Plant Growth Promoting Rhizobacteria: Constraints in Bioformulation, Commercialization, and Future Strategies

Microbial bioformulation: a microbial assisted biostimulating fertilization technique for sustainable agriculture

Addressing the pressing issues of increased food demand, declining crop productivity under varying agroclimatic conditions, and the deteriorating soil health resulting from the overuse of agricultural chemicals, requires innovative and effective strategies for the present era. Microbial bioformulation technology is a revolutionary, and eco-friendly alternative to agrochemicals that paves the way for sustainable agriculture. This technology harnesses the power of potential microbial strains and their cell-free filtrate possessing specific properties, such as phosphorus, potassium, and zinc solubilization, nitrogen fixation, siderophore production, and pathogen protection. The application of microbial bioformulations offers several remarkable advantages, including its sustainable nature, plant probiotic properties, and long-term viability, positioning it as a promising technology for the future of agriculture. To maintain the survival and viability of microbial strains, diverse carrier materials are employed to provide essential nourishment and support. Various carrier materials with their unique pros and cons are available, and choosing the most appropriate one is a key consideration, as it substantially extends the shelf life of microbial cells and maintains the overall quality of the bioinoculants. An exemplary modern bioformulation technology involves immobilizing microbial cells and utilizing cell-free filters to preserve the efficacy of bioinoculants, showcasing cutting-edge progress in this field. Moreover, the effective delivery of bioformulations in agricultural fields is another critical aspect to improve their overall efficiency. Proper and suitable application of microbial formulations is essential to boost soil fertility, preserve the soil's microbial ecology, enhance soil nutrition, and support crop physiological and biochemical processes, leading to increased yields in a sustainable manner while reducing reliance on expensive and toxic agrochemicals. This manuscript centers on exploring microbial bioformulations and their carrier materials, providing insights into the selection criteria, the development process of bioformulations, precautions, and best practices for various agricultural lands. The potential of bioformulations in promoting plant growth and defense against pathogens and diseases, while addressing biosafety concerns, is also a focal point of this study.

Entomopathogen-based biopesticides: insights into unraveling their potential in insect pest management

Global food security is a critical challenge to fulfill the demands of an exponentially growing population. To date, growers rely on chemicals; the broad-spectrum application of synthetic molecules leads to environmental contamination, resistance development, residual toxicity, pest resurgence, and a detrimental effect on human health and cattle. Crop production needs to be improved considering environmental and human health concerns to ensure food security. Furthermore, economically important crops are prone to attack by insect pests, causing considerable yield losses. Microbes are an eco-friendly, versatile alternative, and a potential candidate for combatting destructive pests below the economic injury level and improving the plant's health and productivity. Several microbial pathogens, including parasites, predators, parasitoids, pollinators, and many beneficial microorganisms, possess toxic properties against target organisms but do not cause harm to the non-target organisms. Entomopathogens (ENMs) have great potential for pest suppression due to their remarkable properties. Bacteria are host-specific, but fungi have a broader host range and can be significantly affected by both soil-dwelling and terrestrial insect pests. Virulent pathogens cause mortality in target insect pests known as ENMs and can penetrate through natural openings, ingestions, and integuments to cause a possible effect on target insect pests. The objective of using ENMs is to sustain productivity, improve environmental health, reduce pesticides, and conserve natural resources. Moreover, research is ongoing to discover other possible aspects, especially exploring potential ENMs. Therefore, there is a need for identification, isolation, and bioformulation to overcome the existing issues. This study is mainly focused on the status of bio-formulations, pathogenicity, their mode of action, and the potential application of different types of microbial formulations for sustainable pest management.

Engineering the Plant Microenvironment To Facilitate Plant-Growth-Promoting Microbe Association

New technologies that enhance soil biodiversity and minimize the use of scarce resources while boosting crop production are highly sought to mitigate the increasing threats that climate change, population growth, and desertification pose on the food infrastructure. In particular, solutions based on plant-growth-promoting bacteria (PGPB) bring merits of self-replication, low environmental impact, tolerance to biotic and abiotic stressors, and reduction of inputs, such as fertilizers. However, challenges in facilitating PGPB delivery in the soil still persist and include survival to desiccation, precise delivery, programmable resuscitation, competition with the indigenous rhizosphere, and soil structure. These factors play a critical role in microbial root association and development of a beneficial plant microbiome. Engineering the seed microenvironment with protein and polysaccharides is one proposed way to deliver PGPB precisely and effectively in the seed spermosphere. In this review, we will cover new advancements in the precise and scalable delivery of microbial inoculants, also highlighting the latest development of multifunctional rhizobacteria solutions that have beneficial impact on not only legumes but also cereals. To conclude, we will discuss the role that legislators and policymakers play in promoting the adoption of new technologies that can enhance the sustainability of crop production.

Prospect and Challenges for Sustainable Management of Climate Change-Associated Stresses to Soil and Plant Health by Beneficial Rhizobacteria

Climate change imposes biotic and abiotic stresses on soil and plant health all across the planet. Beneficial rhizobacterial genera, such as Bacillus, Pseudomonas, Paraburkholderia, Rhizobium, Serratia, and others, are gaining popularity due to their ability to provide simultaneous nutrition and protection of plants in adverse climatic conditions. Plant growth-promoting rhizobacteria are known to boost soil and plant health through a variety of direct and indirect mechanisms. However, various issues limit the wider commercialization of bacterial biostimulants, such as variable performance in different environmental conditions, poor shelf-life, application challenges, and our poor understanding on complex mechanisms of their interactions with plants and environment. This study focused on detecting the most recent findings on the improvement of plant and soil health under a stressful environment by the application of beneficial rhizobacteria. For a critical and systematic review story, we conducted a non-exhaustive but rigorous literature survey to assemble the most relevant literature (sorting of a total of 236 out of 300 articles produced from the search). In addition, a critical discussion deciphering the major challenges for the commercialization of these bioagents as biofertilizer, biostimulants, and biopesticides was undertaken to unlock the prospective research avenues and wider application of these natural resources. The advancement of biotechnological tools may help to enhance the sustainable use of bacterial biostimulants in agriculture. The perspective of biostimulants is also systematically evaluated for a better understanding of the molecular crosstalk between plants and beneficial bacteria in the changing climate towards sustainable soil and plant health.

Comparison of the Bulk and Rhizosphere Soil Prokaryotic Communities Between Wild and Reintroduced Manglietiastrum sinicum Plants, a Threatened Species with Extremely Small Populations

Huagaimu (Manglietiastrum sinicum) trees are critically endangered species and classified as a plant species with extremely small populations in China. Rhizospheres and bulk soils prokaryotic communities play an important role to protect and promote plants health and growth. However, the compositions and structures of prokaryotic communities in wild and reintroduced M. sinicum rhizospheres and bulk soils are still poorly understood. In the present study, prokaryotic communities in wild and reintroduced M. sinicum rhizospheres and bulk soils were compared using high-throughput sequencing. Thirty-two phyla, 76 classes, 193 orders, 296 families, and 470 genera of prokaryotes were obtained. Proteobacteria and Acidobacteria were the two most abundant phyla in all soil samples. The compositions and structures of prokaryotic communities were overall similar, and the abundance of some taxa varied significantly among soil samples. Soil prokaryotic communities were significantly affected by soil pH, total nitrogen, total phosphorus, and total potassium. Eleven of predicted functions were significantly different among the four soil groups. This study provides for the first insights into the compositions, structures, and potential functions of prokaryotic communities associated with wild and reintroduced M. sinicum rhizospheres and bulk soils, and providing a foundation for future research to help protect this endangered species.

Current biological approaches for management of crucifer pests

Cabbage is considered as one of the most commonly found vegetables and it has been cultivated in large areas throughout the year. As it is mostly grown in large areas, higher rate of pest infestation likely to occur, which hinder its total production and consumption. However, continuous use of synthetic pesticides in agricultural pest management often leads to various negative impacts such as development of resistance by the pest, adverse effect on non-target organisms and hazardous effect on environment. These drawbacks led to an alternative approaches for control of crucifer pests that are cost effective, biodegradable, low toxic effect on non-target organisms and eco-friendly. This review brings together all the information of different biological practices for management of crucifer pests and list of botanical insecticides and entomopathogenic organisms that are being reported. This will help in establishing the knowledge of limited studies on pest management using different biological control methods to more challenging research and conveys the importance of pest management system for taking research forward.

Biochemical and economical effect of application biostimulants containing seaweed extracts and amino acids as an element of agroecological management of bean cultivation

The implementation of agronomic activities, based on the use of biostimulants, is an important element of agroecological practices. Therefore, comprehensive research was carried on the use of biostimulants. A field experiment was performed in 2016-2018 with common bean of Mexican Black cultivar. In particular growing seasons, bean plants were treated with Kelpak SL (seaweed extracts) and Terra Sorb Complex (free amino acids) in the form of single and double spraying with two solutions concentrations. According to the obtained data, application of biostimulants increased the yield of bean. Better results were observed after the use of Kelpak SL. The application of preparations influenced nutritional and nutraceutical quality of bean seeds. Terra Sorb Complex caused the highest increase in proteins level. In the light of achieved data, biostimulants in similar level decreased the starch accumulation. The most promising results, in the context of nutraceutical value of bean, were obtained in the case of increasing level of fiber. A positive impact of biostimulants on the seeds antioxidant potential was noted, expressed by the increased synthesis of phenolics, flavonoid, anthocyanins and antioxidant activities. Results of this study, directly indicate economic benefits from the use of biostimulants, which are extremely important to the farmers.

Immobilization of Bradyrhizobium and Azospirillum in alginate matrix for long time of storage maintains cell viability and interaction with peanut

Immobilizarion of PGPR for agricultural applications aims to provide temporary physical protection from stressful environmental conditions and the gradual release of cells for successful root colonization, release the cells gradually. In this work, we immobilized Bradyrhizobium sp. SEMIA6144 or Azospirillum brasilense Az39 cells in 2% alginate beads prepared by ionic gelation process, and then stored up to 12 months at 4 °C. Alginate matrix showed interaction with the immobilized bacteria (FTIR), allowed a constant release of cells, and improved their viability and capability to interact with Arachis hypogaea. Cell number into beads reached 10⁷ CFU.bead^-1; however, viability decreased from 4 months of storage for Az39, while it was maintained up to 12 months for SEMIA6144, showing a low metabolic activity measured by the MTT assay. Adhesion of SEMIA6144 and Az39 from new beads to peanut root was 11.5% and 16%, respectively, higher than non-immobilized bacteria. Peanut inoculation with 12 months storage SEMIA6144 beads significantly increased root length and biomass at 30 days of growth, and under restrictive water condition (RWC), nodulation and total plant N content increased compared with liquid inoculation. Our results demonstrate that immobilization of SEMIA6144 and Az39 in alginate matrix is a potential alternative to enhance peanut growth even under RWC. KEY POINTS: • Alginate encapsulation enhances viability of SEMIA6144 or Az39 under storage at 4 °C for 1 year. • Alginate beads 2% ensure the gradual release of the microorganisms. • Cells from beads stored for long periods present chemotaxis and adhesion to peanut root. • Peanut inoculation with 1-year-old SEMIA6144 beads improves nodulation and growth in RWC.

Exploring the efficacy of antagonistic rhizobacteria as native biocontrol agents against tomato plant diseases

As the environmental and health concerns alert the necessity to move towards a sustainable agriculture system, biological approach using indigenous plant growth-promoting rhizobacteria (PGPR) gains a strong impetus in the field of plant disease control. In this context, the present review article addresses the usage of rhizospheric antagonistic bacteria as a suitable alternative to control tomato fungal diseases namely Fusarium wilt and early blight disease. Biological control has been considered to be an eco-friendly, safe and effective method for disease management. The inherent traits of PGPR to antagonize a pathogen through various mechanisms has been investigated extensively to utilize them as potent biocontrol agents (BCA). Hence, the article provides a detailed account on different biocontrol mechanisms displayed by BCA. It is also suggested that the use of bacterial consortium ensures consistent performance by BCA in field conditions. Likewise, this review also deals with the opportunities and obstacles faced during commercialization of these antagonistic bacteria as biocontrol agents in the market.

Biopesticides in India: technology and sustainability linkages

Despite enhancing the crop yields, the so called green revolution (GR) has proven unsuccessful in assuring long term agricultural sustainability. The methods used for productivity enhancement during GR have not only proven to be problematic but have also resulted in deterioration of soil quality and several other issues related to ecosystems and health issues. The damage was mainly caused by the indiscriminate use of chemical fertilizers and insecticides. Various types of pesticides, are now known to be causing huge problems in the agro-ecosystems. In such a situation, where chemicals have caused or are causing irreversible impacts on agroecosystems, the use of biopesticides has emerged as a sustainable alternative leading to safe organic farming. At the global level, environmentally benign nature and target-specificity of biopesticides are gaining wide popularity. However, in developing countries like India usage of biopesticides is still minuscule in comparison to conventional chemical pesticides. Although the Indian government has encouraged the use of biopesticides by placing them into many of the agricultural schemes, at the grassroots level, biopesticides are facing many challenges. The lower adaptability and declining interest of farmers towards biopesticides have become a matter of concern. However, technological challenges related to production, manufacture and application in agroecosystems have also raised a question on their long-term sustainability. The main objective of this review is to highlight the developing trend in the field of biocontrol products in India. Apart from this, the review also focuses on the technological perspectives that are required for the long-term sustainability of biological control products in Indian agriculture and market.

Development of Bacillus safensis-based liquid bioformulation to augment growth, stevioside content, and nutrient uptake in Stevia rebaudiana

The application of chemical fertilizers to enhance crop production is a major concern due to associated environmental pollution and health hazards. Hence, there is an urgent need to develop an eco-friendly solution to improve crop production and promote sustainable agriculture simultaneously. Stevia rebaudiana is an important medicinal crop being substitute for sugar, superior flavor outline, extensive medicinal properties, and also of agronomic interest. In the present study, bacterium STJP isolated from the rhizospheric soil of S. rebaudiana and identified as Bacillus safensis on the basis of 16S rRNA gene sequencing, showed good amount of zinc (4.4 mg/L) and potassium (5.4 mg/L) solubilization. Paneer-whey (a dairy waste) based bioformulation (P-WBF) was developed utilizing isolate B. safensis STJP (accession number NAIMCC TB-2833) and inspected for the quality and ability to enhance the growth, nutrients uptake, and stevioside content in S. rebaudiana. The application of P-WBF displayed a significantly higher concentration (153.12%) of stevioside in S. rebaudiana as compared to control. P-WBF treated Stevia plants showed significantly higher fresh and dry weight as well (as compared to control). Further, enhancement of phosphorous, nitrogen, potassium, and zinc uptake in plant tissue was also recorded by application of P-WBF. This study suggests the use of P-WBF based biofertilizer using B. safensis STJP to increase stevioside content in Stevia plant by a nutrient(s) linked mechanism. This novel approach can also be beneficial for utilization of a dairy waste in preparation of bioformulation and, for enhancement of crop yield by an ecofriendly manner leading to sustainable agriculture.

Effect of bioformulations on the biocontrol efficacy, microbial viability and storage stability of a consortium of biocontrol agents against Fusarium wilt of banana

AIMS

This study sought to investigate the effect of bioformulation on the biocontrol efficacy, microbial viability and storage stability of a consortium of Pseudomonas aeruginosa DRB1 and Trichoderma harzianum CBF2 against Foc Tropical Race 4 (Foc-TR4).

MATERIALS AND RESULTS

Four bioformulations consisting of dry (pesta granules, talc powder and alginate beads) and liquid formulations were evaluated for their ability to control Foc-TR4, sustain microbial populations after application and maintain microbial stability during storage. All tested bioformulations reduced disease severity (DS) by more than 43·00% with pesta granules producing the highest reduction in DS by 66·67% and the lowest area under the disease progress curve value (468·75) in a glasshouse trial. Microbial populations of DRB1 and CBF2 were abundant in the rhizosphere, rhizoplane and within the roots of bananas after pesta granules application as compared to talc powder, alginate beads and liquid formulations 84 days after inoculation (DAI). The stability of both microbial populations after 180 days of storage at 4°C was the greatest in the pesta granule formulation.

CONCLUSION

The pesta granule formulation was a suitable carrier of biological control agents (BCA) without compromising biocontrol efficacy, microbial population and storage stability as compared to other bioformulations used in this study.

SIGNIFICANCE AND IMPACT OF THE STUDY

Pesta granules could be utilized to formulate BCA consortia into biofertilizers. This formulation could be further investigated for possible applications under agricultural field settings.

Biofertilizers: a potential approach for sustainable agriculture development

The worldwide increase in human population raises a big threat to the food security of each people as the land for agriculture is limited and even getting reduced with time. Therefore, it is essential that agricultural productivity should be enhanced significantly within the next few decades to meet the large demand of food by emerging population. Not to mention, too much dependence on chemical fertilizers for more crop productions inevitably damages both environmental ecology and human health with great severity. Exploitation of microbes as biofertilizers is considered to some extent an alternative to chemical fertilizers in agricultural sector due to their extensive potentiality in enhancing crop production and food safety. It has been observed that some microorganisms including plant growth promoting bacteria, fungi, Cyanobacteria, etc. have showed biofertilizer-like activities in the agricultural sector. Extensive works on biofertilizers have revealed their capability of providing required nutrients to the crop in sufficient amounts that resulted in the enhancement of crop yield. The present review elucidates various mechanisms that have been exerted by biofertilizers in order to promote plant growth and also provides protection against different plant pathogens. The aim of this review is to discuss the important roles and applications of biofertilizers in different sectors including agriculture, bioremediation, and ecology.