Screening of sunflower associated bacteria as biocontrol agents for plant growth promotion

Invisible Inhabitants of Plants and a Sustainable Planet: Diversity of Bacterial Endophytes and their Potential in Sustainable Agriculture

Uncontrolled usage of chemical fertilizers, climate change due to global warming, and the ever-increasing demand for food have necessitated sustainable agricultural practices. Removal of ever-increasing environmental pollutants, treatment of life-threatening diseases, and control of drug-resistant pathogens are also the need of the present time to maintain the health and hygiene of nature, as well as human beings. Research on plant-microbe interactions is paving the way to ameliorate all these sustainably. Diverse bacterial endophytes inhabiting the internal tissues of different parts of the plants promote the growth and development of their hosts by different mechanisms, such as through nutrient acquisition, phytohormone production and modulation, protection from biotic or abiotic challenges, assisting in flowering and root development, etc. Notwithstanding, efficient exploitation of endophytes in human welfare is hindered due to scarce knowledge of the molecular aspects of their interactions, community dynamics, in-planta activities, and their actual functional potential. Modern "-omics-based" technologies and genetic manipulation tools have empowered scientists to explore the diversity, dynamics, roles, and functional potential of endophytes, ultimately empowering humans to better use them in sustainable agricultural practices, especially in future harsh environmental conditions. In this review, we have discussed the diversity of bacterial endophytes, factors (biotic as well as abiotic) affecting their diversity, and their various plant growth-promoting activities. Recent developments and technological advancements for future research, such as "-omics-based" technologies, genetic engineering, genome editing, and genome engineering tools, targeting optimal utilization of the endophytes in sustainable agricultural practices, or other purposes, have also been discussed.

Potential Biocontrol Agents of Corn Tar Spot Disease Isolated from Overwintered Phyllachora maydis Stromata

Tar spot disease in corn, caused by Phyllachora maydis, can reduce grain yield by limiting the total photosynthetic area in leaves. Stromata of P. maydis are long-term survival structures that can germinate and release spores in a gelatinous matrix in the spring, which are thought to serve as inoculum in newly planted fields. In this study, overwintered stromata in corn leaves were collected in Central Illinois, surface sterilized, and caged on water agar medium. Fungi and bacteria were collected from the surface of stromata that did not germinate and showed microbial growth. Twenty-two Alternaria isolates and three Cladosporium isolates were collected. Eighteen bacteria, most frequently Pseudomonas and Pantoea species, were also isolated. Spores of Alternaria, Cladosporium, and Gliocladium catenulatum (formulated as a commercial biofungicide) reduced the number of stromata that germinated compared to control untreated stromata. These data suggest that fungi collected from overwintered tar spot stromata can serve as biological control organisms against tar spot disease.

Bioprospecting and Challenges of Plant Microbiome Research for Sustainable Agriculture, a Review on Soybean Endophytic Bacteria

This review evaluates oilseed crop soybean endophytic bacteria, their prospects, and challenges for sustainable agriculture. Soybean is one of the most important oilseed crops with about 20-25% protein content and 20% edible oil production. The ability of soybean root-associated microbes to restore soil nutrients enhances crop yield. Naturally, the soybean root endosphere harbors root nodule bacteria, and endophytic bacteria, which help increase the nitrogen pool and reclamation of another nutrient loss in the soil for plant nutrition. Endophytic bacteria can sustain plant growth and health by exhibiting antibiosis against phytopathogens, production of enzymes, phytohormone biosynthesis, organic acids, and secondary metabolite secretions. Considerable effort in the agricultural industry is focused on multifunctional concepts and bioprospecting on the use of bioinput from endophytic microbes to ensure a stable ecosystem. Bioprospecting in the case of this review is a systemic overview of the biorational approach to harness beneficial plant-associated microbes to ensure food security in the future. Progress in this endeavor is limited by available techniques. The use of molecular techniques in unraveling the functions of soybean endophytic bacteria can explore their use in integrated organic farming. Our review brings to light the endophytic microbial dynamics of soybeans and current status of plant microbiome research for sustainable agriculture.

Metabolic profile and molecular characterization of endophytic bacteria isolated from Pinus sylvestris L. with growth-promoting effect on sunflower

Endophytic bacteria inhabit plant tissues such as roots, stems, leaves, fruits, and seeds and can multiply inside plant tissue without damaging them. This study involves the isolation, characterization, metabolic profiling, and effect of endophytic bacteria isolated from the roots of Scots pine (Pinus sylvestris), on the growth of sunflower. In the current study, fifteen isolates of endophytic bacteria were obtained from the roots of Scots pine, and their molecular characterization was performed using 16 s rRNA ribotyping. The molecular characterization revealed that the strains belonged to Bacillus spp., Pseudomonas spp., Micrococcus sp., Serratia sp., Enterobacter sp., Pantoea sp., Staphylococcus sp., and Microbacterium sp. Among the isolated strains, 9 strains showed positive results for ammonium production, 12 strains for calcium solubilization, 11 strains for magnesium solubilization, 5 strains for zinc solubilization, 12 strains for phosphate solubilization, 8 strains for potassium solubilization, 10 strains for indole acetic acid (IAA) production, 9 strains for siderophore, and 6 strains for hydrogen cyanide (HCN) production. The greenhouse experiment results demonstrated that all isolated endophytic bacteria improved the shoot length, dry weight, and chlorophyll content of sunflower, whereas a significant increase was observed by PS-3 (Bacillus cereus), PS-6 (Serratia marcescens), and PS-8 (Pseudomonas putida). Besides, the concentration of nitrogen, phosphorus, and potassium were also measured in sunflower shoots, and results asserted that bacterial inoculation increased the bioavailability of these essential nutrients to plants compared to uninoculated control. Thus, these endophytic bacteria could be used as an encouraging option to improve plant growth and performance.

Bioinoculants as mitigators of multiple stresses: A ray of hope for agriculture in the darkness of climate change

Plant encounters various biotic and abiotic stresses, that affect agricultural productivity and reduce farmer's income especially under changing global climate. These environmental stresses can advance plant senescence by inducing osmotic stress, nutrient stress, hormonal imbalance, production of oxygen radicals, and ion toxicity, etc. Additionally, these stresses are not limited to plant health but also deteriorate soil health by affecting the microbial diversity of soil. To tackle this global delinquent of agriculture, several methods are suggested to ameliorate the negative effect of different types of stresses, the application of beneficial microorganisms or bioinoculants is one of them. Beneficial microorganisms that are used as bioinoculants not only facilitate plant growth by fulfilling the nutrient requirements but also assist the plant to withstand these stresses. These microorganisms produce certain chemicals such as 1-aminocyclopropane-1-carboxylate (ACC) deaminase, phytohormones, antioxidants, extracellular polysaccharide (EPS), siderophores, antibiotics, and volatile organic compounds (VOCs), etc. which help the plants to mitigate various stresses. Besides, these microbes also activate plant defence responses. Thus, these bioinoculants can effectively replace chemical inputs to supplement nutrient requirements and mitigation of multiple stresses in plants.

Root Exudates: Mechanistic Insight of Plant Growth Promoting Rhizobacteria for Sustainable Crop Production

The breaking silence between the plant roots and microorganisms in the rhizosphere affects plant growth and physiology by impacting biochemical, molecular, nutritional, and edaphic factors. The components of the root exudates are associated with the microbial population, notably, plant growth-promoting rhizobacteria (PGPR). The information accessible to date demonstrates that PGPR is specific to the plant's roots. However, inadequate information is accessible for developing bio-inoculation/bio-fertilizers for the crop in concern, with satisfactory results at the field level. There is a need to explore the perfect candidate PGPR to meet the need for plant growth and yield. The functions of PGPR and their chemotaxis mobility toward the plant root are triggered by the cluster of genes induced by the components of root exudates. Some reports have indicated the benefit of root exudates in plant growth and productivity, yet a methodical examination of rhizosecretion and its consequences in phytoremediation have not been made. In the light of the afore-mentioned facts, in the present review, the mechanistic insight and recent updates on the specific PGPR recruitment to improve crop production at the field level are methodically addressed.