Defense response induced by essential oils in tomato plants against Xanthomonas vesicatoria

The control of Xanthomonas vesicatoria (Xv), one of the aetiological agents of tomato bacterial leafspot, is limited to copper compounds, few resistance inducers and biocontrol agents (BCA). Moreover, the main prophylactic strategy still consists of diagnostic analyses of seeds. Among BCA, natural compounds as essential oils (EO) and plant extracts were studied to find out eco-friendly tools to counteract phytopatogenic bacteria. An essential oil from Origanum compactum (OR; 48.13% carvacrol) and an hydrolate (HyAA; 47.7% linalool) obtained from EO of Citrus aurantium var. amara were assayed for their ability to induce resistance against Xv. The root apparatuses of Solanum lycopersicum cv. VF10 plants were treated with OR (0.03%) or HyAA (4.5%) by 10 minutes soaking and, after 48h, were inoculated with the Xv strain DISTAL 2684 (approx. 107 CFU/ml) by leaf spraying. Acibenzolar-S-methyl (75 ppm) and sterile distilled water (SDW) were used as controls. Neither the treatment with OR nor that with HyAA were phytotoxic.The disease severity (DS) was evaluated by counting the number of leaf spots 14 days after Xv inoculation. In the plants treated with both OR and HyAA (approx. 57 spots/leaf), the DS resulted lower compared to the SDW-treated control samples (approx. 98 spots/leaf). It is likely that the root treatment with the OR or HyAA might have stimulated the host defense responses against Xv since a relative protection of about 42% was detected. Further transcriptomic analyses will be undertaken to support the hypothesis of the activation of the defense responses of the plant.