Assessment of Bacterial Inoculant Delivery Methods for Cereal Crops

Diversity and Plant Growth-Promoting Activities of Culturable Seed Endophytes in Abies pindrow (Royle ex D. Don) Royle: Their Role in Seed Germination and Seedling Growth

Abies pindrow, a vital conifer in the Kashmir Himalayan forests, faces threats from low regeneration rates, deforestation, grazing, and climate change, highlighting the urgency for restoration efforts. In this context, we investigated the diversity of potential culturable seed endophytes in A. pindrow, assessed their plant growth-promoting (PGP) activities, and their impact on seed germination and seedling growth. We cultured 729 microbial isolates that were resolved into 30 bacterial and 18 fungal species across various phyla. All 48 isolates exhibited various PGP activities. Specifically, all the cultured isolates showed IAA activity with concentrations ranging from 2.07 to 8.453 μg/ml, while ammonia production ranged from 0.936 to 3.436 mM/ml. Only 18 isolates, predominantly fungi, tested positive for phosphate solubilisation. Additionally, 20 isolates exhibited the ability to inhibit the growth of Fusarium oxysporum f.sp. pini. We selected four bacterial and six fungal isolates, which showed positive results for all PGP activities, to evaluate their effects on seed germination and seedling growth. Notably, seed germination rates increased by 750.9% under bacterial and consortium treatments and by 550.45% under fungal treatment. The consortium treatment also led to a 96% increase in needle count, while bacterial treatment enhanced stem length by 55.4%. Furthermore, shoot biomass also showed a significant increase with both bacterial and fungal treatments, underscoring the potential of harnessing seed endophytes to boost A. pindrow seedling health and resilience. This study underscores the crucial role of seed endophytic diversity in enhancing seed germination, seedling growth, and forest restoration efforts.

Biotechnological advances in plant growth-promoting rhizobacteria for sustainable agriculture

The rhizosphere, the soil zone surrounding plant roots, serves as a reservoir for numerous beneficial microorganisms that enhance plant productivity and crop yield, with substantial potential for application as biofertilizers. These microbes play critical roles in ecological processes such as nutrient recycling, organic matter decomposition, and mineralization. Plant growth-promoting rhizobacteria (PGPR) represent a promising tool for sustainable agriculture, enabling green management of crop health and growth, being eco-friendly alternatives to replace chemical fertilizers and pesticides. In this sense, biotechnological advancements respecting genomics and gene editing have been crucial to develop microbiome engineering which is pivotal in developing microbial consortia to improve crop production. Genome mining, which involves comprehensive analysis of the entire genome sequence data of PGPR, is crucial for identifying genes encoding valuable bacterial enzymes and metabolites. The CRISPR-Cas system, a cutting-edge genome-editing technology, has shown significant promise in beneficial microbial species. Advances in genetic engineering, particularly CRISPR-Cas, have markedly enhanced grain output, plant biomass, resistance to pests, and the sensory and nutritional quality of crops. There has been a great advance about the use of PGPR in important crops; however, there is a need to go further studying synthetic microbial communities, microbiome engineering, and gene editing approaches in field trials. This review focuses on future research directions involving several factors and topics around the use of PGPR putting special emphasis on biotechnological advances.

Development of bacterial bioformulations using response surface methodology

AIM

Bacterial consortia exhibiting plant growth promoting properties have emerged as a sustainable approach for crop improvement. As the main challenge associated with them is loss of viability and performance under natural conditions, a robust approach for designing bioformulation is needed. In this study, an efficient bioformulation was developed using spontaneous mutants of three bacterial strains for growth promotion of Cajanus cajan.

METHODS AND RESULTS

Optimization of additives for solid [carboxymethylcellulose (CMC), and glycerol] and liquid [polysorbate, CMC, and polyvinyl pyrrolidone (PVP)] bioformulations was done by response surface methodology using central composite design. The stability of each bioinoculant in the formulation was assessed at 30°C and 4°C. The efficiency of the liquid bioformulation was checked in planta in sterile, and subsequently in non-sterile, soil. The maximum cell count was observed in solid bioformulation with 0.1 g l-1 CMC and 50% glycerol (8.10 × 108, 3.69 × 108, and 7.39 × 108 CFU g-1 for Priestia megaterium, Azotobacter chroococcum, and Pseudomonas sp. SK3, respectively) and in liquid bioformulation comprising 1% PVP, 0.1 g l-1 CMC, and 0.025% polysorbate (8 × 109, 3.8 × 109, and 6.82 × 109 CFU ml-1 for P. megaterium, A. chroococcum, and Pseudomonas sp. SK3, respectively). The bioinoculants showed a higher viability (6 months) at 4°C compared to 30°C. Triple culture consortium enhanced plant growth in comparison to the control. The strains could be detected in soil till 45 days after sowing.

CONCLUSIONS

The study established a systematic process for developing a potent bioformulation to promote agricultural sustainability. Using mutant strains, the bioinoculants could be tracked. In planta assays revealed that the triple culture consortium out-performed mono and dual cultures in terms of impact on plant growth.

The Impact of Inoculation of Two Strains of Rhizobacteria on Radionuclide Transfer in Sesbania Grandiflora

Transfer factors of some naturally-occurring and artificial radionuclides from an agricultural soil to rhizobacteria-treated Sesbania grandiflora, a small leguminous tree, were studied. Two plant growth promoting rhizobacteria (PGPR) strains (SCR17 and PCE2) were used to carry out an agricultural experiment in pots in semi-arid region (Syria). The results showed the bacterial strain (SCR17) increased the transfer and accumulation of 238U and 40K in Sesbania grandiflora, while both bacterial strains showed no effect on the accumulation of 234Th, 226Ra, 210Po and 210Pb in the treated plants. The transfer factor of 137Cs from soil to rhizobacteria-treated Sesbania grandiflora was negligible. The values of the transfer factors of 234Th, 226Ra, 210Po and 40K were found to be within the global values, while the values of the transfer factors of 238U and 210Pb were found to be relatively higher. This study highlights the importance of using Phytoremediation by PGPR strains for radionuclides-contaminated soils. Therefore, this method is a promising technique for the restoration and rehabilitation of contaminated sites with radionuclides, as it is low cost, easy to apply, and environmentally friendly.

Bioprospecting for Isoetes cangae Endophytes with Potential to Promote Plant Growth

Isoetes cangae is a native plant found only in a permanent pond in Serra dos Carajás in the Amazon region. Plant-associated microbial communities are recognized to be responsible for biological processes essential for the health, growth, and even adaptation of plants to environmental stresses. In this sense, the aims of this work were to isolate, identify, and evaluate the properties of endophytic bacteria isolated from I. cangae. The bioprospecting of potentially growth-promoting endophytes required the following steps to be taken: isolation of endophytic colonies, molecular identification by 16S rDNA sequence analysis, and evaluation of the bacterial potential for nitrogen fixation, production of indole acetic acid and siderophores, as well as phosphate solubilization and mineralization. Bacillus sp., Rhizobium sp., Priestia sp., Acinetobacter sp., Rossellomorea sp., Herbaspirillum sp., Heyndrickxia sp., and Metabacillus sp., among other bacterial species, were identified. The isolates showed to be highly promising, evidencing the physiological importance for the plant and having the potential to promote plant growth.

Phyto-Friendly Soil Bacteria and Fungi Provide Beneficial Outcomes in the Host Plant by Differently Modulating Its Responses through (In)Direct Mechanisms

Sustainable agricultural systems based on the application of phyto-friendly bacteria and fungi are increasingly needed to preserve soil fertility and microbial biodiversity, as well as to reduce the use of chemical fertilizers and pesticides. Although there is considerable attention on the potential applications of microbial consortia as biofertilizers and biocontrol agents for crop management, knowledge on the molecular responses modulated in host plants because of these beneficial associations is still incomplete. This review provides an up-to-date overview of the different mechanisms of action triggered by plant-growth-promoting microorganisms (PGPMs) to promote host-plant growth and improve its defense system. In addition, we combined available gene-expression profiling data from tomato roots sampled in the early stages of interaction with Pseudomonas or Trichoderma strains to develop an integrated model that describes the common processes activated by both PGPMs and highlights the host's different responses to the two microorganisms. All the information gathered will help define new strategies for the selection of crop varieties with a better ability to benefit from the elicitation of microbial inoculants.

Rhizobacteria Mitigate the Negative Effect of Aluminum on Pea Growth by Immobilizing the Toxicant and Modulating Root Exudation

High soil acidity is one of the main unfavorable soil factors that inhibit the growth and mineral nutrition of plants. This is largely due to the toxicity of aluminum (Al), the mobility of which increases significantly in acidic soils. Symbiotic microorganisms have a wide range of beneficial properties for plants, protecting them against abiotic stress factors. This report describes the mechanisms of positive effects of plant growth-promoting rhizobacteria Pseudomonas fluorescens SPB2137 on four pea (Pisum sativum L.) genotypes grown in hydroponics and treated with 80 µM AlCl3. In batch culture, the bacteria produced auxins, possessed 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, alkalized the medium and immobilized Al, forming biofilm-like structures and insoluble phosphates. Inoculation with Ps. fluorescens SPB2137 increased root and/or shoot biomass of Al-treated plants. The bacteria alkalized the nutrient solution and transferred Al from the solution to the residue, which contained phosphorus that was exuded by roots. As a result, the Al concentration in roots decreased, while the amount of precipitated Al correlated negatively with its concentration in the solution, positively with the solution pH and negatively with Al concentration in roots and shoots. Treatment with Al induced root exudation of organic acids, amino acids and sugars. The bacteria modulated root exudation via utilization and/or stimulation processes. The effects of Al and bacteria on plants varied depending on pea genotype, but all the effects had a positive direction and the variability was mostly quantitative. Thus, Ps. fluorescens SPB2137 improved the Al tolerance of pea due to immobilization and exclusion of toxicants from the root zone.