Determination of Plant Growth-Promoting Activity of Plant Growth-Promoting Bacteria: Testing Princip

The determination of plant growth-promoting (PGP) activity is a core analytical process used to characterize beneficial bacteria—such as rhizobacteria and endophytes—by quantifying traits like indole-3-acetic acid production, phosphate solubilization, nitrogen fixation, and siderophore secretion. This testing supports strain screening, microbial fertilizer registration, and product quality control in accordance with recognized standards such as GB 20287 and NY/T 1113. This article provides a technical overview of the samples, testing items, methodologies, and applications involved in evaluating the growth-promoting effects of beneficial microorganisms.

Technical Overview

Plant growth-promoting bacteria (PGPB), including plant growth-promoting rhizobacteria (PGPR), are beneficial microorganisms that colonize the rhizosphere, rhizoplane, or internal plant tissues and enhance host growth through direct and indirect mechanisms. Direct mechanisms include phytohormone production (indole-3-acetic acid, gibberellins, cytokinins), mineral nutrient mobilization (phosphate, potassium, zinc solubilization), biological nitrogen fixation, siderophore-mediated iron acquisition, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity that lowers plant ethylene levels. Indirect mechanisms include antagonism against phytopathogens through the secretion of hydrolytic enzymes, antibiotics, hydrogen cyanide, and induced systemic resistance.

Determination of plant growth-promoting activity is a standardized laboratory evaluation process that combines in vitro biochemical assays, molecular detection of functional genes, and in vivo plant-based trials. In China, microbial fertilizer products are governed by the national standard GB 20287 and supporting agricultural industry standards such as NY/T 1113 and NY 227, and strain-level functional characterization is a prerequisite for product registration, quality control, and claims substantiation. Accredited third-party laboratories and research institutions typically perform these evaluations using validated qualitative and quantitative protocols.

Testing Samples

Pure Bacterial Isolates and Culture Collections

Newly isolated strains from rhizosphere soil, root interiors, or environmental samples, as well as preserved strains from culture collections, are the primary subjects for functional characterization. Isolates must be purified, verified for viability, and identified to the species level (typically by 16S rRNA gene sequencing) before trait testing begins.

Fermentation Broths and Cell-Free Supernatants

Liquid fermentation cultures and their filtered supernatants are tested to quantify extracellular metabolites such as indole-3-acetic acid, siderophores, ammonia, organic acids, and hydrolytic enzymes. These samples reflect the functional output of a strain under controlled fermentation conditions.

Microbial Inoculant Products and Biofertilizers

Commercial products, including solid powder, granular, and liquid formulations, are evaluated to confirm that the declared functional strains retain their growth-promoting traits after formulation, storage, and transportation. Testing supports conformity assessment against GB 20287 and related registration requirements.

Rhizosphere Soil, Root, and Plant Tissue Samples

Field or greenhouse samples are analyzed to verify root colonization competence, measure population dynamics of inoculated strains, and assess plant growth responses such as germination rate, root architecture, biomass accumulation, and nutrient uptake.

Testing Items

Common testing items for determining plant growth-promoting activity include:

• Indole-3-acetic acid (IAA) and total auxin production, with and without L-tryptophan supplementation
• Phosphate solubilization (inorganic and organic phosphorus) and quantification of soluble phosphorus release
• Biological nitrogen fixation activity, including acetylene reduction (nitrogenase) assay and ammonia production
• Siderophore production (hydroxamate, catecholate, and carboxylate types)
• ACC deaminase activity and quantification of alpha-ketobutyrate generation
• Potassium and zinc solubilization capacity
• Gibberellin and cytokinin production
• Extracellular enzyme activities, including cellulase, protease, chitinase, phytase, and amylase
• Hydrogen cyanide (HCN) production and antifungal antagonism against soil-borne pathogens
• Exopolysaccharide production and biofilm formation ability
• Root colonization competence and population persistence
• Plant response parameters: seed germination rate, root and shoot length, fresh and dry biomass, chlorophyll content, and nutrient uptake in pot trials
• Functional gene detection, such as nifH (nitrogen fixation), ipdC (IAA biosynthesis), acdS (ACC deaminase), and pqq genes (mineral solubilization)

Testing Methods

Qualitative Plate and Solid Media Assays

Screening-level assays deploy bacteria onto selective agar media to visualize functional halos or color changes. Pikovskaya medium with tricalcium phosphate reveals phosphate-solubilizing halos; chrome azurol S (CAS) agar detects siderophore production through a color shift from blue to orange; N-free semi-solid malate medium (Nfb) identifies diazotrophs by forming a typical pellicle; and skim milk, carboxymethyl cellulose, and colloidal chitin plates detect protease, cellulase, and chitinase activity, respectively.

Quantitative Spectrophotometric and Colorimetric Methods

Quantification is typically performed by UV-visible spectrophotometry. IAA is measured with the Salkowski reagent at approximately 530 nm against a standard curve; soluble phosphorus is determined by the molybdenum blue (ascorbic acid) method at 700–880 nm; siderophores are quantified using the CAS liquid assay at 630 nm; ACC deaminase activity is assayed by measuring alpha-ketobutyrate at 540 nm; and ammonia is detected with Nessler's reagent. Organic acid profiles released during mineral solubilization can be further characterized by high-performance liquid chromatography (HPLC).

Molecular Biological Methods

Polymerase chain reaction (PCR) amplification and sequencing of functional marker genes confirm the genetic potential of a strain for nitrogen fixation (nifH), IAA biosynthesis (ipdC, iaAM), ACC deaminase (acdS), and mineral solubilization pathways. Quantitative real-time PCR can estimate gene expression levels, while whole-genome sequencing provides comprehensive annotation of plant growth-promoting gene clusters.

In Vivo Plant Bioassays

Seed germination tests, gnotobiotic pouch or agar-based seedling assays, and greenhouse pot trials validate laboratory findings under plant-interacting conditions. Inoculated plants are compared with non-inoculated controls across parameters such as germination percentage, root and shoot elongation, dry matter accumulation, and nutrient content. Statistically designed replicated trials (typically with at least three replications) determine the significance of growth promotion effects.

Standards and Reference Protocols

Testing is conducted with reference to applicable national and industry standards, including GB 20287 for microbial fertilizers, NY/T 1113 for the determination of microbial fertilizer efficacy, and NY 227 for liquid microbial fertilizer specifications, supplemented by internationally recognized methodological references from organizations such as ISO where applicable.

Applications

Determination of plant growth-promoting activity serves multiple purposes across research, industry, and regulatory contexts:

• Screening and selection of high-efficiency functional strains for biofertilizer and biostimulant development
• Registration support for microbial fertilizer products requiring documented functional efficacy
• Quality control during fermentation scale-up, formulation, shelf-life studies, and batch release
• Verification of functional stability after processing, storage, and field application
• Scientific research on plant-microbe interactions, microbiome engineering, and sustainable agriculture
• Support for organic farming systems, reduced chemical fertilizer input programs, and soil health restoration projects
• Evaluation of combined inoculant formulations, including consortium products containing multiple compatible strains

FAQ

What is the difference between qualitative and quantitative determination of growth-promoting traits?

Qualitative methods confirm the presence or absence of a trait by observing halo zones or color reactions on selective media, making them suitable for rapid screening. Quantitative methods measure the actual amount of the metabolite released (for example, micrograms of IAA per milliliter or micrograms of soluble phosphorus per milliliter), providing numerical data suitable for strain comparison, product specifications, and regulatory submissions.

Why must in vitro results be confirmed by plant trials?

In vitro assays demonstrate the biochemical potential of a strain under artificial conditions, but effective plant growth promotion also depends on root colonization ability, competitiveness with native microbes, environmental tolerance, and compatibility with the host plant. Pot and field trials confirm that laboratory traits translate into measurable growth benefits under realistic conditions.

How long does a complete growth-promoting activity evaluation take?

Individual in vitro assays typically require several days of culture plus analysis. A comprehensive characterization covering multiple traits usually takes two to four weeks, while greenhouse pot trials require an additional four to eight weeks depending on the plant species and growth cycle.

Which traits are most important when selecting a plant growth-promoting strain?

Priority traits generally include IAA production, phosphate solubilization, nitrogen fixation, siderophore secretion, and ACC deaminase activity, as these directly influence nutrient availability and root development. The optimal trait profile depends on the target crop, soil conditions, and intended application scenario, and multi-trait strains with confirmed root colonization competence are usually preferred.

Can formulated commercial products be tested for retained growth-promoting activity?

Yes. Finished inoculant products are analyzed by re-isolating the declared functional strains, verifying their identity, and re-running the relevant functional assays. This verifies that the strains retain their declared beneficial traits at or above specified thresholds throughout the product shelf life, in line with the requirements of GB 20287 and related standards.