Bacillus for Plant Growth Promotion and Stress Resilience: What Have We Learned?

Abstract

The rhizosphere is a thin film of soil that surrounds plant roots and the primary location of nutrient uptake, and is where important physiological, chemical, and biological activities are occurring. Many microbes invade the rhizosphere and have the capacity to promote plant growth and health. Bacillus spp. is the most prominent plant growth promoting rhizobacteria due to its ability to form long-lived, stress-tolerant spores. Bacillus-plant interactions are driven by chemical languages constructed by a wide spectrum of metabolites and lead to enhanced plant growth and defenses. Thus, this review is a synthesis and a critical assessment of the current literature on the application of Bacillus spp. in agriculture, highlighting gaps that remain to be explored to improve and expand on the Bacillus-based biostimulants. Furthermore, we suggest that omics sciences, with a focus on metabolomics, offer unique opportunities to illuminate the chemical intercommunications between Bacillus and plants, to elucidate biochemical and molecular details on modes of action of Bacillus-based formulations, to generate more actionable insights on cellular and molecular events that explain the Bacillus-induced growth promotion and stress resilience in plants.

Keywords: Bacillus; biostimulant; metabolomics; plant growth; stress resilience.

Figure 1

Intra-and extracellular biosynthesis of metal-nanoparticles by Bacillus spp. Extracellular biosynthesis of metal-nanoparticles is carried out by trapping metal ions on the cell surface and reducing them in the presence of secreted enzymes or metabolite and/or membrane-bound enzymes. In the intracellular biosynthesis of metal-nanoparticles, after transfer of metal ions into cell cytoplasm, the metal ions are reduced as a result of metabolic reactions with enzymes such as alpha-NADPH-dependent nitrate reductase.

Figure 2

Some molecules involved in Bacillus-plant interactions. Plants produce various compounds including the hormones salicylic acid (SA), jasmonic acid (JA), cytokinins (CK), and indole acetic acid. Plants also produce organic acids, flavonoids, amino acids, and 1-aminocyclopropane-1-carboxylate (ACC) as signaling molecules. On the other hand, Bacillus produce volatiles such as 2,3 butanediol, lipopeptides such as surfactin, and phytohormones such as gibberellic acid and acyl homoserine lactone (AHL) as signaling molecules.

Figure 3

An overview of mechanisms employed by Bacillus spp. in the mitigation of biotic and abiotic stresses. Bacillus produce cyclic lipopetides which activate pathways regulated by jasmonic acid and ethylene and thus elicit induced systemic resistance (ISR). They also produce hydrogen cyanide (HCN) and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase which lower the levels of plant ethylene. Siderophores produced by Bacillus chelate iron thus making it unavailable to the pathogen. Several root associated Bacillus spp. bacteria produce zeatin, gibberellic acid (GA), indole-3-acetic acid (IAA), salicylic acid (SA), abscisic acid (ABA) as well as volatile organic acids (VOCs) which help plants to withstand stress by enhancing its antioxidant potential, by up-regulation of the antioxidant system and by accumulation of compatible osmolytes thus reducing oxidative stress-induced damage; improving photosynthetic capacity and membrane stability; promoting cell division and stomatal regulation; stimulating growth of root system, and acquisition of water and nutrients.

Figure 4

Omics technologies in systems biology. The omics cascade shows the flow of information from gene level to the metabolome with metabolomics being the closest link to the phenotype.