What is Plant Growth-Promoting Rhizobacteria (PGPR)?

Among free-living bacteria in plant-microbe co-evolution are Plant Growth-Promoting Rhizobacteria (PGPR). They exert beneficial effects on plants through direct and indirect mechanisms.

Beneficial rhizobacteria have been utilized to improve water and nutrient uptake, abiotic and biotic stress tolerance, for example, salt stress. Also numerous soil bacteria have been reported to promote plant growth and development. However, the mode of action by which the bacteria exhibit beneficial activities are often not well understood.

it is suggested that many rhizosphere bacteria excrete hormones for root uptake or manipulate hormone balance in the plants to boost growth and stress response.

Many PGPR produce cytokinins and gibberellins (Gupta et al., 2015; Kumar et al., 2015) but the role of bacterially synthesized hormones in plants, and bacterial mechanism of synthesis, are not yet completely understood (Garcia de Salamone et al., 2001; Kang et al., 2009). Some strains of PGPR can promote relatively large amounts of gibberellins, leading to enhanced plant shoot growth (Jha and Saraf, 2015). Interactions of these hormones with auxins can alter root architecture (Vacheron et al., 2013). Production of cytokinins by PGPR can also lead to enhanced root exudate production by the plant (Ruzzi and Aroca, 2015) potentially increasing the presence of PGPR associated with the plant.

Why is Rhizobacteria important for grain legumes?

In 2020, a group of Indian scientists reviewed significance of plant growth promoting rhizobacteria in grain legumes.

Grain legumes as an important component of sustainable agri-food systems establish symbiotic association with rhizobia and arbuscular mycorrhizal fungi. This relationship reduces the use of chemical fertilizers.

Several other free-living microbial communities (PGPR—plant growth promoting rhizobacteria) residing in the soil-root interface are also known to influence biogeochemical cycles and improve legume productivity.

The growth and function of these microorganisms are affected by root exudate molecules secreted in the rhizosphere region.

Advantages of Plant Growth-Promoting Rhizobacteria (PGPR) in Legumes

PGPRs produce the chemicals which stimulate growth and functions of leguminous crops at different growth stages. They promote plant growth by nitrogen fixation, solubilization as well as mineralization of phosphorus, and production of phytohormone(s).

PGPRs interact with plants through various direct and indirect mechanisms which are functions of PGPR activities and biotic as well as abiotic factors present in the surroundings:

The co-inoculation of PGPRs along with rhizobia has shown to enhance nodulation and symbiotic interaction. The recent molecular tools are helpful to understand and predict the establishment and function of PGPRs and plant response. In this review, we provide an overview of various growth promoting mechanisms of PGPR inoculations in the production of leguminous crops.

What is Quinoa crop?

Quinoa (Chenopodium quinoa) is a member of the amaranth family, along with beets and spinach.

It is a multipurpose agricultural crop with great genetic diversity that is famous for its tolerance to harsh environmental conditions. Quinoa has the capability to adapt to diverse agroecosystems and tolerates different environmental conditions from sea level to nearly 4000 m in elevation.

Quinoa is closely related to the weed known as “lambsquarters” (fat han). At the same time, the quinoa leaves on maturing plants also seem to be broader than the lambsquarters.

Quinoa is a pseudograin?

Quinoa is known as a pseudograin. It has this status because its grains are an achene type similar to cereals that can be ground into flour and used as a grain crop. Although quinoa does not belong to the Poaceae botanical family.

At the same time, the seeds can be consumed for human food and the crop residues may be utilized for animal feed, in addition to several valuable compounds such as saponin, oil, and protein concentrate.

Salinity tolerance for crops

Salinity is one of the negative effects of urbanisation of agricultural lands. Salinity increases the competition between crops and energy plant species.

Indeed, salt stress is a major encountered issue that affects crop production. Saline and dry soils are poorly accessible for agricultural cultivation. Salinity negatively influences the physicochemical properties of soils and suppresses the growth of both plants and soil microbes.

Several approaches have been developed to reduce the toxic effects caused by high salinity on plants. These include the use of:

• conventional breeding,
• halophyte varieties,
• transgenic editing.

However, these strategies are time-consuming, unsustainable, and labour-intensive [12].

In contrast plant-associated microorganisms can be harnessed to promote plant performance and yield under such conditions. Thus, the bioremediation of saline soils requires the use of plants and microbes that can tolerate salinity.

In fact, salt tolerant PGPB mediates several mechanisms within plants facing salinity. As a result, this enhances nutrient assimilation, promotes homeostasis, and increases antioxidant response during salty conditions.

Abstract of the research

Plant Growth-Promoting Rhizobacteria (PGPR) have attracted much attention in agriculture biotechnology as biological inputs to sustain crop production. The present study describes a halotolerant phosphate solubilizing bacterium associated with quinoa plant roots.

Based on a metabolic screening, one bacterial isolate, named QA2, was selected and screened for PGPR traits. This isolate solubilized both inorganic phosphate and zinc, produced indole-3-acetic acid, ammonia, hydrogen cyanide, cellulase, and (to be deleted) protease, and induced biofilm formation. We demonstrated that QA2 exhibited both antimicrobial and ion metabolism activities and tolerated high salt concentration at up to 11% NaCl.

Genotyping analyses, using 16S rRNA and chaperonin cpn60 genes, revealed that QA2 belongs to the species of Bacillus velezensis.

Using the quinoa model cultivated under a saline condition, we demonstrated that QA2 promoted plant growth and mitigated the saline irrigation effects.

Analysis of harvested plants revealed that QA2 induced a significant increase of both leaf chlorophyll index by 120.86% (p < 0.05) and P uptake by 41.17% (p < 0.05), while the content of Na+ was drastically decreased.

Lastly, a bibliometric data analysis highlighted the panoramic view of studies carried out so far on B. velezensis strains. Our investigation presents a holistic view of the potential application of B. velezensis as a biological inoculant to promote plant growth, control pathogen attacks, and mitigate the salinity effect of quinoa plants. Further investigations are still needed to demonstrate these effects in field conditions.

Materials and Methods of this Research

1. Soil sampling

2.  Isolation, Screening, and Purification of Phosphate Solubilizing Rhizobacteria on Plates

3. Screening for Salt Tolerance

4. Quantitative Assay of P Solubilization in Liquid Media

5. DNA Amplification and Phylogenetic Identification of Selected Rhizobacterium

6. In Vitro Evaluation of PGP Traits

7. In-Vivo Inoculation Experiment. On the stage of plant vegetative attributes measurement, the plant research team determined leaf area using the Petiole mobile application.

8. Bibliometric Analysis of Bacillus velezensis Strains

9. Statistical Analysis

Official credentials

Mahdi, I.; Allaoui, A.; Fahsi, N.; Biskri, L. Bacillus velezensis QA2 Potentially Induced Salt Stress Tolerance and Enhanced Phosphate Uptake in Quinoa Plants. Microorganisms 2022, 10, 1836. https://doi.org/10.3390/microorganisms10091836

Received: 27 August 2022
Revised: 8 September 2022
Accepted: 8 September 2022
Published: 14 September 2022

This article belongs to the Special Issue Microbial-Based Plant Biostimulants

Other resources about Plant Growth-Promoting Rhizobacteria (PGPR) for Quinoa plants published in 2020 - 2022

1. El-Shamy, M.A.; Alshaal, T.; Mohamed, H.H.; Rady, A.M.S.; Hafez, E.M.; Alsohim, A.S.; Abd El-Moneim, D. Quinoa Response to Application of Phosphogypsum and Plant Growth-Promoting Rhizobacteria under Water Stress Associated with Salt-Affected Soil. Plants 2022, 11, 872. https://doi.org/10.3390/plants11070872

2. Cai D, Xu Y, Zhao F, Zhang Y, Duan H, Guo X. Improved salt tolerance of Chenopodium quinoa Willd. contributed by Pseudomonas sp. strain M30-35. PeerJ. 2021 Jan 13;9:e10702. https://doi.org/10.7717/peerj.10702

3. Carolina Chumpitaz-Segovia, Débora Alvarado, Katty Ogata-Gutiérrez, and Doris Zúñiga-Dávila. Bioprospection of native psychrotolerant plant-growth-promoting rhizobacteria from Peruvian Andean Plateau soils associated with Chenopodium quinoa. Canadian Journal of Microbiology. 66(11): 641-652. https://doi.org/10.1139/cjm-2020-0036