Phosphate solubilizing epilithic and endolithic bacteria isolated from clastic sedimentary rocks, Murree lower Himalaya, Pakistan

Root metabolites regulated by FERONIA promote phosphorus-solubilizing rhizobacteria enrichment induced by Arabidopsis thaliana coping with phosphorus deficiency

The recruitment of the phosphorus-solubilizing rhizobacteria plays an important role in response to phosphorus deficiency. Through the treatments of Arabidopsis thaliana (Col-0) and the FERONIA (FER) functional deficient mutants (fer-4 and fer-5) with the soil suspension in various phosphorus conditions, we discovered that FER could promote phosphorus-solubilizing rhizobacteria enrichment to rescue the defective plant during phosphorus deficiency. The amplicon sequencing data reflected that the phosphorus-solubilizing rhizobacterial genus Alcaligenes was significantly enriched of Col-0 than fer-4 in low phosphorus conditions. Metabolomics analysis revealed that there were more α-D-Glucose (α-D-Glc) and L-Leucine (L-Leu) in Col-0 roots than those in fer-4 roots. The alterations of α-D-Glc and L-Leu mediated by FER had high-positive correlations to the enrichment of Alcaligenes. We successfully isolated a phosphorus-solubilizing rhizobacteria strain identified as Alcaligenes faecalis PSB15. The α-D-Glc and L-Leu could promote the strain PSB15 growth on LB agar plates and assist fer-4 in recovering from phosphorus starvation in the low phosphorus (LP) liquid medium vermiculite with tricalcium phosphate (TCP). The α-D-Glc and L-Leu could be considered as promising compounds to enrich beneficial phosphorus-solubilizing rhizobacteria, such as Alcaligenes, and provide a reference for overcoming the plight of phosphorus deficiency in crops in the field of agricultural production in the future.

Isolation and characterization of cadmium-resistant Bacillus cereus strains from Cd-contaminated mining areas for potential bioremediation applications

Cadmium (Cd) is a naturally occurring heavy metal found in the soil. However, its concentrations can be substantially increased by anthropogenic activities, presenting considerable environmental challenges. One effective remediation strategy is soil bioremediation, which employs indigenous bacteria to mitigate contamination. This study aimed to identify Cd-resistant bacteria and assess their potential for bioremediating Cd-contaminated soil. Two Cd-resistant bacterial strains, designated C9 and C27, were isolated from Cd-contaminated soil at concentrations ranging from 100 to 500 mg/L. Morphological analysis and 16S rDNA sequencing identified both strains as Bacillus cereus. The strains' capacity to adsorb and remove Cd from solutions was assessed, as well as their resistance to other heavy metals, including Zinc (Zn) and Thallium (Tl). Optimal Cd adsorption was observed at 36 h for strain C9 and at 48 h for strain C27, with maximum removal rates achieved at a Cd concentration of 70 μM. Both strains demonstrated substantial resistance to heavy metals in the order Zn > Cd > Tl on solid media. Additionally, they exhibited strong salt tolerance, starch hydrolysis, citrate utilization, and ammonia production capabilities. Notably, both strains produced significantly higher levels of siderophores compared to the model bacterium Bacillus subtilis 3,610, with strain C9 exhibiting superior siderophore production. This enhanced siderophore activity is hypothesized to contribute to Cd resistance. Collectively, these findings suggest that strains C9 and C27 have significant potential for the bioremediation of Cd-contaminated environments. Future research will focus on elucidating the molecular mechanisms underlying heavy-metal resistance and optimizing their application in large-scale bioremediation strategies.

Probing the potential of salinity-tolerant endophytic bacteria to improve the growth of mungbean [Vigna radiata (L.) Wilczek]

Soil salinity is one of the major limiting factors in plant growth regulation. Salinity-tolerant endophytic bacteria (STEB) can be used to alleviate the negative effects of salinity and promote plant growth. In this study, thirteen endophytic bacteria were isolated from mungbean roots and tested for NaCl salt-tolerance up to 4%. Six bacterial isolates, TMB2, TMB3, TMB5, TMB6, TMB7 and TMB9, demonstrated the ability to tolerate salt. Plant growth-promoting properties such as phosphate solubilization, indole-3-acetic acid (IAA) production, nitrogen fixation, zinc solubilization, biofilm formation and hydrolytic enzyme production were tested in vitro under saline conditions. Eight bacterial isolates indicated phosphate solubilization potential ranging from 5.8-17.7 μg mL-1, wherein TMB6 was found most efficient. Ten bacterial isolates exhibited IAA production ranging from 0.3-2.1 μg mL-1, where TMB7 indicated the highest potential. All the bacterial isolates except TMB13 exhibited nitrogenase activity. Three isolates, TMB6, TMB7 and TMB9, were able to solubilize zinc on tris-minimal media. All isolates were capable of forming biofilm except TMB12 and TMB13. Only TMB2, TMB6 and TMB7 exhibited cellulase activity, while TMB2 and TMB7 exhibited pectinase production. Based on in vitro testing, six efficient STEB were selected and subjected to the further studies. 16S rRNA gene sequencing of efficient STEB revealed the maximum similarity between TMB2 and Rhizobium pusense, TMB3 and Agrobacterium leguminum, TMB5 and Achromobacter denitrificans, TMB6 and Pseudomonas extremorientalis, TMB7 and Bradyrhizobium japonicum and TMB9 and Serratia quinivorans. This is the first international report on the existence of A. leguminum, A. denitrificans, P. extremorientalis and S. quinivorans inside the roots of mungbean. Under controlled-conditions, inoculation of P. extremorientalis TMB6, B. japonicum TMB7 and S. quinivorans TMB9 exhibited maximum potential to increase plant growth parameters; specifically plant dry weight was increased by up to 52%, 61% and 45%, respectively. Inoculation of B. japonicum TMB7 displayed the highest potential to increase plant proline, glycine betaine and total soluble proteins contents by 77%, 78% and 64%, respectively, compared to control under saline conditions. It is suggested that the efficient STEB could be used as biofertilizers for mungbean crop productivity under saline conditions after field-testing.

Phosphate-Solubilizing Bacteria with Low-Solubility Fertilizer Improve Soil P Availability and Yield of Kikuyu Grass

Inoculation with phosphate-solubilizing bacteria (PSB) and the application of phosphorus (P) sources can improve soil P availability, enhancing the sustainability and efficiency of agricultural systems. The implementation of this technology in perennial grasses, such as Kikuyu grass, for cattle feed in soils with high P retention, such as Andisols, has been little explored. The objective of this study was to evaluate the productive response of Kikuyu grass and soil P dynamics to BSF inoculation with different P sources. The experiment was conducted on a Kikuyu pasture, which was evaluated for 18 months (September 2020 to March 2022). Three P fertilizers with different solubility levels were applied: diammonium phosphate (DAP) (high-solubility), rock phosphate (RP), and compost (OM) (low-solubility). Moreover, the inoculation of a PSB consortium (Azospirillum brasilense D7, Rhizobium leguminosarum T88 and Herbaspirillum sp. AP21) was tested. Inoculation with PSB and fertilization with rock phosphate (RP) increased soil labile P and acid phosphomonoesterase activity. Increased grass yield and quality were related with higher soil inorganic P (Pi) availability. This study validated, under field conditions, the benefits of PSB inoculation for soil P availability and Kikuyu grass productivity.