Abstract
Plant growth-promoting rhizobacteria (PGPR) are increasingly appreciated for their contributions to primary productivity through promotion of growth and triggering of induced systemic resistance in plants. Here we focus on the beneficial effects of one particular species of PGPR (Pseudomonas fluorescens) on plants through induced plant defense. This model organism has provided much understanding of the underlying molecular mechanisms of PGPR-induced plant defense. However, this knowledge can only be appreciated at full value once we know to what extent these mechanisms also occur under more realistic, species-diverse conditions as are occurring in the plant rhizosphere. To provide the necessary ecological context, we review the literature to compare the effect of P. fluorescens on induced plant defense when it is present as a single species or in combination with other soil dwelling species. Specifically, we discuss combinations with other plant mutualists (bacterial or fungal), plant pathogens (bacterial or fungal), bacterivores (nematode or protozoa), and decomposers. Synergistic interactions between P. fluorescens and other plant mutualists are much more commonly reported than antagonistic interactions. Recent developments have enabled screenings of P. fluorescens genomes for defense traits and this could help with selection of strains with likely positive interactions on biocontrol. However, studies that examine the effects of multiple herbivores, pathogens, or herbivores and pathogens together on the effectiveness of PGPR to induce plant defenses are underrepresented and we are not aware of any study that has examined interactions between P. fluorescens and bacterivores or decomposers. As co-occurring soil organisms can enhance but also reduce the effectiveness of PGPR, a better understanding of the biotic factors modulating P. fluorescens–plant interactions will improve the effectiveness of introducing P. fluorescens to enhance plant production and defense.
Highlights
Plant growth-promoting rhizobacteria (PGPR) are a diverse group of microorganisms that are increasingly appreciated for their contributions to primary productivity through promotion of growth and triggering of induced systemic resistance (ISR) in plants
To test the dependence of PGPR–plant interactions on direct and indirect biotic interactions with other rhizosphere biota, we compare studies in which effects of P. fluorescens on plant defense are examined for a single P. fluorescens isolate with studies in which these effects are examined for a P. fluorescens isolate in combination with other isolates and/or species
As we argue that the effect of PGPR on induced plant defense cannot be considered in isolation from the effects of other organisms that are present in the soil such as nematodes, fungi, earthworms, or protozoa on the PGPR or on the plant, we provide a brief overview of interactions between bacteria and other soil dwelling organisms in the rhizosphere
Summary
Plant growth-promoting rhizobacteria (PGPR) are a diverse group of microorganisms that are increasingly appreciated for their contributions to primary productivity through promotion of growth and triggering of induced systemic resistance (ISR) in plants. We will discuss these interactions in increasing order of complexity, starting with single introductions of P. fluorescens with introductions of multiple P. fluorescens isolates, with other PGPR, with other plant growth-promoting fungi, bacterivores, and with decomposing organisms.
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