Abstract
Plant beneficial microbes (PBMs), such as plant growth-promoting bacteria, rhizobia, arbuscular mycorrhizal fungi, and Trichoderma, can reduce the use of agrochemicals and increase plant yield, nutrition, and tolerance to biotic–abiotic stresses. Yet, large-scale applications of PBM have been hampered by the high amounts of inoculum per plant or per cultivation area needed for successful colonization and consequently the economic feasibility. Seed coating, a process that consists in covering seeds with low amounts of exogenous materials, is gaining attention as an efficient delivery system for PBM. Microbial seed coating comprises the use of a binder, in some cases a filler, mixed with inocula, and can be done using simple mixing equipment (e.g., cement mixer) or more specialized/sophisticated apparatus (e.g., fluidized bed). Binders/fillers can be used to extend microbial survival. The most reported types of seed coating are seed dressing, film coating, and pelleting. Tested in more than 50 plant species with seeds of different dimensions, forms, textures, and germination types (e.g., cereals, vegetables, fruits, pulses, and other legumes), seed coating has been studied using various species of plant growth-promoting bacteria, rhizobia, Trichoderma, and to a lesser extent mycorrhizal fungi. Most of the studies regarding PBM applied via seed coating are aimed at promoting crop growth, yield, and crop protection against pathogens. Studies have shown that coating seeds with PBM can assist crops in improving seedling establishment and germination or achieving high yields and food quality, under reduced chemical fertilization. The right combination of biological control agents applied via seed coating can be a powerful tool against a wide number of diseases and pathogens. Less frequently, studies report seed coating being used for adaptation and protection of crops under abiotic stresses. Notwithstanding the promising results, there are still challenges mainly related with the scaling up from the laboratory to the field and proper formulation, including efficient microbial combinations and coating materials that can result in extended shelf-life of both seeds and coated PBM. These limitations need to be addressed and overcome in order to allow a wider use of seed coating as a cost-effective delivery method for PBM in sustainable agricultural systems.
Highlights
More than 1/3 of the Earth’s land surface is occupied by agriculture, with a total net production value estimated in 2.6 × 109 US dollar (FAOSTAT, 2016)
This review mostly focuses on two main groups of soil microorganisms, bacteria and fungi, on plant growth-promoting bacteria (PGPB), arbuscular mycorrhizal (AM) fungi, and Trichoderma, due to their importance as microbial inoculants in agroecosystems
Despite the well-known and proven benefits of Plant beneficial microbes (PBMs) in improving yield, quality, and stress resistance of agricultural crops, few studies were focused on feasible delivery systems to apply microbial inocula in large-scale agriculture, frequently hampering its commercial exploitation and scale-up
Summary
More than 1/3 of the Earth’s land surface is occupied by agriculture, with a total net production value estimated in 2.6 × 109 US dollar (FAOSTAT, 2016). Plant beneficial microbes (PBMs) are considered to be a natural alternative path to ease the pressure on the environment resulting from conventional farming These microbes can help plants maintain or increase productivity while reducing the input of agrochemicals, restoring soil fertility, and/or overcoming problems caused by abiotic and biotic stresses (Malusá et al, 2012; Nadeem et al, 2014). Various genera of bacteria (e.g., Azospirillum, Azotobacter, Pseudomonas, Bacillus and Burkholderia) contain species that have positive effects on plant growth and development These beneficial bacteria, designated as PGPB, are responsible for protecting plants from biotic and abiotic stresses, enhancing plant growth and performance through direct and indirect mechanisms (Glick, 1995; Timmusk et al, 2017). Inoculation of plants through root dipping and foliar is currently being used, these techniques demand large
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