Abstract
Plant growth-promoting bacteria (PGPB) afford plants several advantages (i.e., improvement of nutrient acquisition, growth, and development; induction of abiotic and biotic stress tolerance). Numerous PGPB strains have been isolated and studied over the years. However, only a few of them are available on the market, mainly due to the failed bacterial survival within the formulations and after application inside agroecosystems. PGPB strains with these challenging limitations can be used for the formulation of cell-free supernatants (CFSs), broth cultures processed through several mechanical and physical processes for cell removal. In the scientific literature there are diverse reviews and updates on PGPB in agriculture. However, no review deals with CFSs and the CFS metabolites obtainable by PGPB. The main objective of this review is to provide useful information for future research on CFSs as biostimulant and biocontrol agents in sustainable agriculture. Studies on CFS agricultural applications, both for biostimulant and biocontrol applications, have been reviewed, presenting limitations and advantages. Among the 109 articles selected and examined, the Bacillus genus seems to be the most promising due to the numerous articles that support its biostimulant and biocontrol potentialities. The present review underlines that research about this topic needs to be encouraged; evidence so far obtained has demonstrated that PGPB could be a valid source of secondary metabolites useful in sustainable agriculture.
Highlights
Plant growth-promoting bacteria (PGPB) are a widespread group of bacteria generally living in association with plants, having several beneficial effects related to (i) improvement of plant nutrient acquisition [1], (ii) promotion of plant growth and development [2], and (iii) induction of tolerance towards abiotic and biotic stress [3]
This review provides useful information for future research on cell-free supernatants (CFSs) as biostimulant and biocontrol agents in sustainable agriculture
It is evident that the literature contains only a few reports useful for the creation of valid scientific evidence to support the development of CFS formulations
Summary
Plant growth-promoting bacteria (PGPB) are a widespread group of bacteria generally living in association with plants, having several beneficial effects related to (i) improvement of plant nutrient acquisition [1], (ii) promotion of plant growth and development [2], and (iii) induction of tolerance towards abiotic and biotic stress [3]. The mechanisms behind these effects are complex and not fully known, most of the effects can be ascribed to the bacterial ability to produce metabolites with stimulant and/or protective effects. A meaningful role is played by phytohormones (i.e., abscisic acid, auxins, cytokinins, ethylene, and gibberellins). These substances regulate plant growth at all stages of development, by stimulating growth, coordination between cells, tissues and organs, and by preserving certain functions [4]. Stimulant effects are ascribed to organic acids, which induce the release of nutrients from insoluble complexes by lowering soil pH, chelation, and mineralization [5,6]. The promotion of plant growth and development are induced by several other secondary metabolites, volatile compounds, and exopolysaccharides [2,7]
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