Satureja montana L. essential oil and montmorillonite nanoclay modulate the phenylpropanoid pathway and polyphenols biosynthesis of tomato plants suffering from bacterial spot disease.

Main conclusion: Plant-derived GRAS Essential Oils (EOs) may be used to control plant diseases. X. euvesicatoria decreased flavonols and flavan-3-ols and their transcripts in tomato. S. montana EO downregulated the phenylpropanoid pathways' genes. Besides its antibacterial effect S. montana EO may act as defense elicitor in tomato. New efficient control methods are needed to control Xanthomonas spp. pathogens, including X. euvesicatoria (Xeu), the etiological agent of bacterial spot on tomato. Satureja montana essential oils (EOs) and nanoformulations have previously shown antibacterial activity and capacity to modulate plant hormone responses in Xeu-infected tomato plants. Still, their effects on plant key defense pathways, like the phenylpropanoid pathway, remain unknown. To assess the impact of these treatments on the phenylpropanoid pathway, uninfected and Xeu-infected tomato plants (var. cerasiforme) were treated with S. montana EO, alone or in a formulation with montmorillonite nanoclay (EO + NMT). The transcripts' levels of genes related to the phenylpropanoid pathway (c4h, hct, f5h, f3h, anr) and polyphenolics (caffeoyl-O-glucaric acid, caffeoyl quinic acid, quercetin-O-xyloside-O-rutinoside, rutin, and kaempferol rutinoside) were quantified by RT-qPCR and HPLC-DAD, respectively. EO/EO + NMT treatments significantly downregulated those genes in healthy plants. Also, the infection downregulated these pathways, which resulted in a reduction of the concentration of certain phenolics in the leaves. The EO application to infected plants shifted the levels of caffeoyl acids, which may be related to defense responses. This is the first work demonstrating that foliar applications of S. montana EOs can shift the plant production of phenolics. Our results also contribute to valorizing the use of S. montana EO as GRAS (Generally Regarded As Safe) biopesticides with dual roles (antimicrobial and host defense inducers).