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Phosphate and potash solubilizing bacteria from Moroccan phosphate mine showing antagonism to bacterial canker agent and inducing effective tomato growth promotion

Frontiers in Plant Science · 9 authors, 2 centres

AI SUMMARY

FIDELITY 100%

POPULATIONCherry tomato plants (Solanum lycopersicum var. cerasiforme) grown in greenhouse conditions

INTERVENTIONSoil inoculation with bacterial isolates Bg22c (Serratia sp.), Bg32c (Pseudomonas sp.), or their consortium (Bg22c + Bg32c) at 10⁶ CFU/mL

COMPARISONNegative control (no biofertilizer), non-phosphate-solubilizing Pseudomonas sp. Bg15d, and conventional NPK chemical fertilizer

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TL;DR

Two bacterial isolates (Bg22c and Bg32c) from Moroccan phosphate rock were identified as Serratia sp. and Pseudomonas sp., respectively, and shown to solubilize phosphate, potassium, and zinc while producing indole-acetic acid. In greenhouse trials, Pseudomonas sp. Bg32c significantly improved cherry tomato growth, fruit yield, stomatal conductance, and biochemical parameters (sugars, proteins, phenolics) compared to untreated controls, with performance comparable to conventional NPK fertilizer. Isolates Bg22c and Bg15d also demonstrated in vitro antagonism against Clavibacter michiganensis subsp. michiganensis, the causal agent of tomato bacterial canker, suggesting dual biofertilizer and biopesticide potential.

Full summary

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**Background:** Phosphorus (P) deficiency limits crop yields globally; most soil P is insoluble and unavailable to plants. Phosphate-solubilizing microorganisms (PSM) offer an eco-friendly alternative to chemical fertilizers. Moroccan phosphate mines represent an underexplored source of adapted PSM. This study aimed to isolate PSM from Moroccan rock phosphate, characterize their plant growth-promoting (PGP) traits, assess antagonism against Clavibacter michiganensis subsp. michiganensis (tomato bacterial canker), and evaluate their effects on cherry tomato growth under greenhouse conditions.

**Methods:** Bacterial strains were isolated from soil of a Moroccan phosphate mine on King B medium. Three isolates were selected: Bg22c, Bg32c (both showing high phosphate solubilization on NBRIP medium), and Bg15d (non-solubilizing control). Phosphate solubilization index (PSI) was calculated; quantitative P release was measured colorimetrically (Fiske-Subbarow method). Organic acids were identified by HPLC-MS. Potassium and zinc solubilization were tested on Aleksandrov and Bunt-Rovira media, respectively. IAA production was quantified by Salkowski reagent. Antagonism against C. michiganensis subsp. michiganensis was assessed by agar cylinder diffusion. Molecular identification used 16S rRNA gene sequencing (FD1/RP2 primers, ~1500 bp). Greenhouse experiment: cherry tomato plants were grown for 120 days in sterilized soil (sand 51%, clay 19%, silt 30%; pH 8.6; available P 11 ppm) with six treatments: negative control, NPK chemical fertilizer, Bg22c, Bg32c, Bg15d, and Bg22c+Bg32c consortium (8 replicates each). Bacterial inoculum (10⁶ CFU/mL) was applied at 15 and 30 days post-transplant. Measured parameters included shoot/root length, fresh/dry biomass, fruit number and weight, stomatal conductance (porometer), chlorophyll fluorescence (Fv/Fm, OS30p fluorometer), chlorophyll a/b/total and carotenoids (Arnon method), leaf P content (Olsen method), total soluble sugars (Dubois method), proteins (Bradford method), and total phenolics (Folin-Ciocalteu). Statistical analysis used ANOVA with Tukey's post-hoc test (p<0.05) and PCA.

**Key Results:** Bg22c and Bg32c showed strong P solubilization (PSI 3.83 and 2.85, respectively), K solubilization (KSI 2.50 and 4.70), and Zn solubilization (ZSI 3.5 and 2.4), while Bg15d showed negligible activity. Medium pH dropped from 6.8 to 4.73 (Bg22c) and 5.12 (Bg32c) after 48 h. HPLC identified gluconic acid (Bg22c) and oxalic acid (Bg32c) as major organic acids. IAA production was 155.00 ± 1.83 µg/mL (Bg22c), 90.62 ± 0.21 µg/mL (Bg32c), and 116.00 ± 0.25 µg/mL (Bg15d). Bg22c and Bg15d inhibited C. michiganensis subsp. michiganensis (inhibition zones 2.75 ± 0.05 cm and 2.9 ± 0.1 cm, respectively); Bg32c showed no inhibition. 16S rDNA sequencing identified Bg32c and Bg15d as Pseudomonas spp. (Bg32c closest to P. gessardii at 99.28% identity; Bg15d closest to P. putida at 98.35%) and Bg22c as Serratia sp. (98.76% identity with S. quinivorans). In greenhouse trials, Bg32c produced the greatest improvements: shoot length increased 19.2%, root length 35%, shoot fresh weight 13.5%, shoot dry weight 18.4%, root fresh weight 18.4%, root dry weight 21%, fruit fresh weight 34.6%, and fruit number 65.1% vs. negative control. Stomatal conductance increased by 21.3% at 120 days. Chlorophyll a increased 7.2%, chlorophyll b 10.0%, total chlorophyll 8.6%, and carotenoids 32.0%. Leaf P content increased 75.7% with Bg32c. Leaf sugars increased 30%, fruit sugars 60%, fruit proteins 120.2%, and total phenolics 4.8-fold vs. control. Bg32c performance was statistically comparable to NPK fertilizer for most parameters. PCA explained 92.18% of total variation (PC1 80.52%, PC2 11.66%), confirming positive correlations between Bg32c/Bg22c treatments and growth, physiological, and biochemical parameters.

**Clinical Implications:** Pseudomonas sp. Bg32c demonstrates strong potential as a biofertilizer for cherry tomato, matching conventional NPK fertilizer efficacy while providing additional benefits (Zn/K solubilization, IAA production, phenolic induction). The dual functionality of Bg22c and Bg15d (P solubilization + pathogen antagonism) suggests they could serve as integrated biofertilizer-biopesticide agents. Moroccan phosphate mines represent a valuable source of novel Pseudomonas and Serratia species for sustainable agriculture. Further research is needed to evaluate field performance, formulation stability, and efficacy against bacterial canker disease in vivo.

PICO

PPOPULATION

Cherry tomato plants (Solanum lycopersicum var. cerasiforme) grown in greenhouse conditions

IINTERVENTION

Soil inoculation with bacterial isolates Bg22c (Serratia sp.), Bg32c (Pseudomonas sp.), or their consortium (Bg22c + Bg32c) at 10⁶ CFU/mL

OOUTCOME

Shoot/root length, shoot/root fresh and dry weight, number of leaves, number of fruits, fruit fresh weight, stomatal conductance, chlorophyll fluorescence (Fv/Fm), chlorophyll a/b/total, carotenoids, total soluble sugars, total soluble proteins, total phenolic compounds, and leaf phosphorus content

STUDY TYPE

other

SPECIALTY

agriculture

SUMMARISED BY

AI pipeline

FIDELITY CHECK

100% · A

ORIGINAL PAPER

PMC10030999

Phosphate and potash solubilizing bacteria from Moroccan phosphate mine showing antagonism to bacterial canker agent and inducing effective tomato growth promotion

Frontiers in Plant Science

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ABSTRACT

Most agricultural soils are facing limited phosphorus availability that challenges modern agriculture. Phosphate solubilizing microbia (PSM) has been explored extensively as potential biofertilizers for plant growth and nutrition, and harnessing phosphate rich areas could provide such beneficial microorganisms. Isolation of PSM from Moroccan rock phosphate led to the selection of two bacterial isolates, Bg22c and Bg32c, showing high solubilization potential. The two isolates were also tested for other in vitro PGPR effects and compared to a non-phosphate solubilizing bacterium Bg15d. In addition to phosphates, Bg22c and Bg32c were able to solubilize insoluble potassium and zinc forms (P, K, and Zn solubilizers) and produce indole-acetic acid (IAA). Mechanisms of solubilization involved production of organic acids as demonstrated by HPLC. In vitro , the isolates Bg22c and Bg15d were able to antagonize the phytopathogenic bacteria Clavibacter michiganensis subsp. michiganensis , causal agent of tomato bacterial canker disease. Phenotypic and molecular identification by 16S rDNA sequencing demonstrated delineation of Bg32c and Bg15d as members of the genus Pseudomonas and Bg22c as member of the genus Serratia . The two isolates Bg22c and Bg32c were further tested either alone or in a consortium and compared to the non-P, K, and Zn solubilizing Pseudomonas strain Bg15d for their efficacy to promote tomato growth and yield. They were also compared to treatment with a conventional NPK fertilizer. Under greenhouse conditions, Pseudomonas strain Bg32c remarkably improved the growth of whole plant height, root length, shoot and root weight, number of leaves and fruits, as well as fruit fresh weight. This strain also induced stomatal conductance enhancement. The strain also improved total soluble phenolic compounds, total sugars, protein, phosphorus and phenolic compounds contents compared to the negative control. All increases were more pronounced in plants inoculated with strain Bg32c in comparison with control and strain Bg15d. The strain Bg32c could be considered a potential candidate for formulation of a biofertilizer in order to improve tomato growth.

CITATION EXPORT

B. Bouizgarne, M. Bakki, A. Boutasknit et al.. Phosphate and potash solubilizing bacteria from Moroccan phosphate mine showing antagonism to bacterial canker agent and inducing effective tomato growth promotion. Frontiers in Plant Science. (2023). https://doi.org/10.3389/fpls.2023.970382

AUTHORS

9 · 2 CENTRES

B. Bouizgarne, M. Bakki, A. Boutasknit, B. Banane, H. El Ouarrat, S. Ait El Maalem, A. Amenzou, A. Ghousmi, A. Meddich

1 Laboratory of Plant Biotechnology, Department of Biology, Faculty of Sciences, Ibn Zohr University (UIZ), Agadir, Morocco
2 Laboratory of Agro-Food, Biotechnologies & Valorization of Plant Bioresources (AGROBIOVAL), Department of Biology, Faculty of Science Semlalia, Cadi Ayyad University (UCA), Marrakesh, Morocco