in members of the genus Burkholderia. These CSIs distinguish the genus from all other bacteria. Additionally, the CSIs that were found to be shared by Burkholderia species are absent in Paraburkholderia, providing evidence of separate lineages.
Promicromonospora iranensis is a bacterium from the genus of Promicromonospora which has been isolated from rhizospheric soil in the Fars Province in Iran.
Caballeronia is a genus of bacteria from the family of Burkholderiaceae which has been reported to perform biological nitrogen fixation and promote plant growth
Asanoa hainanensis is a Gram-positive and non-motile bacterium from the genus of Asanoa which has been isolated from the rhizospheric soil of the fern Acrostichum speciosum from Wenchang in China.
