Plant Growth-Promoting Rhizobacteria: Context, Mechanisms of Action, and Roadmap to Commercialization of Biostimulants for Sustainable Agriculture

Abstract

Microbes of the phytomicrobiome are associated with every plant tissue and, in combination with the plant form the holobiont. Plants regulate the composition and activity of their associated bacterial community carefully. These microbes provide a wide range of services and benefits to the plant; in return, the plant provides the microbial community with reduced carbon and other metabolites. Soils are generally a moist environment, rich in reduced carbon which supports extensive soil microbial communities. The rhizomicrobiome is of great importance to agriculture owing to the rich diversity of root exudates and plant cell debris that attract diverse and unique patterns of microbial colonization. Microbes of the rhizomicrobiome play key roles in nutrient acquisition and assimilation, improved soil texture, secreting, and modulating extracellular molecules such as hormones, secondary metabolites, antibiotics, and various signal compounds, all leading to enhancement of plant growth. The microbes and compounds they secrete constitute valuable biostimulants and play pivotal roles in modulating plant stress responses. Research has demonstrated that inoculating plants with plant-growth promoting rhizobacteria (PGPR) or treating plants with microbe-to-plant signal compounds can be an effective strategy to stimulate crop growth. Furthermore, these strategies can improve crop tolerance for the abiotic stresses (e.g., drought, heat, and salinity) likely to become more frequent as climate change conditions continue to develop. This discovery has resulted in multifunctional PGPR-based formulations for commercial agriculture, to minimize the use of synthetic fertilizers and agrochemicals. This review is an update about the role of PGPR in agriculture, from their collection to commercialization as low-cost commercial agricultural inputs. First, we introduce the concept and role of the phytomicrobiome and the agricultural context underlying food security in the 21st century. Next, mechanisms of plant growth promotion by PGPR are discussed, including signal exchange between plant roots and PGPR and how these relationships modulate plant abiotic stress responses via induced systemic resistance. On the application side, strategies are discussed to improve rhizosphere colonization by PGPR inoculants. The final sections of the paper describe the applications of PGPR in 21st century agriculture and the roadmap to commercialization of a PGPR-based technology.

Persistence ofPseudomonas fluorescensLBUM677 in the rhizosphere of corn gromwell (Buglossoides arvensis) under field conditions and its impact on seed oil and stearidonic acid bioaccumulation

“…Previous results have shown that LBUM677 is able to solubilize phosphate, produce IAA-like compounds, ACC deaminase and siderophores (Novinscak and Filion 2018), which are all mechanisms that have previously been demonstrated to increase plant biomass (Compant et al 2005;Beauregard 2015;Backer et al 2018). The increase in seed, oil and SDA yields in B. arvensis reported in this study could be due in part to the benefits brought on by these mechanisms in LBUM677, or it could be due to other currently uncharacterized mechanisms of plant growth promotion by the bacterium.…”

“…; Beauregard ; Backer et al . ). The increase in seed, oil and SDA yields in B. arvensis reported in this study could be due in part to the benefits brought on by these mechanisms in LBUM677, or it could be due to other currently uncharacterized mechanisms of plant growth promotion by the bacterium.…”

“…Unfortunately, PGPR are often used in quantities that do now allow efficient field rhizosphere colonization, potentially due to competition with resident micro‐organisms (Backer et al . ). The threshold to obtain a positive effect on plant health and/or growth has been shown to differ among different PGPR (Bashan et al .…”

“…Studies have found that one of the most important characteristics of PGPR for field application is rhizosphere colonization, or the ability to establish themselves on the plant's roots and move along while the roots are growing. Unfortunately, PGPR are often used in quantities that do now allow efficient field rhizosphere colonization, potentially due to competition with resident micro-organisms (Backer et al 2018). The threshold to obtain a positive effect on plant health and/or growth has been shown to differ among different PGPR (Bashan et al 2014).…”

A suite of complementary biocontrol traits allows a native consortium of root‐associated bacteria to protect their host plant from a fungal sudden‐wilt disease

“…Biofilm formation was synergistically enhanced in the consortium for two of the media tested (NB, Figure 2; TSB, Supporting Information Figure S2). In the past, most studies used single strains of Pseudomonas or Bacillus to study biofilm formation and bioprotection -see, for example, the reviews of Backer et al (2018) and Pandin et al (2017) -although the idea of using defined bacterial communities is not new (Lebeis et al, 2015;Xu, Jeffries, Pautasso, & Jeger, 2011). Nevertheless the number of studies investigating the biofilm formation and bioprotection mechanism of a bacterial consortium is still very limited and their practical use remains an agricultural goal (Mazzola & Freilich, 2016).…”