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.
Highlights
Some bacteria isolated from the root microbiome have been selected for their remarkable beneficial effect provided to their host plant and are referred to as plant growth-promoting rhizobacteria (PGPR) (Backer et al, 2018; Singh et al, 2019)
These PGPR favor plant growth notably by facilitating nutrient availability and modulating the host’s hormonal balance and display plant protective activity toward pathogen ingress. This biocontrol potential relies on several traits including the ability to efficiently compete for space and nutrients with pathogens, a strong direct antagonistic activity based on secretion of low-size antimicrobials or hydrolytic enzymes and the capacity to stimulate induced systemic resistance (ISR) (Pieterse et al, 2014; Köhl et al, 2019)
We provide an updated overview of those PGPR determinants responsible for ISR elicitation that are in most cases small-size compounds secreted by the colonizing bacteria even if some proteins isolated from Brevibacillus laterosporus, B. amyloliquefaciens, and Saccharothrix yanglingensis have been recently proposed as bacterial triggers (Table 1) (Wang et al, 2015, 2016; Zhang et al, 2018)
Summary
Some bacteria isolated from the root microbiome have been selected for their remarkable beneficial effect provided to their host plant and are referred to as plant growth-promoting rhizobacteria (PGPR) (Backer et al, 2018; Singh et al, 2019). When applied as pure compound on roots, in micromolar amounts (5–10 μM), surfactin demonstrated to induce ISR in bean, tomato, tobacco, against B. cinerea, in melon against Podosphaera fusca, and peanut, against Sclerotium rolfsii (Ongena et al, 2007; GarcíaGutiérrez et al, 2013; Cawoy et al, 2014; Rodríguez et al, 2018).
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