Introduction
Induced resistance (IR) refers to a phenotypic state in which an exogenous stimulus conditions the plant for reduced susceptibility to future biotic challenges (De Kesel et al. 2021). IR stimuli include chemical compounds, beneficial microbes and various (a)biotic stresses (Conrath et al. 2006; Martinez-Medina et al.2016; Mauch-Mani, Baccelli, Luna & Flors 2017). Once triggered, IR is memorized through long-term epigenetic, proteomic and metabolic alterations (Balmer, Pastor, Gamir, Flors & Mauch-Mani 2015; Conrath, Beckers, Langenbach & Jaskiewicz 2015) and may last the plant’s entire life cycle (Conrath et al. 2015).
IR has been successfully used in crop protection (Walters & Fountaine 2009), and several IR-inducers are available commercially. Examples include beneficial microbes such as Trichoderma spp. (Perazzolli, Roatti, Bozza & Pertot 2011; Martínez-Medina et al. 2017), functional analogs of the defense hormone salicylic acid (SA) such as acibenzolar-S-methyl (Romero, Kousik & Ritchie 2001) or probenazole (Yoshioka, Nakashita, Klessig & Yamaguchi 2001), as well as oligosaccharide-based products such as COS-OGA, a combination of chito-oligosaccharides and oligogalacturonides (van Aubel, Buonatesta & Van Cutsem 2014; van Aubel, Cambier, Dieu & Van Cutsem 2016).
IR involves both direct activation of plant defense mechanisms and priming, enhanced induction of defense responses upon later challenge (Mauch-Mani et al. 2017; De Kesel et al. 2021). Examples of defense mechanisms involved in IR include accumulation of proteins with anti-pathogen activity (van Loon, Rep & Pieterse 2006), production of phytoalexins (Ahuja, Kissen & Bones 2012; Desmedt, Mangelinckx, Kyndt & Vanholme 2020) and cell wall reinforcement (Lunaet al. 2011; Malinovsky, Fangel & Willats 2014; Veronicoet al. 2018).
Both defense metabolite accumulation and cell wall reinforcement depend at least partially on the phenylpropanoid pathway (PPP) (Dixon et al. 2002; La Camera et al. 2004; Miedes, Vanholme, Boerjan & Molina 2014). In this pathway, phenylalanine is deaminated by PHENYLALANINE AMMONIA LYASE (PAL) to trans -cinnamic acid, which is para -hydroxylated by CINNAMIC ACID-4-HYDROXYLASE (C4H) topara -coumaric acid. Para -Coumaric acid is activated by 4-COUMAROYL-CoA-LIGASE to form para -coumaroyl-CoA, a reactive intermediary in the biosynthesis of lignin monomers and numerous other PPP-derived metabolites (Vogt 2010). Several phenylpropanoids are phytoanticipins or phytoalexins in their own right, as are numerous PPP-derived metabolites such as flavonoids, coumarins, stilbenoids and diarylheptanoids (Mathesius 2018; Desmedt et al. 2020). Finally, the PPP is also, together with the shikimate pathway, involved in SA biosynthesis (Dempsey, Vlot, Wildermuth & Klessig 2011).
Given the PPP’s diverse roles in plant defense, one might expect PPP inhibition to impair plant immunity. Chemical or genetic PAL inhibition is indeed associated with increased susceptibility in various pathosystems, and high basal or induced PAL activity is often positively correlated with resistance (Wang, Wang & Ning 2019; Yadav et al.2020). Downstream of the core PPP, stress-induced lignification and accumulation of PPP-derived phytoalexins are common resistance mechanisms (Dixon et al. 2002; Obermeier et al. 2013; Chezem, Memon, Li, Weng & Clay 2017; Veronico et al. 2018; Ranjan et al. 2019; Lee et al. 2019; Desmedt et al.2020). Accordingly, it seems logical that C4H inhibition would also reduce disease resistance. However, Schoch and colleagues hinted that the opposite might be true by showing that co-application of piperonylic acid (PA), a C4H inhibitor (Schalk et al. 1998), and β-megaspermin, a Phytophthora megasperma elicitor protein,to tobacco BY-2 cells dramatically increased SA accumulation – something not seen when PA or β-megaspermin were applied individually (Schoch, Nikov, Alworth & Werck-Reichhart 2002). This led the authors to suggest that SA accumulation after combined PA and β-megaspermin elicitation might induce SA-dependent defense responses (Schoch et al. 2002). However, the hypothesis that PA might induce plant resistance has, to the best of our knowledge, never been tested.
We investigated if and how PA induces plant resistance by performing bioassays with various pathogens and pests, transcriptome and metabolome analysis, RT-qPCR and biochemical assays. As a model plant, we focused on tomato, which is the world’s most widely consumed horticultural crop and is susceptible to numerous agriculturally important pests and diseases (Gould 1992; Blancard 2013).