Introduction

Since several chemical pesticides have been shown to be detrimental on human health and ecosystems, considerable research has been done to find more environment-friendly plant protection solutions (Gay, 2012; Hernández et al. , 2013; Wang et al. , 2013; Carvalho, 2017). Elicitors, defined as molecules able to stimulate defence responses in a host plant, are one of the emerging alternatives (Paréet al. , 2005; Henry et al. , 2012; Thakur and Sohal, 2013). They can 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)) (Yu et al. , 2017; Malik et al. , 2020). For example, flagellin (flg22) or cryptogein are a bacterial and a fungal elicitor respectively (Gómez-Gómez and Boller, 2002; Gerbeau-Pissotet al. , 2014), or chemically synthesized molecules like rhamnolipids (RLs) or COS-OGA also act as elicitors (Clinckemaillieet al. , 2017; Luzuriaga-Loaiza et al. , 2018). They can hence be of different chemical natures like carbohydrate polymers, lipids, peptides or proteins (Boller and Felix, 2009; Thakur and Sohal, 2013; Jogaiah et al. , 2019; Pršić and Ongena, 2020). Recognition of elicitors by the plant cells first triggers early defence responses among which the release of reactive oxygen species (ROS) (superoxide anion (O2¯), hydrogen peroxide (H2O2) and hydroxyl radical (OH)), also known as the oxidative burst (Mittler, 2017; Camejo et al. , 2019; Wang et al. , 2019; Zaid and Wani, 2019). The perception of elicitors by plant cells further 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 most of elicitors are perceived by membrane proteic receptors, the involvement of the lipid part of the cell membrane has not been ruled out (Nimchuk et al. , 2003; Deleu et al. , 2018). Indeed, many danger signals or invasion patterns are recognized by specific pattern recognition receptors within PPM (Schellenberger et al. , 2019; Pršić and Ongena, 2020), and some amphiphilic elicitors, like surfactin fromBacillus and RLs from Pseudomonas , have been strongly suggested to be perceived by the lipid fraction of PPM (Henry et al. , 2011; Gerbeau-Pissot et al. , 2014; Haba et al. , 2014; Luzuriaga-Loaiza et al. , 2018; Monnier et al. , 2018; Herzog et al. , 2020; Come et al. , 2021). Vey recently, it was demonstrated that the RL-triggered immune response is affected by the sphingolipid composition of the plasma membrane and thus independent of a receptor (Schellenberger et al. , 2021).
Fatty acid hydroperoxides (HPOs) are amphiphilic molecules naturally produced by plants in response to (a)biotic stresses by the oxidative catabolism of polyunsaturated fatty acids. They belong to the large family of plant oxylipins (Blée, 2002; Wasternack and Feussner, 2018; Genva et al. , 2019). Oxylipins are ubiquitous in the plant kingdom and can either be esterified, notably in biological membranes, or be found in free form. Among them, the best known are the jasmonates, a family of molecules including jasmonic acid (JA), its derivatives and some JA precursors such as 12-oxo-phytodienoic acid (OPDA) and 12-dinor-oxo-phytodienoic acid (dnOPDA) (Wasternack and Strnad, 2018). JA is obtained from 13-hydroperoxy-9,11,15-octadecatrienoic acid (13-HPOT) following the successive action of various enzymes (lipoxygenase, etc.) (Deboever et al. , 2020a ). 13-HPOT and its dienoic equivalent, 13-hydroperoxy-9,11-octadecadienoic acid (13-HPOD), are key intermediates in the synthesis of jasmonates and other oxylipins, and have been extensively studied for their signalling properties (Deboever et al. , 2020b ). In the recent years, HPOs have emerged as a promising plant defence solution and their exogenous application to protect plants against phytopathogens has been considered (Granér et al. , 2003; Prostet al. , 2005; Deboever et al. , 2020a ). However, their biological activities in plant defence and hence their potential as elicitors still remain elusive. In addition, in our previous study (Deleu et al. , 2019), we have shown that HPOs are able to interact with PPM lipids inducing a perturbation of their lateral organization. We have suggested that, by this interaction, HPOs could activate cellular signaling involved in plant defense mechanisms.
In the present study, we first explored the potential of exogenously applied HPOs to protect Arabidopsis thaliana plants againstBotrytis cinerea by a systemic signalling mechanism. Their eliciting activity was evaluated by measuring ROS production byArabidopsis thaliana cells in their presence. In a second part, the molecular mechanism of HPO perception by the PPM was further investigated on plant biomimetic lipid systems by using a panel of complementary biophysical tools. More particularly, we analysed the effects of HPOs on the transversal organization and on the structure of the PPM bilayer.
The relationship between the results of biological assays and the effect of HPOs on model plant plasma membrane suggests that their action on the lipids might trigger early plant defence events.