Plant growth-promoting rhizobacteria (PGPR) are widely viewed as a valuable emerging asset for improving the future of food security. Among thousands of taxa of soil bacteria, the species within the genera Bacillus and Pseudomonas are particularly abundant in strains with proven PGPR effects. These beneficial bacteria excrete enzymes and growth regulators that promote plant growth, act as biological control agents and induce systemic resistance to infections and pests, they facilitate nutrient uptake and detoxification of pollutants. Although these plant-microbe interactions are often viewed as mutualistic, increasing evidence has shown their greater complexity, calling for in-depth analysis of anatomical and molecular interactions between the host plants and bacteria inhabiting their rhizosphere. When research is focused on the interaction between the host plant and a single bacterial species of interest, "one-on-one"; systems are used to study the plant-microbe interactions in axenic conditions, or complex in vivo comparisons can be made between plant systems in presence and absence of the studied beneficial bacterial strains.

Research on the beneficial effects of Bacillus and Pseudomonas species on plant growth has been progressively multiplying over the last decade. Reports confirming previously established mechanisms, as well as discoveries of novel beneficial effects of these plant growth-promoting bacteria on crop plants, are being continuously published. There is a need to discover, explain, and systematically summarize the mechanisms of the multiple beneficial effects of Bacillus and Pseudomonas on crop plants in order to optimally exploit these rhizosphere bacteria for maximizing crop yields and ensuring food security.

Thus, this Research Topic welcomes a wide spectrum of original research and review articles that will contribute to broadening and summarizing the knowledge on how the plant-associated Bacillus and Pseudomonas species affect the life of their host plants. Interference of Bacillus and Pseudomonas with phytohormonal regulation, their elicitation of plant systemic resistance, their biocontrol activity, and their other molecular, physiological, and ecological effects, as well as their underlying mechanisms, are expected to be analyzed, explained, and summarized under the scope of this Research Topic.

Another intriguing and less explored property is the capability of certain species to colonize the phyllosphere, a hostile habitat where diverse factors such as UV radiation, high and low temperatures, and hydric stress affect the bacterial populations established on the phyllosphere. Particularly, the Bacillus and Pseudomonas genera are producers of a vast diversity of antimicrobial compounds, among them cyclic lipopeptides and volatiles are promising as fungicides and bactericides.

This Research Topic will welcome the following topics, but not limited to:

- Particular interactions on the species (strain) and molecular level.
- Metagenomic analyses that show how environmental changes may affect plants through altering their associated microbiome
- Reflecting on the relative abundance of beneficial Bacillus and Pseudomonas species.
- Interference of Bacillus and Pseudomonas with phytohormonal regulation.
- Elicitation of plant systemic resistance by Bacillus and Pseudomonas.
- Biocontrol activity of Bacillus and Pseudomonas.
- Survival of Bacillus and Shelf life of antimicrobials on the phyllosphere.
- Mechanisms of stomata opening by Bacillus and Pseudomonas.
- Molecular mechanisms of cyclic lipopeptides biosynthesis and regulation.

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Frontiers in Microbiology

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