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

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

The fatty acid binding protein 6 gene (Fabp6) is expressed in murine granulosa cells and is involved in ovulatory response to superstimulation

The fatty acid binding protein 6 (Fabp6) is commonly regarded as a bile acid binding protein found in the distal portion of the small intestine and has been shown to be important in maintaining bile acid homeostasis. Previous studies have also reported the presence of Fabp6 in human, rat and fish ovaries, but the significance of Fabp6 in this organ is largely unknown. Therefore, we surveyed murine ovaries for Fabp6 gene expression and evaluated its role in ovarian function using mice with whole body Fabp6 deficiency. Here we show that the Fabp6 gene is expressed in granulosa and luteal cells of the mouse ovary. Treatment with gonadotropins stimulated Fabp6 gene expression in large antral follicles. The ovulation rate in response to superovulatory treatment in Fabp6-deficient mice was markedly decreased compared to wildtype (C57BL/6) mice. The results of this study suggest that expression of Fabp6 gene in granulosa cells serves an important and previously unrecognized function in fertility

The Ileal Lipid Binding Protein Is Required for Efficient Absorption and Transport of Bile Acids in the Distal Portion of the Murine Small Intestine

The ileal lipid binding protein (ilbp) is a cytoplasmic protein that binds bile acids with high affinity. However evidence demonstrating the role of this protein in bile acid transport and homeostasis is missing. We created a mouse strain lacking ilbp (Fabp6−/− mice) and assessed the impact of ilbp deficiency on bile acid homeostasis and transport in vivo. Elimination of ilbp increased fecal bile acid excretion (54.2%, P<0.05) in female but not male Fabp6−/− mice. The activity of cholesterol 7α-hydroxylase (cyp7a1), the rate-controlling enzyme of the classical bile acid biosynthetic pathway, was significantly increased in female (63.5%, P<0.05) but not in male Fabp6−/− mice. The amount of [3H]taurocholic acid (TCA) excreted by 24 h after oral administration was 102% (P<0.025) higher for female Fabp6−/− mice whereas it was 57.3% (P<0.01) lower for male Fabp6−/− mice, compared to wild-type mice

The Ileal Lipid Binding Protein Is Required for Efficient Absorption and Transport of Bile Acids in the Distal Portion of the Murine Small Intestine

<div><p>The ileal lipid binding protein (ilbp) is a cytoplasmic protein that binds bile acids with high affinity. However evidence demonstrating the role of this protein in bile acid transport and homeostasis is missing. We created a mouse strain lacking ilbp (<em>Fabp6^−/−</em> mice) and assessed the impact of ilbp deficiency on bile acid homeostasis and transport in vivo. Elimination of ilbp increased fecal bile acid excretion (54.2%, <em>P</em><0.05) in female but not male <em>Fabp6^−/−</em> mice. The activity of cholesterol 7α-hydroxylase (cyp7a1), the rate-controlling enzyme of the classical bile acid biosynthetic pathway, was significantly increased in female (63.5%, <em>P</em><0.05) but not in male <em>Fabp6^−/−</em> mice. The amount of [³H]taurocholic acid (TCA) excreted by 24 h after oral administration was 102% (<em>P</em><0.025) higher for female <em>Fabp6^−/−</em> mice whereas it was 57.3% (<em>P</em><0.01) lower for male <em>Fabp6^−/−</em> mice, compared to wild-type mice. The retained fraction of the [³H]TCA localized in the small and large intestines was increased by 22% (<em>P</em><0.02) and decreased by 62.7% (<em>P</em><0.01), respectively, in male <em>Fabp6^−/−</em> mice relative wild-type mice, whereas no changes were seen in female <em>Fabp6^−/−</em> mice. Mucosal to serosal bile acid transport using everted distal gut sacs was decreased by 74% (<em>P</em><0.03) in both sexes of <em>Fabp6^−/−</em> mice as compared to wild-type mice. The results demonstrate that ilbp is involved in the apical to basolateral transport of bile acids in ileal enterocytes, and is vital for the maintenance of bile acid homeostasis in the enterohepatic circulation (EHC) in mice.</p> </div

Hepatic cholesterol concentration and enzyme activities of <i>Fabp6</i>^+/+ (black bars) and <i>Fabp6^−/−</i> (white bars) mice.

<p>(A) Unesterified cholesterol concentration and (B) Cholesteryl ester concentration, (n = 3). (C) Relative cyp7a1 activity (n = 5). (D) Relative HMGR activity (n = 5). Groups are compared relative to male <i>Fabp6</i>^+/+ mice.</p

Survey of gene expression in the liver.

<p>qPCR analysis of liver RNA from <i>Fabp6</i>^+/+ (black bars) and <i>Fabp6^−/−</i> (white bars) mice were done in duplicates. The abundance (mean ± SEM) of target mRNAs was normalized to internal standards and expressed relative male wild-type mice. *<i>P</i><0.05, vs. wild-type.</p

Plasma chemistry.

*<p>P<0.01,</p>**<p>P<0.05, vs. wild-type, n = 10 per group. Values for ALT and AST are mean ± SEM.</p

Survey of gene expression in the small intestine.

<p>(A) qPCR analysis of RNA from small intestines of <i>Fabp6</i>^+/+ (black bars) and <i>Fabp6^−/−</i> (white bars) were done in duplicates. The normalized abundance (mean±SEM) of target mRNAs is expressed relative to male <i>Fabp6</i>^+/+ mice. <i>P</i><0.05, vs. wild-type of the same sex. (B) Protein blots of small intestine homogenates of <i>Fabp6</i>^+/+ and <i>Fabp6^−/−</i> mice were probed with antisera to ilbp, asbt and L-FABP.</p

Targeted disruption of the murine <i>Fabp6</i> gene.

<p>(A) Structure of wild-type (wt) and disrupted <i>Fabp6</i> alleles. The targeting vector was designed to replace the region of the <i>Fabp6</i> gene encompassing exons 2 and 3 with the neo resistance (neo^r) gene cassette. X, Xba I site. (B) RNA blot of small intestine RNA from <i>Fabp6</i>^+/+ and <i>Fabp6^−/−</i> mice probed with [³²P]-labeled ilbp cDNA. (C) Protein blot of small intestine homogenates from <i>Fabp6</i>^+/+ and <i>Fabp6^−/−</i> mice probed with antiserum to murine ilbp.</p

Transport of bile acids in everted gut sacs.

<p>(A) Amount of TCA accumulated in the serosal fluid after a 30 min incubation of gut sacs from the proximal one-third and distal one-third portions of the small intestine of male <i>Fabp6</i>^+/+ (n = 4), female <i>Fabp6</i>^+/+ (n = 3–4) mice (black bars), male <i>Fabp6^−/−</i> (n = 4) and female <i>Fabp6^−/−</i> (n = 5) mice (white bars). (B) Amount of TCA accumulated in the gut sac tissue at the end of the assay incubation period. *<i>P</i><0.025, **<i>P</i><0.01, vs. wild-type of the same sex.</p