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Plant growth-promoting rhizobacteria

Abbreviation
PGPR
Also known as
PGPR, plant growth-promoting bacteria

Plant growth-promoting rhizobacteria (PGPR) are bacteria that live in the soil around roots, on root surfaces or inside roots and benefit plant growth, for example by making nutrients available, producing plant hormones or protecting against disease.

What are plant growth-promoting rhizobacteria?

The soil immediately around roots, the rhizosphere, is rich in bacteria feeding on substances the roots release. Some of those bacteria benefit the plant. Researchers Kloepper and Schroth coined the term plant growth-promoting rhizobacteria in the late 1970s for these beneficial root-associated bacteria. Some PGPR live in the soil near roots, some on root surfaces, and some inside root tissue as endophytes.

How PGPR help plants

Mechanisms are usually grouped as direct and indirect.

Direct effects

  • Nutrient availability, including biological nitrogen fixation by some species and phosphorus solubilization.
  • Iron supply, through siderophores, compounds that bind iron and make it available.
  • Plant hormones, such as auxins, which stimulate root growth.
  • Stress relief, by lowering levels of the stress hormone ethylene in roots through the enzyme ACC deaminase.

Indirect effects

  • Biocontrol, by competing with pathogens or producing antimicrobial compounds.
  • Induced systemic resistance, priming the plant’s defenses throughout.

Common PGPR genera

Well-studied PGPR include species of Bacillus, Pseudomonas, Azospirillum and Azotobacter. Performance is strongly strain-specific.

PGPR in agricultural products

PGPR are the basis of many biostimulants, biofertilizers and microbial seed and soil treatments. Results depend on whether the bacteria survive application and establish on the roots, which varies with soil type, moisture, temperature, crop and the existing soil microbial community. That variability is one reason replicated field testing is important for these products.

Frequently asked questions

Rhizobia benefit legumes by fixing nitrogen in root nodules, and they are sometimes counted as PGPR. More often they are discussed separately, because their nodule-forming partnership is so specialized.

PGPR is defined by effect: bacteria that promote plant growth. Endophyte is defined by location: microbes living inside plant tissue. Some PGPR are endophytes; others live only in the soil or on the root surface.

  • Rhizosphere

    The rhizosphere is the thin zone of soil immediately surrounding plant roots that is directly influenced by them. Roots release sugars, amino acids and other compounds there, which feed a dense, active community of microorganisms that affects nutrient availability and plant health.

  • Bacillus

    Bacillus is a genus of rod-shaped, spore-forming bacteria common in soil. Selected strains of species such as Bacillus subtilis and Bacillus amyloliquefaciens are widely used in agricultural biologicals to promote plant growth, improve nutrient availability and suppress plant diseases.

  • Endophyte

    An endophyte is a bacterium or fungus that lives inside a plant's tissues for at least part of its life without causing disease. Some endophytes benefit their host, for example by improving nutrient uptake, growth or tolerance to stress.

  • Phosphorus solubilization

    Phosphorus solubilization is the release of phosphorus that is bound in soil minerals and organic matter into soluble forms that plant roots can absorb. Certain soil bacteria and fungi do this by producing organic acids and enzymes.

  • Biofertilizer

    A biofertilizer is a product containing living microorganisms that increase the supply or availability of nutrients to plants, for example by fixing nitrogen from the air, solubilizing phosphorus, or extending the reach of roots. The microbes, not the product's own nutrient content, deliver the benefit.

Sources

  1. Plant growth-promoting bacteria: mechanisms and applications, B. R. Glick, Scientifica, 2012
  2. Plant growth-promoting rhizobacteria in agriculture, Peer-reviewed reviews