Bacillus subtilis and Pseudomonas fluorescens Trigger Common and Distinct Systemic Immune Responses in Arabidopsis thaliana Depending on the Pathogen Lifestyle.

Aziz Aziz,Patricia Trotel-Aziz,Ngoc Huu Nguyen,Adrian Schwarzenberg,Eric Nguema-Ona,Fanja Rabenoelina,Fabienne Baillieul,Christophe Clément,Sandra Villaume

doi:10.3390/vaccines8030503

Abstract

Plants harbor various beneficial bacteria that modulate their innate immunity, resulting in induced systemic resistance (ISR) against various pathogens. However, the immune mechanisms underlying ISR triggered by Bacillus spp. and Pseudomonas spp. against pathogens with different lifestyles are not yet clearly elucidated. Here, we show that root drenching of Arabidopsis plants with Pseudomonas fluorescens PTA-CT2 and Bacillus subtilis PTA-271 can induce ISR against the necrotrophic fungus B. cinerea and the hemibiotrophic bacterium Pseudomonas syringae Pst DC3000. In the absence of pathogen infection, both beneficial bacteria do not induce any consistent change in systemic immune responses. However, ISR relies on priming faster and robust expression of marker genes for the salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) signaling pathways upon pathogen challenge. These responses are also associated with increased levels of SA, JA, and abscisic acid (ABA) in the leaves of bacterized plants after infection. The functional study also points at priming of the JA/ET and NPR1-dependent defenses as prioritized immune pathways in ISR induced by both beneficial bacteria against B. cinerea. However, B. subtilis-triggered ISR against Pst DC3000 is dependent on SA, JA/ET, and NPR1 pathways, whereas P. fluorescens-induced ISR requires JA/ET and NPR1 signaling pathways. The use of ABA-insensitive mutants also pointed out the crucial role of ABA signaling, but not ABA concentration, along with JA/ET signaling in primed systemic immunity by beneficial bacteria against Pst DC3000, but not against B. cinerea. These results clearly indicate that ISR is linked to priming plants for enhanced common and distinct immune pathways depending on the beneficial strain and the pathogen lifestyle.

Highlights

In hostile habitats, plants are subjects to various pathogens, including necrotrophs, biotrophs, and hemibiotrophs [1]
We examined the differences and similarities of B. subtilis and P. fluorescens-induced priming immune response and analyzed signaling pathways involved in induced systemic resistance (ISR) against pathogen infection
We especially focused on the expression of defense-related genes, phytohormone amounts, and the roles of salicylic acid (SA), jasmonic acid (JA)/ET, NPR1, and abscisic acid (ABA) signaling pathways in P. fluorescens- and B. subtilis-mediated ISR

Summary

Introduction

Plants are subjects to various pathogens, including necrotrophs, biotrophs, and hemibiotrophs [1]. To restrict the pathogen infection, plants rapidly activate different layers of defenses depending on its own ability to recognize conserved microbial molecules that are characteristic of microorganisms [2,3]. The recognition of these microbial molecules known as “microbe-associated molecular patterns” (MAMPs) is mediated by a set of receptors referred to as pattern-recognition receptors (PRRs). Once pathogen or non-pathogen microbes are perceived by the plant, immune responses are often triggered in distal plant parts to protect undamaged tissues against subsequent pathogen infection This long-lasting induced immunity is referred to as systemic acquired resistance (SAR) or induced systemic resistance (ISR) [4,5]. Accumulating evidence has shown that treatment of plant roots with beneficial bacteria enhance plant health through ISR [5], that mostly linked to a stronger and faster systemic immune response after pathogen infection, a phenomenon known as priming that allows plants to alert their immune system, reducing their energy consumption [6,7]

Methods

Results

Conclusion