Date Published: May 31, 2019
Publisher: Public Library of Science
Author(s): Set Madian Perez, Enrico Biondi, Romolo Laurita, Mariarita Proto, Fabio Sarti, Matteo Gherardi, Assunta Bertaccini, Vittorio Colombo, Zonghua Wang.
Plant bacterial diseases are routinely managed with scheduled treatments based on heavy metal compounds or on antibiotics; to reduce the negative environmental impact due to the use of such chemical compounds, as pollution or selection of antibiotic resistant pathogens, the integrated control management is required. In the frame of a sustainable agriculture the use of bacterial antagonists, biological agents, plant defence response elicitors or resistant host plant genotypes are the most effective approaches. In this work, cold atmospheric pressure plasma (CAP) was applied to sterile distilled water, inducing the production of a hydrogen peroxide, nitrite and nitrate, and a pH reduction. In particular, an atmospheric pressure dielectric barrier discharge (DBD) has been used to produce plasma activated water (PAW), that was firstly assayed in in vitro experiments and then in planta through application at the root apparatus of tomato plants, against Xanthomonas vesicatoria (Xv), the etiological agent of bacterial leaf spot. Moreover, the transcription abundance of five genes related to the plant defense was investigated in response to PAW treatment.
Bacterial leaf spot of tomato caused by Xanthomonas vesicatoria (Xv), is widely spread in all the areas where tomato is cultivated [1,2]. The lack of effective pesticides or resistant tomato cultivars for the management of this bacterial disease, stimulates the efforts to develop sustainable strategies in the framework of the integrated control management strategies [3,4]. The control programmes for bacterial diseases of tomato plants are mainly based on prophylaxis trough diagnostic analysis carried out on seeds, to detect the pathogen presence (latent infections) . Moreover, a certain degree of seed sanitation is achieved through the application of fermentation methods during the seed extraction from tomatoes and/or through physical disinfection procedures (i.e. heat treatments) directly applied to the seeds batches [6,7]. In glasshouse and open field, preventive treatments applied to the plants are based on copper compounds, antibiotics (where it is allowed) and antagonistic bacteria, that are able to reduce the pathogen population and avoid its penetration inside the plant. However, microbial resistance to copper/antibiotics poses a threat to the continued successful use of copper/antibiotics sprays for disease control [8–10]. Therefore, alternative sustainable control methods, as the use of nanoparticles or natural compounds directly active against Xv were studied and employed to control the infections [11–15]. Moreover, since the end of the nineties, the efficacy and the mechanism of action of resistance inducer molecules, acting against the pathogens through responses mediated by the host, were investigated as an alternative to the use of copper/antibiotics compounds . Among all known resistance inducers, those based on the active principle acibenzolar-S-methyl (ASM), a benzo-thiadiazole (BTH) derivative, were found effective against bacterial plant diseases by strengthening the physical barriers, the pre-infectional defenses (e.g. lignins and callose production), and by producing several compounds, directly active against the pathogens (post-infectional defenses). ASM, in fact, mimics the role of salycilic acid (SA), the natural plant activator of systemic acquired resistance (SAR), which culminates in the expression of pathogenesis related proteins (PRs), in particular the PR1a is also kept as SAR marker ; through synergistic cross-talking pathways, ASM is also able to elicit the productions of PRs and compounds, dependant to the jasmonic acid/ethylene (JA/ET) pathway, which brings to induced systemic resistance (ISR) .
The plasma treatment of SDW induced the production of NO2-, NO3- and H2O2. As reported by Laurita et al. , NO2- completely disappeared few minutes after the plasma treatment, due to its reaction with H2O2 in acidic liquids. One hour after treatment, H2O2 and NO3-concentrations (approx. 20 mg/L and 120 mg/L, respectively) in PAW resulted stable for at least 2 hours at room temperature (Table 3); so the delay time between the production and the PAW treatment of plants was mantained between 1 h and 3 h in order to have stable concentrations of H2O2 and NO3-.
PAW did not show antimicrobial activity against Xv in the in vitro experiments using the diffusion and dilution methods.