Abstract
In the last 10–15 years, the wide application of bioformulated plant beneficial microorganisms is accepted as an effective alternative of chemical agro-products. Two main problems can be distinguished in their production and application: (a) economical competiveness based on the overall up-stream and down-stream operational costs, and (b) development of commercial products with a high soil-plant colonization potential in controlled conditions but not able to effectively mobilize soil nutrients and/or combat plant pathogens in the field. To solve the above problems, microbe-based formulations produced by immobilization methods are gaining attention as they demonstrate a large number of advantages compared to other solid and liquid formulations. This mini-review summarizes the knowledge of additional compounds that form part of the bioformulations. The additives can exert economical, price-decreasing effects as bulking agents or direct effects improving microbial survival during storage and after introduction into soil with simultaneous beneficial effects on soil and plants. In some studies, combinations of additives are used with a complex impact, which improves the overall characteristics of the final products. Special attention is paid to polysaccharide carriers and their derivates, which play stimulatory role on plants but are less studied. The mini-review also focuses on the potential difficulty in evaluating the effects of complex bio-formulations.
Highlights
Different groups of soil microorganisms, such as root endophytic fungi, mycorrhizal fungi, plant growth-promoting rhizobacteria, rhizobia, and phosphate solubilizers affect plant growth through direct and plant-mediated mechanisms, including in stressed conditions (van der Heijden et al, 2008; Berg, 2009; Shilev et al, 2019)
The main roles of the formulation of inoculants are: (i) to provide a more suitable micro-environment for the microbial strain/s, combined with physical or chemical protection over a prolonged period, in order to avoid a rapid decrease of the cells’ viability during storage, (ii) to support the strain/s competition with the better-adapted native soil microflora, and (iii) to reduce losses due to the depredation by the micro-fauna after being introduced into soil. All these functions are aiming at providing a reliable source of living cells available to interact with plants and soil microbiome (Bashan, 1998; Herrmann and Lesueur, 2013; Bashan et al, 2014; Malusá and Vassilev, 2014)
The gelcell immobilized approach is the technological solution that can better assure a standardization of the formulated inoculum as well as its quality. In this mini-review, we analyze the immobilized-cell approach underlying the possibilities for its improvement and some specific characteristics of the carrier structure and formulation, the role of additional compounds introduced into the cell-gel structures and the effect of the gel-forming polysaccharides and their derivates on plant health and growth
Summary
Different groups of soil microorganisms, such as root endophytic fungi, mycorrhizal fungi, plant growth-promoting rhizobacteria, rhizobia, and phosphate solubilizers affect plant growth through direct and plant-mediated mechanisms, including in stressed conditions (van der Heijden et al, 2008; Berg, 2009; Shilev et al, 2019). In this mini-review, we analyze the immobilized-cell approach underlying the possibilities for its improvement and some specific characteristics of the carrier structure and formulation, the role of additional compounds introduced into the cell-gel structures and the effect of the gel-forming polysaccharides and their derivates on plant health and growth. There is a wide selection of additional materials used in bioimmobilized systems, which can serve as carrier bulking agents, enhance the formulation stability, protect and feed microbial cells or spores.
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