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The role of salicylic acid (SA), a small phenolic compound, in plant defense against a wide range of pathogens is highly significant. The SA function in plants has been widely investigated for years. The presence of SA is critical in responding to various abiotic and biotic stress conditions. Because of its crucial role in developing and regulating plant immunity, it has been extensively studied in plants. The results have shown the role of numerous plant genes in signaling salicylic acid. Much of the efforts have focused on enhancing yield under various unfavorable circumstances and agricultural productivity; therefore, more knowledge about SA signaling can help scientists and other end users to understand the physiological processes of SA and the importance of SA immunity.

Each plant coexists with its associated microbes as a holobiont. Microbes inhabit all parts of the plant, including roots, stems, and leaves. The ability of microbes to colonize a particular plant species is fueled by release of signals by either or both partners that are only recognized by the right partner. It is now becoming increasingly clear that at least some microbe-to-plant signals can promote plant growth. This chapter focuses on the current trends in the use of plant–microbe signal compounds for sustainable agriculture, and also gives a brief description of potential areas for future research. The chapter begins with an introduction to the holobiont, with major focus on the beneficial plant–microbe interactions. The chapter then goes on to describe the various signals involved in the well studied legume–rhizobia and plant–mycorrhizal symbioses. Signals involved in other beneficial plant–microbe interactions; microbe–microbe signal interactions are also described, however, these areas are only beginning to be understood. Further into the chapter, the role of plant–microbe signals in plant growth and development under stressed and non-stressed conditions is described, listing various examples of microbes that promote growth of various crop species. The authors also mention limitations to microbial inoculant efficacy especially under field conditions, as well as the pros and cons of using single microbes, microbial consortia, and plant–microbial signals. The conclusion summarizes the chapter and gives suggestions on what future research on signal compounds needs to focus on, for optimal utilization in agricultural production.

Terrestrial plants evolution occurred in the presence of microbes, the phytomicrobiome. The rhizosphere microbial community is the most abundant and diverse subset of the phytomicrobiome and can include both beneficial and parasitic/pathogenic microbes. Prokaryotes of the phytomicrobiome have evolved relationships with plants that range from non-dependent interactions to dependent endosymbionts. The most extreme endosymbiotic examples are the chloroplasts and mitochondria, which have become organelles and integral parts of the plant, leading to some similarity in DNA sequence between plant tissues and cyanobacteria, the prokaryotic symbiont of ancestral plants. Microbes were associated with the precursors of land plants, green algae, and helped algae transition from aquatic to terrestrial environments. In the terrestrial setting the phytomicrobiome contributes to plant growth and development by (1) establishing symbiotic relationships between plant growth-promoting microbes, including rhizobacteria and mycorrhizal fungi, (2) conferring biotic stress resistance by producing antibiotic compounds, and (3) secreting microbe-to-plant signal compounds, such as phytohormones or their analogues, that regulate aspects of plant physiology, including stress resistance. As plants have evolved, they recruited microbes to assist in the adaptation to available growing environments. Microbes serve themselves by promoting plant growth, which in turn provides microbes with nutrition (root exudates, a source of reduced carbon) and a desirable habitat (the rhizosphere or within plant tissues). The outcome of this coevolution is the diverse and metabolically rich microbial community that now exists in the rhizosphere of terrestrial plants. The holobiont, the unit made up of the phytomicrobiome and the plant host, results from this wide range of coevolved relationships. We are just beginning to appreciate the many ways in which this complex and subtle coevolution acts in agricultural systems.

Biochar can improve crop yields and nutrient uptake by altering soil properties and root growth. The goals of this study were to determine (1) how biochar alters soil NH4-N availability and corn root development and (2) whether the biochar-induced changes in the plant-soil system coincide with increased nitrogen fertilizer recovery efficiency (FRE) when plants reach the reproductive stage. Corn was grown in two soils amended (or not) with biochar, at five N fertilizer rates (ranging from 0 to 300 kg N ha−1). Soil chemical properties, root and shoot biomass and shoot N uptake were measured at the V3 and R1 stages, roots traits at the V3 stage and FRE at the R1 stage. Biochar increased soil CEC and root growth at the V3 stage. Biochar increased soil available NH4-N, CEC, root growth and root metabolic activity and N uptake at the R1 stage. Fertilizer recovery efficiency was increased in the presence of biochar when N fertilizer was applied at 75 and 150 kg N ha−1. Biochar stimulated early root development, allowing plants to take advantage of increased NH4-N retention concentration in soil by biochar, thereby increasing FRE at the R1 stage at lower N fertilizer application rates.

Switchgrass ( Panicum virgatum L.) is a fast growing native C 4 perennial and a lignocellulosic biomass crop for North America. In combination with biochar, an active plant growth promoting rhizobacterial (PGPR) community can contribute to the long-term sequestration of carbon in soil, fix nitrogen, and enhance the availability of other nutrients to plants. Biochar and PGPR have the potential to improve grass biomass production, but they have not been tested together under high-latitude temperate zone field conditions. Therefore, the objective of this three-year field study was to determine whether there were effects on biomass yield and yield components of switchgrass (cv. Cave-in-Rock) due to a rhizobacterium that was able to mobilize soil phosphorus ( Pseudomonas rhodesiae ), a bacterial consortium that was able to supply nitrogen ( Paenibacillus polymyxa , Rahnella sp., and Serrati sp.), and pine wood chip biochar applied as a soil amendment at 20 Mg ha −1 . The incorporation of biochar, or inoculation with the N-fixing consortium, and the combined inoculation of the experimental bacteria had positive effects on switchgrass height. At a loam soil site in Sainte-Anne-de-Bellevue, Quebec, when nitrogen fertilizer was not applied, the addition of biochar had a positive effect on stand count (tillers m −1 row). On the sandy soil in Sainte-Anne-de-Bellevue, when biochar was applied with 100 kg N ha −1 , biomass yield increased over the control but did not provide additional benefits over plots receiving only 50 kg N ha −1 . It remains unclear whether or not the increased C sequestration of this management system justifies increased N fertilizer usage.

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 use of thermally treated biomass, including biochar, as soil amendments can improve soil fertility by providing nutrients, stable C and improving soil water-holding capacity. However, if the degree of carbonization is low, these soil amendments can lower crop productivity as a result of high salinity or organic compounds. The overall effect of these soil amendments is mediated by complex relationships between production conditions, soil properties and environmental conditions. This study aimed to 1) characterize the physiochemical properties and organic compounds released by three soil amendments (softwood biochar or pyrogenic carbonaceous biosolids), 2) determine the effects of these amendments on maize (Zea mays) seedling productivity, and 3) relate properties of these amendments to effects on maize seedling productivity under controlled environment conditions. Physicochemical properties and mobile organic compounds (water-soluble and volatile organic compounds were determined. The amendments were tested in maize germination and greenhouse experiments. Chemical fingerprinting of volatile and water-soluble compounds revealed over 100 mobile organic species. Increasing treatment temperature from 270 to 320°C reduces phytotoxicity of pyrogenic carbonaceous biosolids soil amendments. Water-soluble components of pyrogenic carbonaceous biosolids produced at 270°C (inorganic N, Na and/or organic compounds) were associated with reduced maize seedling productivity. Volatile components of pyrogenic carbonaceous biosolids produced at 320°C were associated with improved maize seedling productivity; nitrogen uptake was increased in spite of smaller root systems as a result of increased mineralization of soil or amendment N and/or uptake of organic N compounds. These results suggest that pyrogenic carbonaceous biosolids have potential benefits to provide plant nutrients when the amount of organic and inorganic species are limited during early growth stages, under greenhouse conditions. Future studies should examine these effects under field conditions to confirm whether controlled environment results translate into effects on yield.