Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to agricultural improvement along with their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this review.

Plant growth-promoting rhizobacteria (PGPR): emergence in agriculture

Mechanisms and applications of plant growth promoting rhizobacteria: Current perspective

The rhizosphere is a complex environment where roots interact with physical, chemical and biological properties of soil. Structural and functional characteristics of roots contribute to rhizosphere processes and both have significant influence on the capacity of roots to acquire nutrients. Roots also interact extensively with soil microorganisms which further impact on plant nutrition either directly, by influencing nutrient availability and uptake, or indirectly through plant (root) growth promotion. In this paper, features of the rhizosphere that are important for nutrient acquisition from soil are reviewed, with specific emphasis on the characteristics of roots that influence the availability and uptake of phosphorus and nitrogen. The interaction of roots with soil microorganisms, in particular with mycorrhizal fungi and non-symbiotic plant growth promoting rhizobacteria, is also considered in relation to nutrient availability and through the mechanisms that are associated with plant growth promotion.

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

Soil bacteria are very important in biogeochemical cycles and have been used for crop production for decades. Plant–bacterial interactions in the rhizosphere are the determinants of plant health and soil fertility. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are also termed plant health promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria (NPR). These are associated with the rhizosphere, which is an important soil ecological environment for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria include the cyanobacteria of the genera Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium. Free-living nitrogen-fixing bacteria or associative nitrogen fixers, for example bacteria belonging to the species Azospirillum, Enterobacter, Klebsiella and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Generally, PGPR function in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening or preventing the plants from diseases. Plant growth promotion and development can be facilitated both directly and indirectly. Indirect plant growth promotion includes the prevention of the deleterious effects of phytopathogenic organisms. This can be achieved by the production of siderophores, i.e. small metal-binding molecules. Biological control of soil-borne plant pathogens and the synthesis of antibiotics have also been reported in several bacterial species. Another mechanism by which PGPR can inhibit phytopathogens is the production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes, e.g., chitinase and s-1,3-glucanase. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR which function through production of plant hormones such as auxins, cytokinins, gibberellins, ethylene and abscisic acid. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, have been reported for several bacterial genera. Some PGPR function as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into α-ketobutyrate and ammonia, and in this way promote root growth by lowering indigenous ethylene levels in the micro-rhizo environment. PGPR also help in solubilization of mineral phosphates and other nutrients, enhance resistance to stress, stabilize soil aggregates, and improve soil structure and organic matter content. PGPR retain more soil organic N, and other nutrients in the plant–soil system, thus reducing the need for fertilizer N and P and enhancing release of the nutrients.

Soil beneficial bacteria and their role in plant growth promotion: a review

Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion. Inoculation of crop plants with certain strains of PGPR at an early stage of development improves biomass production through direct effects on root and shoots growth. Inoculation of ornamentals, forest trees, vegetables, and agricultural crops with PGPR may result in multiple effects on early-season plant growth, as seen in the enhancement of seedling germination, stand health, plant vigor, plant height, shoot weight, nutrient content of shoot tissues, early bloom, chlorophyll content, and increased nodulation in legumes. PGPR are reported to influence the growth, yield, and nutrient uptake by an array of mechanisms. They help in increasing nitrogen fixation in legumes, help in promoting free-living nitrogen-fixing bacteria, increase supply of other nutrients, such as phosphorus, sulphur, iron and copper, produce plant hormones, enhance other beneficial bacteria or fungi, control fungal and bacterial diseases and help in controlling insect pests. There has been much research interest in PGPR and there is now an increasing number of PGPR being commercialized for various crops. Several reviews have discussed specific aspects of growth promotion by PGPR. In this review, we have discussed various bacteria which act as PGPR, mechanisms and the desirable properties exhibited by them.

Plant Growth Promoting Rhizobacteria: A Critical Review

Quorum Sensing in Bacteria: Potential in Plant Health Protection

Soil microbial populations are the key element in recycling the plant nutrients and thus play a major role in the maintenance of soil fertility and soil health. Various rhizospheric and nonrhizospheric soil samples of agricultural fields of Aligarh revealed the significant viable plate count of various functional groups of soil microorganisms. The population density of aerobic heterotrophic bacteria, actinomycetes, filamentous fungi did not significantly differ in the different field soil irrespective of crop under cultivation. Rhizospheric soil yielded almost ten times more microbial biomass as compared to non-rhizospheric soils. On the other hand population density of aerobic asymbiotic nitrogen fixers particularly Azotobacter chroococcum was poor in soil receiving longterm application of wastewaters. A total of 48 isolates of Azotobacter recovered from different crop rhizospheric soils were tentatively grouped into three morphological types on the basis of pigmentation and cultural characteristics. Isolates of group -A was tentatively identified as A. chroococcum and group B & C as the other species of Azotobacter. Carbohydrates like glucose, fructose, lactose, maltose, adonitol and rhamnose were commonly utilized by the fair number of isolates. Antibiotic resistance profile against 8 antibiotics revealed 60.4% strains resistant to nitrofurantoin followed by nalidixic acid (58.3%), co-trimazine (37.5%), novabiocin (31.2%), cloxacillin (20.8%) and least to chloramphenicol (12.5%). All test strains were found sensitive to streptomycin and doxycycline. All isolates produced ammonia, and showed varying degree of positive influence on germination of moong seeds in vitro. HCN production could be detected in five isolates. IAA production in the selected isolates varied from 3.0 to 7.6 μg/ml of culture filtrate. These isolates showed tolerance to salt (2.5-3.5% NaCl conc.) and pH value (710). It is expected that isolates exhibiting multi-PGP activities and elevated tolerance to environmental factors may be suited for further assessment and development as the effective PGPRinoculants.

Microbial Diversity Of Rhizospheric Soil With Special Reference To Plant Growth Promoting Isolates Of Azotobacter

Threats to the environment and the economy significantly worsened human health, greatly outweighing any gains associated with increasing food production. Numerous heavily fertilized crops are used as animal feed, contributing to global food inequalities and unbalanced diets, even in wealthy countries. Hazardous fertilizers have been linked to an excessive burden of communicable and non-communicable diseases, which significantly increase yields and pose considerable environmental and public health risks. The rate at which nitrogen fertilizers are applied is closely related to the buildup of toxins in the surrounding area, groundwater, and leafy and root crops. Consumption of diets including these ingredients has contributed to a significant risk to human health.

Farah Ahmad

Papers

Quorum Sensing in Bacteria: Potential in Plant Health Protection

Microbial Diversity Of Rhizospheric Soil With Special Reference To Plant Growth Promoting Isolates Of Azotobacter

Food safety and the effect of fertilizers on human health