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

A . K H A L I D , M . A R S H A D A N D Z . A . Z A H I R . 2004. Aims: Plant growth promoting rhizobacteria (PGPR) are commonly used as inoculants for improving the growth and yield of agricultural crops, however screening for the selection of effective PGPR strains is very critical. This study focuses on the screening of effective PGPR strains on the basis of their potential for in vitro auxin production and plant growth promoting activity under gnotobiotic conditions. Methods and Results: A large number of bacteria were isolated from the rhizosphere soil of wheat plants grown at different sites. Thirty isolates showing prolific growth on agar medium were selected and evaluated for their potential to produce auxins in vitro. Colorimetric analysis showed variable amount of auxins (ranging from 1AE 1t o 12AE 1m g l )1 ) produced by the rhizobacteria in vitro and amendment of the culture media with L-tryptophan (L-TRP), further stimulated auxin biosynthesis (ranging from 1AE 8t o 24AE 8m g l )1 ). HPLC analysis confirmed the presence of indole acetic acid (IAA) and indole acetamide (IAM) as the major auxins in the culture filtrates of these rhizobacteria. A series of laboratory experiments conducted on two cv. of wheat under gnotobiotic (axenic) conditions demonstrated increases in root elongation (up to 17AE3%), root dry weight (up to 13AE5%), shoot elongation (up to 37AE7%) and shoot dry weight (up to 36AE3%) of inoculated wheat seedlings. Linear positive correlation (r ¼ 0AE99) between in vitro auxin production and increase in growth parameters of inoculated seeds was found. Based upon auxin biosynthesis and growth-promoting activity, four isolates were selected and designated as plant growth-promoting rhizobacteria (PGPR). Auxin biosynthesis in sterilized vs nonsterilized soil inoculated with selected PGPR was also monitored that revealed superiority of the selected PGPR over indigenous microflora. Peatbased seed inoculation with selected PGPR isolates exhibited stimulatory effects on grain yields of tested wheat cv. in pot (up to 14AE7% increase over control) and field experiments (up to 27AE5% increase over control); however, the response varied with cv. and PGPR strains. Conclusions: It was concluded that the strain, which produced the highest amount of auxins in nonsterilized soil, also caused maximum increase in growth and yield of both the wheat cv. Significance and Impact of Study: This study suggested that potential for auxin biosynthesis by rhizobacteria could be used as a tool for the screening of effective PGPR strains.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.2003.02161.x

Screening plant growth-promoting rhizobacteria for improving growth and yield of wheat.

Publisher Summary The chapter presents a discussion on plant growth-regulating (PGR) substances in the rhizosphere, with emphasis on microbial production and functions. The chapter discusses the rhizosphere as a site of plant-microbe interactions; plant growth-regulating substances and their sources; biochemistry of microbial production of PGRs; production of PGRs by rhizosphere microorganisms; metabolism of PGRs in soil; and ecological significance of PGRs produced in the rhizosphere. The chapter provides a better understanding of the mechanisms of actions of microbially derived PGRs and their interactions with plants. Moreover, development of hormone-deficient plant mutants can provide opportunities to clearly define the role of microbially produced PGRs in plan-microbe interactions. An understanding of these aspects can aid in the utilization of microbial PGRs for the betterment and benefit of sustainable agriculture.

Plant growth-regulating substances in the rhizosphere: microbial production and functions

Inoculation of zinc solubilizing Bacillus aryabhattai strains for improved growth, mobilization and biofortification of zinc in soybean and wheat cultivated in Vertisols of central India

Involvement of siderophores in the reduction of metal-induced inhibition of auxin synthesis in Streptomyces spp

The presence of auxins in soil may have an ecological impact affecting plant growth and development. A rapid and simple colorimetric method was used to assess California soils for their potential to produce auxins upon the addition of L-tryptophan (L-TRP). The auxin content measured by colorimetry was expressed as indole-3-acetic acid (IAA)-equivalents. A substrate (L-TRP) concentration of 5.3 g kg-1, glucose concentration of 6.7 g kg-1, no nitrogen, pH 7.0, 40°C, shaking (aeration) and 48 h incubation time were selected as standardized conditions to assay for auxin biosynthesis in soil. IAA was confirmed as a major microbial metabolite derived from L-TRP in soil by use of high performance liquid chromatography (HPLC). Under standardized conditions, L-TRP-derived auxins in 19 soils varied greatly ranging from 18.2 to 303.2 mg IAA equivalents (auxins) kg-1 soil. This study suggests that the phenotypic character of the soil microbiota has more of an influence on auxin production than the soil physicochemical properties (e.g., pH, organic C content, CEC, etc.).

Tryptophan-dependent biosynthesis of auxins in soil

We describe the iterative co-design process and evaluation of an early autism screening tool (EAST). EAST is an intermediary interactive tablet based app that assists in the early-detection of Autism Spectrum Disorder (ASD) by screening preschoolers in Pakistan through play-based activities in a home, school or clinical setting. Medical professionals, parents of autistic children and teachers were surveyed through focus groups to understand the reasons that contribute to the increasing number of missed early detections, and late- or misdiagnoses. We also evaluate the acceptability, usability and validity of our tool. We tested EAST with both typically developed and autistic children on how they relate to people, imitation, motor skills, visual and intellectual response. They were scored via time taken, the number of wrong attempts, or incorrect answers and audiovisual feedback. This paper contributes towards a digital autism screening tool that delivers insights into the child's behaviour and enables collaboration among parents, teachers and medical professionals.

EAST: Early Autism Screening Tool for Preschoolers

Glasshouse experiments were conducted to evaluate the influence of L-TRP in comparison with indole-3-acetamide (IAM), tryptophol (TOL) and indole-3-acetic acid (IAA) on the growth of Zea mays L. var. Early Sunglow. L-TRP (25 to 2.5×10−5 mg kg−1 soil), IAM (22 to 2.2×10−5 mg kg−1 soil), TOL (20 to 2.0×10−5 mg kg−1 soil), and IAA (22 to 2.2×10−5 mg kg−1 soil) were applied as a soil drench to established uniform seedlings. All treatments were applied in a completely randomized design with 10 replicates. IAM had no significant effect on the plant growth parameters. Shoot height, uppermost leaf collar base distance, internodal distance, and shoot dry and fresh weights were significantly improved upon the addition of TOL (2.0×10−2 mg kg−1 soil), however, the highest concentration (20 mg kg−1 soil) caused a 14.6% reduction in leaf width. L-TRP (2.5×10−3 mg kg-1 soil) also had a significant influence on shoot height, uppermost leaf collar base distance, internodal distance and fresh weight of shoot compared with the control. The highest concentration of L-TRP (25=mg kg−1 soil) had a negative effect on leaf width and dry weight of the shoot. The most pronounced response on the corn growth parameters was observed with the application of IAA at lower concentrations (2.2×10−5 to 2.2×10−2 mg kg−1 soil) specifically improving root growth. The highest concentration (22 mg kg−1 soil) of IAA had a significant negative effect on plant height, leaf width, stem diameter, shoot fresh and dry weight. These findings indicate that L-TRP applied at the appropriate concentrations can have positive effects on corn growth comparable to pure auxins (TOL and IAA).

Influence of L-tryptophan and auxins applied to the rhizosphere on the vegetative growth of Zea mays L

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Tryptophan-dependent biosynthesis of auxins in soil

EAST: Early Autism Screening Tool for Preschoolers

Influence of L-tryptophan and auxins applied to the rhizosphere on the vegetative growth of Zea mays L