Several microbes promote plant growth, and many microbial products that stimulate plant growth have been marketed. In this review we restrict ourselves to bacteria that are derived from and exert this effect on the root. Such bacteria are generally designated as PGPR (plant-growth-promoting rhizobacteria). The beneficial effects of these rhizobacteria on plant growth can be direct or indirect. This review begins with describing the conditions under which bacteria live in the rhizosphere. To exert their beneficial effects, bacteria usually must colonize the root surface efficiently. Therefore, bacterial traits required for root colonization are subsequently described. Finally, several mechanisms by which microbes can act beneficially on plant growth are described. Examples of direct plant growth promotion that are discussed include (a) biofertilization, (b) stimulation of root growth, (c) rhizoremediation, and (d) plant stress control. Mechanisms of biological control by which rhizobacteria can promote plant growth indirectly, i.e., by reducing the level of disease, include antibiosis, induction of systemic resistance, and competition for nutrients and niches.

Plant-growth-promoting rhizobacteria.

The influence of stonewool substrate on the exudation of the major soluble carbon nutrients and of the auxin precursor tryptophane for Pseudomonas biocontrol agents was studied. To this end, the composition of the organic acids and sugars, as well that of tryptophane, of axenically collected exudates of seed, seedlings, and roots of tomato, cucumber, and sweet pepper was determined. The major results were as follows. i) The total amount of organic acid is much higher than that of total sugar. ii) Exudation of both organic acids and sugars increases during plant growth. iii) Citric, succinic, and malic acids represent the major organic acids, whereas fructose and glucose are the major sugars. iv) Compared with glass beads as a neutral substrate, stonewool substantially stimulates exudation of organic acids and sugars. v) It appeared that enhanced root-tip-colonizing bacteria isolated previously from the rhizosphere of tomato and cucumber grow much better in minimal medium with citrate as the sole carbon so...

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-19-0250

Organic acids, sugars, and L-tryptophane in exudates of vegetables growing on stonewool and their effects on activities of rhizosphere bacteria.

/pdf/bacteria-in-agrobiology-crop-ecosystems-4itmo41pw2.pdf

Bacteria in Agrobiology: Crop Ecosystems

Pseudomonas fluorescens PCL1751 is a rod-shaped Gram-negative bacterium isolated from the rhizosphere of a greenhouse-grown tomato plant in Uzbekistan. It controls several plant root diseases caused by Fusarium fungi through the mechanism of competition for nutrients and niches (CNN). This mechanism does not rely on the production of antibiotics, so it avoids the concerns of resistance development and is environmentally safe. Additionally, this bacterium promotes plant growth by alleviating salt stress for its plant host. To investigate the genetic mechanisms that may explain these observations, we determined the complete genome sequence of this bacterium, examined its gene content, and performed comparative genomics analysis with other Pseudomonas strains. The genome of P. fluorescens PCL1751 consisted of one circular chromosome that is 6,143,950 base-pairs (bp) in size; no plasmid was found. The annotation included 19 rRNA, 70 tRNA, and 5,534 protein-coding genes. The gene content analysis identified a large number of genes involved in chemotaxis and motility, colonization of the rhizosphere, siderophore biosynthesis, and osmoprotectant production. In contrast, the pathways involved in the biosynthesis of phytohormones or antibiotics were not found. Comparison with other Pseudomonas genomes revealed extensive variations in their genome size and gene content. The presence and absence of secretion system genes were highly variable. As expected, the synteny conservation among strains decreased as a function of phylogenetic divergence. The integration of prophages appeared to be an important driver for genome rearrangements. The whole-genome gene content analysis of this plant growth-promoting rhizobacterium (PGPR) provided some genetic explanations to its phenotypic characteristics. The extensive and versatile substrate utilization pathways, together with the presence of many genes involved in competitive root colonization, provided further support for the finding that this strain achieves biological control of pathogens through effective competition for nutrients and niches.

/pdf/genome-analysis-of-pseudomonas-fluorescens-pcl1751-a-4ho6kzuc1x.pdf

Genome Analysis of Pseudomonas fluorescens PCL1751: A Rhizobacterium that Controls Root Diseases and Alleviates Salt Stress for Its Plant Host.

The world’s population is assumed to increase from 7 billion now to 8.3 billion in 2025. The world will need 70–100% more food by 2050 (Godfray et al., 2010). Therefore, the production of cereals, especially wheat, rice, and maize, which accounts for half of human calorie intake, has to be increased. Currently, plant growth is enhanced by the input of chemicals which act as plant growth regulators (using a hormonal mechanism) and as nutrients. Of the nutrients added to the soil, nitrogen and phosphorus are the major ones. They are, together with potassium, applied as chemical fertilizers to improve grain yield. According to Roberts (2009), the present global annual use of chemical nitrogen, phosphorus, and potash fertilizer is 130, 40, and 35 million tonnes, respectively. The high input of chemicals raises a number of concerns such as water contamination leading to

Plant Growth Promotion by Microbes

The role of competitive root tip colonization in the biological control of tomato foot and root rot.

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The role of competitive root tip colonization in the biological control of tomato foot and root rot.