Role of Bacterial Phytohormones in Plant Growth Regulation and Their Development

Taxonomic description of Micromonospora reichwaldensis sp. nov. and its biosynthetic and plant growth-promoting potential

Micromonospora strains proved to be a model organism for drug discovery and plant growth promotion (PGP). Strain DSM 115977 T was subjected to polyphasic taxonomic analysis and genome mining for biosynthetic gene clusters and PGP-associated genes in order to determine its taxonomic rank and assess its biosynthetic potential. The strain was found to form a novel species within the evolutionary radiation of the genus Micromonospora. The strain contained glucose, mannose, xylose, and ribose as whole-cell sugars and the isomer DL-diaminopimelic acid in its peptidoglycan. Strain DSM 115977T had iso-C15:0, iso-C16:0, C17:1cis 9, C17:0, iso-C17:0, and 10-methyl-C17:0 as fatty acid profile (>5%) and MK10-H4 and MK10-H6 as the predominant menaquinones (>10%). The polar lipid profile consisted of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, glycophosphatidylinositol, glycophospholipids, phosphoaminolipid, unidentified lipids, and phospholipids. The genome of the strain had a size of 7.0 Mbp with a DNA G + C content of 73.4%. It formed a well-supported sub-clade with its close phylogenomic neighbor, Micromonospora echinofusca DSM 43913T (98.7%). Digital DNA-DNA hybridization and average nucleotide identity derived from sequence comparisons between the strain and its close phylogenomic neighbors were below the thresholds of 70 and 95-96% for prokaryotic species demarcation, respectively. Based on these findings, strain DSM 115977T (Asg4T = KCTC 59188T) merits to be considered as the type strain of a new species for which the name Micromonospora reichwaldensis sp. nov. is proposed. Genome mining for biosynthetic gene clusters encoding specialized secondary metabolites highlighted its ability to produce potentially novel therapeutic compounds. The strain is rich in plant growth-promoting genes whose predicted products directly and indirectly affect the development and immune system of the plant.

IMPORTANCE

In view of the significant pharmaceutical, biotechnological, and ecological potentials of micromonosporae, it is particularly interesting to enhance the genetic diversity of this genus by focusing on the isolation of novel strain from underexplored habitats, with the promise that novel bacteria will lead to new chemical entities. In this report, modern polyphasic taxonomic study confirmed the assignment of strain DSM 115977T to a novel species for which the name Micromonospora reichwaldensis sp. nov. is proposed. The strain harbors in its genomic sequence several biosynthetic gene clusters for secondary metabolites and genes associated with plant growth-promoting features. The results of this study provide a very useful basis for launching more in-depth research into agriculture and/or drug discovery.

Integrating Biochar, Bacteria, and Plants for Sustainable Remediation of Soils Contaminated with Organic Pollutants

The contamination of soil with organic pollutants has been accelerated by agricultural and industrial development and poses a major threat to global ecosystems and human health. Various chemical and physical techniques have been developed to remediate soils contaminated with organic pollutants, but challenges related to cost, efficacy, and toxic byproducts often limit their sustainability. Fortunately, phytoremediation, achieved through the use of plants and associated microbiomes, has shown great promise for tackling environmental pollution; this technology has been tested both in the laboratory and in the field. Plant-microbe interactions further promote the efficacy of phytoremediation, with plant growth-promoting bacteria (PGPB) often used to assist the remediation of organic pollutants. However, the efficiency of microbe-assisted phytoremediation can be impeded by (i) high concentrations of secondary toxins, (ii) the absence of a suitable sink for these toxins, (iii) nutrient limitations, (iv) the lack of continued release of microbial inocula, and (v) the lack of shelter or porous habitats for planktonic organisms. In this regard, biochar affords unparalleled positive attributes that make it a suitable bacterial carrier and soil health enhancer. We propose that several barriers can be overcome by integrating plants, PGPB, and biochar for the remediation of organic pollutants in soil. Here, we explore the mechanisms by which biochar and PGPB can assist plants in the remediation of organic pollutants in soils, and thereby improve soil health. We analyze the cost-effectiveness, feasibility, life cycle, and practicality of this integration for sustainable restoration and management of soil.

Influence of Bacteria of the Genus Pseudomonas on Leguminous Plants and Their Joint Application for Bioremediation of Oil Contaminated Soils

The modern approach to the creation of biological products to stimulate plant growth is based on the study of specific inter-bacterial interactions. This study describes the impact that the introduction of strains of the genus Pseudomonas has on annual and perennial leguminous plants and the ecosystem of the leguminous plant-the indigenous microbial community. The objects of research under the conditions of vegetation experiments were plants of field peas (Pisum sativum L.), white lupine (Lupinus albus L.), chickpea (Cicer arietinum L.), alfalfa (Medicago sativa subsp. varia (Martyn) Arcang.), and white sweet clover (Melilotus albus Medik.). For the treatment of plant seeds, a liquid culture of strains of growth-stimulating bacteria Pseudomonas koreensis IB-4, and P. laurentiana ANT 17 was used. The positive effect of the studied strains on the germination, growth and development of plants was established. There was no inhibitory effect of inoculants on rhizobia; on the contrary, an increase in nodule formation was observed. The possibility of recultivation of oil-contaminated soil using chickpea and alfalfa as phytomeliorants and growth-stimulating strains P. koreensis IB-4, P. laurentiana ANT 17 as inoculants was evaluated. It is proved that seed treatment improved the morphological parameters of plants, as well as the efficiency of oil destruction.

Mechanisms underlying cereal/legume intercropping as nature-based biofortification: A review

The deficiencies of micronutrients known as hidden hunger are severely affecting more than one-half of the world’s population, which is highly related to low bioavailability of micronutrients, poor quality diets, and consumption of cereal-based foods in developing countries. Although numerous experiments proved biofortification as a paramount approach for improving hidden hunger around the world, its effectiveness is highly related to various soil factors, climate conditions, and the adoption rates of biofortified crops. Furthermore, agronomic biofortification may result in the sedimentation of heavy metals in the soil that pose another detrimental effect on plants and human health. In response to these challenges, several studies suggested intercropping as one of the feasible, eco-friendly, low-cost, and short-term approaches for improving the nutritional quality and yield of crops sustainable way. Besides, it is the cornerstone of climate-smart agriculture and the holistic solution for the most vulnerable area to solve malnutrition that disturbs human healthy catastrophically. Nevertheless, there is meager information on mechanisms and processes related to soil-plant interspecific interactions that lead to an increment of nutrients bioavailability to tackle the crisis of micronutrient deficiency in a nature-based solution. In this regard, this review tempted to (1) explore mechanisms and processes that can favor the bioavailability of Zn, Fe, P, etc. in soil and edible parts of crops, (2) synthesize available information on the benefits and synergic role of the intercropping system in food and nutritional security, and (3) outline the bottlenecks influencing the effectiveness of biofortification for promoting sustainable agriculture in sub-Saharan Africa (SSA). Based on this review SSA countries are malnourished due to limited access to diverse diets, supplementation, and commercially fortified food; hence, I suggest integrated research by agronomists, plant nutritionists, and agroecologist to intensify and utilize intercropping systems as biofortification sustainably alleviating micronutrient deficiencies.

Graphical Abstract

Plant Growth-promoting Effects of Viable and Dead Spores of Bacillus pumilus TUAT1 on Setaria viridis

Spores are a stress-resistant form of Bacillus spp., which include species that are plant growth-promoting rhizobacteria (PGPR). Previous studies showed that the inoculation of plants with vegetative cells or spores exerted different plant growth-promoting effects. To elucidate the spore-specific mechanism, we compared the effects of viable vegetative cells, autoclaved dead spores, and viable spores of Bacillus pumilus TUAT1 inoculated at 10⁷ CFU plant^–1 on the growth of the C4 model plant, Setaria viridis A10.1. B. pumilus TUAT1 spores exerted stronger growth-promoting effects on Setaria than on control plants 14 days after the inoculation. Viable spores increased shoot weight, root weight, shoot length, root length, and nitrogen uptake efficiency 21 days after the inoculation. These increases involved primary and crown root formation. Additionally, autoclaved dead spores inoculated at 10⁸ or 10⁹ CFU plant^–1 had a positive impact on crown root differentiation, which increased total lateral root length, resulting in a greater biomass and more efficient nitrogen uptake. The present results indicate that an inoculation with viable spores of B. pumilus TUAT1 is more effective at enhancing the growth of Setaria than that with vegetative cells. The plant response to dead spores suggests that the spore-specific plant growth-promoting mechanism is at least partly independent of symbiotic colonization.

Phytohormone Mediation of Interactions Between Plants and Non-Symbiotic Growth Promoting Bacteria Under Edaphic Stresses

The capacity of rhizoshere bacteria to influence plant hormonal status, by bacterial production or metabolism of hormones, is considered an important mechanism by which they promote plant growth, and productivity. Nevertheless, inoculating these bacteria into the plant rhizosphere may produce beneficial or detrimental results depending on bacterial effects on hormone composition and quantity in planta, and the environmental conditions under which the plants are growing. This review considers some effects of bacterial hormone production or metabolism on root growth and development and shoot physiological processes. We analyze how these changes in root and shoot growth and function help plants adapt to their growth conditions, especially as these change from optimal to stressful. Consistent effects are addressed, along with plant responses to specific environmental stresses: drought, salinity, and soil contamination (with petroleum in particular).

Bioengineering of Dioscorea nipponica with rhizospheric Proteus spp. for enhanced tuber size and diosgenin content

Rhizobacterial production of plant hormones, ACC deaminase, fixation of nitrogen, solubilization of phosphate and antimicrobial metabolites play very important role in the health and growth of plants. Hence exploration of plant probiotic prospectives of promising rhizobacterial isolates from biodiversity rich areas can have enormous applications to engineer both the biomass and active ingredients of plants. In the present study, rhizospheric Proteus spp. R6 and R60 isolated from Pseudarthria viscida and Glycosmis arborea were analyzed for tuber and diosgenin enhancement effects in Dioscorea nipponica under field conditions for 1 year. Among the two Proteus spp. selected, both were positive for ACC deaminase, siderophore, nitrogen fixation, IAA and ammonia production. However, the isolate R6 was found to have additional phosphate solubilizing activity. Quantitative analysis of IAA by HPTLC showed its maximum production by Proteus sp. R60 (714.47 ± 8.7 µg/mL) followed by Proteus sp. R6 (588.06 ± 7.0 µg/mL). The tubers formed from the Proteus sp. R6 treated samples were identified to have significant enhancement in size, root number and diosgenin content when compared to control. Interestingly, HPLC analysis has confirmed twofold higher diosgenin content in Proteus sp. R6 treated samples than control during 1 year period of its field growth. The obtained results are of great importance as it involved the utilization of rhizospheric bacteria to improve tuber size which suggests its potential use in developing cost-effective, eco-friendly and multifunctional biofertilizer.