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Advances in Agronomy continues to be recognized as a leading reference and a first-rate source for the latest research in agronomy. As always, the subjects.
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With the need to accelerate the development of improved varieties, genomics-assisted breeding is becoming an important tool in breeding programs. With marker-assisted selection, there has been success in breeding for disease resistance; however, much of this work and research has focused on identifying, mapping, and selecting for major resistance genes that tend to be highly effective but vulnerable to breakdown with rapid changes in pathogen races. In contrast, breeding for minor-gene quantitative resistance tends to produce more durable varieties but is a more challenging breeding objective.

Many different and efficient encapsulation techniques were developed for that purposes Schoebitz et al. Almost none were evaluated in the inoculant field apart from direct polymerization of a few polymers. Many of these emerging technologies from other fields merit evaluation in the agricultural inoculant industry.

It is highly unlikely that farming practices will significantly change, even to accommodate a technology that delivers a high-quality inoculant. Consequently, the goal should be to create formulations that are more than farmer-friendly, as some of the contemporary inoculants are. Agriculture cannot sustain inoculants with high production costs or expensive carrier materials. Consequently, because of the large quantities of inoculants involved and main crops that employ inoculant are staple food crops mainly cereals and legumes and not cash crops, any technology must be developed with constantly lower costs in mind.

It is unlikely that an outstanding formulation with elevated price will find a niche in this market.

Low-cost technologies for extending the shelf life of inoculants at the farm level in developing countries need further development. For example, clay pots covered with wet jute bags give better shelf life for inoculants in India Bhattacharya and Mishra Similar, low-cost technologies need to be developed for better performance of inoculants. Local strains should be used for improved performance because no PGPB strain will perform best in all farming conditions.

Since the effectiveness of inoculation depends on multiple parameters, including the target plant species and soil and weather conditions, inoculants should also be differentiated and matched appropriately. This requirement complicates the task of providing effective inoculants because commercial incentives dictate keeping the variety of products small.

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This industrial requirement is contrary to the reality of diversity among crop species, inoculants species, and soil biotic communities that otherwise support the production and distribution of multiple inoculants. Local production and distribution of selected native PGPB-rhizobia isolates might solve some of the current issues plaguing the field performance of inoculants Selvamukilan et al.

Create specific formulations for use under sub-optimal farming conditions. Those include saline soils, drought, high temperatures, acid soils, marginal eroded soils, or soils with limited access for maintenance of plant cultivation,.


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For such problems, local PGPB-rhizobia strains that are tolerant or resistant to the physical or chemical stressor can be used, such as salt tolerant strains Balasubramanian and Prabhu For transplanted crops, nursery inoculation is easier and usually provides better results. The cost of product and application will be far smaller, compared to field treatment. This is because the volume of the inoculated substrate is small and growth conditions are easier to control.


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Consequently, more research and transfer of technologies to farmers should be targeted to nursery-grown plants. For example, seedling roots dipped in microbial inoculants is effective and easy for microbial inoculation of transplanted crops such as rice Choudhury and Kennedy This treatment needs to be fine-tuned for other transplanted crops. Inoculants made of active fragments of PGPB in the absence of living cells. Many elicitors of plant defenses and secondary metabolites of PGPB are well documented. For example, lipochito-oligosaccharides LCOs nod factors Marks et al.

LCOs play a general role as growth regulators in a wide variety of plants, including non-legumes Prithiviraj et al. Potential commercial LCO products for seed and foliar applications in legumes and non-legumes, such as corn, are currently available in the marketplace in North America. More exploration in this direction is required to get more benefits from PGPB. Publications in formal literature involving evaluation of proprietary formulations are largely unhelpful, apart for the extra public exposure for the manufacturer.

The field experience of the farmer and market forces will determine whether such formulations are worth re-purchasing in the following growing season. Such publications contribute only ambiguity to this field. Formulation and field application of inoculants are a pure technological platform with knowledge that is heavily based on fundamental principles of microbiology and material sciences. Yet, the unification of these fields creates useful products that are, and will be, an important input in sustainable agriculture and environmental solutions.

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Advances in plant growth-promoting bacterial inoculant technology: formulations and practical perspectives — Marschner Review First Online: 19 November Background Inoculation of plants to enhance yield of crops and performance of other plants is a century old, proven technology for rhizobia and a newer venue for plant growth-promoting bacteria and other plant symbionts. Scope An assessment of practical aspects of bacterial inoculants for contemporary agriculture and environmental restoration is critically evaluated from the point of view of their current technological status, current applications, and future use.

Conclusions This review discusses characteristics of a carrier for inoculants, formulations of inoculants including liquid, organic, inorganic, polymeric, and encapsulated formulations. Introduction Inoculation of plants with plant growth-promoting bacteria PGPB and plant symbionts to enhance performance of plants is centuries old.

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The first goal when considering inoculation of plants with PGPB including rhizobia is to find the best strain of bacteria or a microbial consortium for the intended effect on the target crop. The next step is to design a specific inoculant formulation for the target crop and a method of practical application, considering the limitations of the growers. A flow diagram showing the procedures for developing microbial inoculants by the industry is presented in Fig. In practical terms, the chosen formulation and method of application determine the potential success of the inoculant.

The scientific literature abounds with many potentially highly useful strains that did not appear on the commercial market, perhaps because of inappropriate formulation. These are lost or forgotten. Open image in new window. Factors that may influence efficacy of inoculants are those that provide better survival of the inoculant on the seed. Those include: 1 growth phase logarithmic or stationary at the time of mixing bacterial cultures with a carrier because these influence incorporation into the inoculant of either very active cells, spores, cysts, or flocculated cells of various species of PGPB for review, see Bashan , 2 rate of drying and rehydration, 3 appropriate carrier material characteristics, and 4 inoculation technology Date In practice, the formulated carrier inoculant is the sole delivery vehicle of live microorganisms from the factory to the field.

The carrier is the major portion by volume or weight of the inoculant. Table 1 A sample of formulations used for producing inoculants of plant growth-promoting bacteria for plants from to Formulation Additives or treatment Microorganisms used Plant species, or substrate Reference No formulation culture media or water.

None Azospirillum brasilense ; A. Peanut, rice Taurian et al. Acacia mangium , Greengram, Leucaena leucocephala ; Rice Diouf et al. Applications as: seed coating and pellets Azospirillum brasilense ; A. Groundnut Anandham et al. None Li et al. Leucaena leucocephala Forestier et al.

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The five main desirable general characteristics for a good formulation are Bashan : 1 Chemical and physical characteristics. Inoculants using organic carriers Without any doubt, peat is the carrier of choice for rhizobia in North and South America, Europe, and Australia, and the main ingredient of inoculants that are sold in large volumes. Inorganic inoculants Inorganic inoculants can be made from natural inorganic materials, natural polymers, or synthetic materials. Soil-made inoculants In recent years, scientific reports on soil-made inoculants are few.

Introduction

Polymeric inoculants Synthetic formulations based on an assortment of polymers have been continuously evaluated for decades because they offer substantial advantages over peat. Encapsulated formulations The encapsulation of microorganisms in a polymeric matrix also known as immobilization when one microorganism is used and co-immobilization when more than one organism is used is currently experimental in the field of agricultural and environmental bacteria-inoculation technology.

The basic industrial concept underlying immobilizing microbial cells is to entrap live microorganisms into a polymeric matrix and maintain their viability. The immobilized product bacteria-synthetic matrix is then fermented in a bacterial growth medium for different end uses. These formulations can produce many useful compounds for industrial applications, including organic acids, amino acids, enzymes, and vitamins, and environmental applications, including bioremediation of toxic materials for extended time periods.

The desired bacterial products are extracted from the bioreactor while fermentation continues. Immobilized microbial cells are easy to produce, store, and handle during industrial operations. The main goal of these industrial formulations is to maintain the entrapped cells in an active form, at high concentrations, for as long as possible. Any premature release of the microorganisms from these encapsulated forms is undesirable Fig. These industrial-end formulations are not the topic of this review and can be viewed elsewhere Prasad and Kadokawa Encapsulated bacterial formulations for agricultural and environmental applications have at least two distinctly different goals from those of the fermentation industry: 1 They have to provide a temporary shelter for the encapsulated strain from the soil environment and from microbial competitors, both hostile to any change in the biological makeup of the soil, and 2 for colonizing plant roots, they have to release the desired strain gradually.

Liberation of the entrapped bacteria from the beads happens when the polymer is slowly degraded by the native soil microorganisms, thereby releasing the PGPB-rhizobia to the soil where plants that need inoculation are growing Fig. Seed inoculation This is the most common and practical inoculation technique. Because every seed needs to be coated with a threshold number of bacteria, adhesives are used.

Adhesives include gum arabic Wani et al.

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A second role of an adhesive is to prevent the inoculant either dry inoculant as powder or wet inoculant once the moisture evaporates from dislodging during sowing with the seeding equipment, especially the powdered-type when applied with air-seeders. Sometimes, pelleting of seeds with superfine limestone CaCO 3 is added to balance the acidic nature of the soil.

This can be done with an additional adhesive layer under the lime coating for older cases, see: Deaker et al. The seeds are then sown with common seeding equipment. It is commonly agreed that one essential condition to seed coating is adding adhesive materials. Yet, there is no agreement on the best adhesives. Each manufacturer or experimentalist empirically evaluates which adhesive best fits seeds and inoculants Albareda et al.

When seeds are inoculated with liquid inoculant, with or without dissolved adhesive, the inoculant is sprayed directly onto the seeds. After drying, the seeds are sown Fig. Sterilization Sterilization of the carrier material is essential to keep high number of the desired bacteria in the final formulation for long storage periods.

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Shelf life Liquid inoculants produced in the field by fermenters and immediately applied are uncommon and only a few exist for turf grass for golf courses and limited hydroponic cultivation. For common agricultural uses, inoculants made of peat or other organic and inorganic materials and a storage period between manufacturing and use, is usually required. Extended shelf life of the inoculant, while retaining its biological traits intact, is a major challenge for any formulation Fig.