Elicitors of Plant Immunity Triggered by Beneficial Bacteria

Abstract

The molecular basis of plant immunity triggered by microbial pathogens is being well-characterized as a complex sequential process leading to the activation of defense responses at the infection site, but which may also be systemically expressed in all organs, a phenomenon also known as systemic acquired resistance (SAR). Some plant-associated and beneficial bacteria are also able to stimulate their host to mount defenses against pathogen ingress via the phenotypically similar, induced systemic resistance phenomenon. Induced systemic resistance resembles SAR considering its mechanistic principle as it successively involves recognition at the plant cell surface, stimulation of early cellular immune-related events, systemic signaling via a fine-tuned hormonal cross-talk and activation of defense mechanisms. It thus represents an indirect but efficient mechanism by which beneficial bacteria with biocontrol potential improve the capacity of plants to restrict pathogen invasion. However, according to our current vision, induced systemic resistance is specific considering some molecular aspects underpinning these different steps. Here we overview the chemical diversity of compounds that have been identified as induced systemic resistance elicitors and thereby illustrating the diversity of plants species that are responsive as well as the range of pathogens that can be controlled via this phenomenon. We also point out the need for further investigations allowing better understanding how these elicitors are sensed by the host and the diversity and nature of the stimulated defense mechanisms.

Plant–Microbiome Crosstalk: Dawning from Composition and Assembly of Microbial Community to Improvement of Disease Resilience in Plants

“…Rhizobacteria conferred ISR is one form of the inducible immunity in plants, and the molecular mechanisms in ISR are conserved in different plant species [167]. Some of the components present on the cell surface of biocontrol bacteria, i.e., flagella or polysaccharide [168], can trigger ISR in plants [169]. The ISR response in plants can also be triggered when they come into direct contact with compounds secreted by beneficial bacteria, e.g., volatile 2,3-butanediol [170], DAPG, and cyclic lipopeptide surfactants [171].…”

Surfactin stimulated by pectin molecular patterns and root exudates acts as a key driver of Bacillus-plant mutualistic interaction

“…This multifunctional compound is of particular interest because it retains important roles in key developmental processes such as bacterial motility, biofilm formation and root colonization (16)(17)(18), but also because it represents the best described Bacillus triggers for plant immunity (6,8). The potential of surfactin to stimulate ISR has been demonstrated on various plants including Solanaceae like tobacco and tomato on which it acts as main if not sole elicitor formed by B. subtilis and B. velezensis species (10,19). In support to its key role in interaction with the host plant, we also previously reported that surfactin is promptly formed in the course of early colonization and that its production is stimulated upon sensing root tissues (20).…”

Exogenous Fatty Acid Hydroperoxide Perception as Elicitor Is Related to Modulation of Plant Plasma Membrane Structure

“…They can have a biological origin, be derived from plants (termed as Damage-Associated Molecular Patterns (DAMPs)) or microorganisms (referred as Microbe-Associated Molecular Patterns (MAMPs) or Pathogen-Associated Molecular Patterns (PAMPs)) (Malik et al, 2020;Yu et al, 2017) or even be chemically synthesized (Luzuriaga-Loaiza et al, 2018). They can be of different chemical natures like carbohydrate polymers, lipids, peptides and proteins (Boller and Felix, 2009;Jogaiah et al, 2019;Pršić and Ongena, 2020;Thakur and Sohal, 2013). Their perception by plant cells results in protection based on the activation of signalling cascades and defence mechanisms leading to the induction of plant immunity, like the systemic acquired resistance (SAR) and the induced systemic resistance (ISR) (Malik et al, 2020;Pršić and Ongena, 2020).…”

“…They can be of different chemical natures like carbohydrate polymers, lipids, peptides and proteins (Boller and Felix, 2009;Jogaiah et al, 2019;Pršić and Ongena, 2020;Thakur and Sohal, 2013). Their perception by plant cells results in protection based on the activation of signalling cascades and defence mechanisms leading to the induction of plant immunity, like the systemic acquired resistance (SAR) and the induced systemic resistance (ISR) (Malik et al, 2020;Pršić and Ongena, 2020).…”

“…In addition to be a selective barrier between the cell and the extracellular medium, the plasma membrane is a sensor for modification of cellular environment and plays thus a key role in the recognition process of bioactive molecules. While many danger signals or invasion patterns are recognized by specific pattern recognition receptors within PPM (Pršić and Ongena, 2020;Schellenberger et al, 2019), some amphiphilic elicitors, like surfactin from Bacillus and rhamnolipids from Pseudomonas , have been strongly suggested to be perceived by the lipid fraction of PPM (Gerbeau-Pissot et al, 2014;Henry et al, 2011;Luzuriaga-Loaiza et al, 2018;Monnier et al, 2018). Recognition of elicitors by the plant cells first triggers early defence responses among which the release of reactive oxygen species (ROS) (superoxide anion (O 2 * ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical ( * OH)), also known as the oxidative burst (Camejo et al, 2019;Mittler, 2017;Wang et al, 2019;Zaid and Wani, 2019).…”