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).