Characterization of plant growth-promoting rhizobacterial isolates associated with food plants in South Africa

Metagenomics approaches in unveiling the dynamics of Plant Growth-Promoting Microorganisms (PGPM) vis-à-vis Phytophthora sp. suppression in various crop ecological systems

Phytophthora species are destructive pathogens causing yield losses in different ecological systems, such as potato, black pepper, pepper, avocado, citrus, and tobacco. The diversity of plant growth-promoting microorganisms (PGPM) plays a crucial role in disease suppression. Knowledge of metagenomics approaches is essential for assessing the dynamics of PGPM and Phytophthora species across various ecosystems, facilitating effective management strategies for better crop protection. This review discusses the dynamic interplay between PGPM and Phytophthora sp. using metagenomics approaches that sheds light on the potential of PGPM strains tailored to specific crop ecosystems to bolster pathogen suppressiveness.

Isolation and Characterization of Plant-Growth-Promoting, Drought-Tolerant Rhizobacteria for Improved Maize Productivity

Drought is one of the main abiotic factors affecting global agricultural productivity. However, the application of bioinocula containing plant-growth-promoting rhizobacteria (PGPR) has been seen as a potential environmentally friendly technology for increasing plants' resistance to water stress. In this study, rhizobacteria strains were isolated from maize (Zea mays L.) and subjected to drought tolerance tests at varying concentrations using polyethylene glycol (PEG)-8000 and screened for plant-growth-promoting activities. From this study, 11 bacterial isolates were characterized and identified molecularly, which include Bacillus licheniformis A5-1, Aeromonas caviae A1-2, A. veronii C7_8, B. cereus B8-3, P. endophytica A10-11, B. halotolerans A9-10, B. licheniformis B9-5, B. simplex B15-6, Priestia flexa B12-4, Priestia flexa C6-7, and Priestia aryabhattai C1-9. All isolates were positive for indole-3-acetic acid (IAA), siderophore, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, ammonia production, nitrogen fixation, and phosphate solubilization, but negative for hydrogen cyanide production. Aeromonas strains A1-2 and C7_8, showing the highest drought tolerance of 0.71 and 0.77, respectively, were selected for bioinoculation, singularly and combined. An increase in the above- and below-ground biomass of the maize plants at 100, 50, and 25% water-holding capacity (WHC) was recorded. Bacterial inoculants, which showed an increase in the aerial biomass of plants subjected to moderate water deficiency by up to 89%, suggested that they can be suitable candidates to enhance drought tolerance and nutrient acquisition and mitigate the impacts of water stress on plants.

Characterization of plant growth-promoting bacteria isolated from rhizosphere of lentil (Lens culinaris L.) grown in two different soil orders of eastern India

Lentil, which is an important grain legume, can be co-inoculated with plant growth-promoting rhizobia and rhizobacteria to boost nitrogen fixation, increase biomass, and a possibility for early nodulation. The goal of the ongoing study was to identify plant growth-promoting rhizobacteria (PGPR) in the rhizosphere of lentil growing soils in eastern India. Sixteen rhizosphere bacteria were isolated from two different soil orders, and their capacity to solubilize phosphate and generate hydrogen cyanide (HCN), siderophore, and indole acetic acid (IAA) was assessed. The three best strains were selected for compatibility study with twenty Rhizobium isolated from lentil root nodules. The isolated rhizobacteria were able to produce ammonia and different mycolytic enzymes. Isolate B3 produced the highest amount of IAA and siderophore; the highest amount of phosphate solubilized by PSB1 strain; and isolates AB1, AB2, B3, PS2, and PSB2 produced considerable amount of HCN gas. Among all the isolates, B3, PSB1, and PS2 performed better based on different plant growth-promoting abilities. These three bacterial isolates showed compatible reaction with most of the Rhizobium strains. Isolates B3, PS2, and PSB1 were identified as Bacillus subtilis (MT729775), Pseudomonas palmensis (MT729782), and Paraburkholderia caribenis (MZ956803), respectively. Lentil shoot weight, root length, nodule number, N uptake, and P uptake were increased in the pot culture experiment when inoculated with these strains. PGPR strain B3 performed best among the three strains in the pot culture experiment. Strain B3 can be used as potential biofertilizer along with compatible Rhizobium species for better production of lentil.

Cytobacillus citreus sp. nov., isolated from citrus rhizosphere soil

A Gram-stain-positive, rod-shaped and motile strain, designated FJAT-49705^T, was isolated from the citrus rhizosphere soil sample. Strain FJAT-49705^T grew at 20-40 °C (optimum, 30 °C) and pH 6.0-11.0 (optimum, pH 7.0) with 0-5 % (w/v) NaCl (optimum, 2 %). Strain FJAT-49705^T showed high 16S rRNA gene sequence similarity to 'Bacillus dafuensis' FJAT-25496^T (99.7 %) and Cytobacillus solani FJAT-18043^T (98.0 %). In phylogenetic (based on 16S rRNA gene sequences) and phylogenomic trees (based on 71 bacterial single-copy genes), strain FJAT-49705^T clustered with the members of the genus Cytobacillus. MK-7 was the only isoprenoid quinone present. The main polar lipids were diphosphatidylglycerol, phosphatidylethanolamine and an unidentified phospholipid. The major fatty acids were anteiso-C_15 : 0 and iso-C_15 : 0. The genomic DNA G+C content was 36.9 %. The average nucleotide identity (ANI) values between FJAT-49705^T and 'B. dafuensis' FJAT-25496^T and C. solani FJAT-18043^T were below the cut-off level (95-96 %) recommended as the ANI criterion for interspecies identity. Based on the above results, strain FJAT-49705^T represents a novel species of the genus Cytobacillus, for which the name Cytobacillus citreus sp. nov. is proposed. The type strain is FJAT-49705^T (=CCTCC AB 2019243^T= LMG 31580^T).

Combined effects of oxytetracycline and microplastic on wheat seedling growth and associated rhizosphere bacterial communities and soil metabolite profiles

The widespread application of antibiotics and plastic films in agriculture leads to new characteristics of soil pollution with the coexistence of antibiotics and microplastics. However, their combined effects on wheat seedling growth and associated rhizosphere bacterial communities and soil metabolite profiles remain unclear. Here, in the potted experiment, wheat was treated with individual oxytetracycline (0, 5.0, 50.0, and 150.0 mg kg^-1) and the combination of oxytetracycline and polyethylene microplastic (0.2%). Results showed that 150 mg kg^-1 oxytetracycline combined with microplastic significantly reduced the biomass and height of the plant. Compared with CK, all the treatments exposed to the combination of oxytetracycline and polyethylene microplastic significantly promoted carotenoid content and peroxidase activity in wheat leaves. Soil dehydrogenase and urease activities were more sensitive to current pollutant exposure than sucrase activity. Oxytetracycline (150 mg kg^-1) alone and in combination with polyethylene significantly decreased the abundances of certain genera belonging to plant growth-promoting rhizobacteria (PGPR) in soil, such as Arthrobacter, Gemmatimonas, Massilia, and Sphingomonas. Combined exposure of 150 mg kg^-1 oxytetracycline and polyethylene microplastic significantly altered multiple metabolites including organic acids and sugars. Network analysis indicated that co-exposure of 150 mg kg^-1 oxytetracycline and microplastic may affect the colonization and succession of PGPR by regulating soil metabolites, thereby indirectly inhibiting wheat seedling growth. The results help to elucidate the potential mechanisms of phytotoxicity of the combination of oxytetracycline and polyethylene microplastic.