# Low Temperature Plasma Strategies for
## 1 Introduction
In general ROS species triggers an oxidative stress response leading to detrimental oxidative cell damage [27; 28] and the inactivation of microorganisms [29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. Hydroxyl radicals (OH) have a direct impact on the cell membranes of microorganisms [40; 41]. Besides direct decontamination processes, plasma can also activate stress signalling cascades enabling the self-defence mechanism, particularly in plants. RONS produced in response to abiotic and biotic stresses act as signalling molecules [42], leading to the activation of defence genes [43; 44; 45; 46; 47; 48] and the production of defence compounds [49; 50]. Moreover, ROS can also make the plant readily available in a redox state to respond quickly to stress [51]. In recent years plasma-activated water (PAW) has attracted the attention of researchers as a possible activated media to be used in decontamination processes [52]. Many studies on PAW indicated that the main active substances of PAW are reactive oxygen species (ROS) and reactive nitrogen species (RNS) [53; 54]. The main components of ROS include hydroxyl radicals, hydrogen peroxide, singlet oxygen, superoxide anions, and ozone, whereas RNS mainly includes nitrate, nitrite, peroxynitrite, nitric oxide radical, ammonia and nitrogen [55]. Among them, the long-lived reactive species are hydrogen peroxide, nitrate and nitrite. The physical and chemical properties of water, such as conductivity, pH and redox potential are affected by plasma activation. The research paper on PAW as a control strategy for diseases and insect pests is sporadic and experimental. Two fields of research are followed: one regarding the direct decontamination by PAW and the other the activation of self-defence in the plant. PAW was found to reduce surface yeast [56] and inhibit the mycelium growth of _Fusarium granimearum_[57], to protect tomato plants against leaf spot disease caused by _Xanthomonas vesiatoria_[58]. Irrigation of seedlings and seeds with PAW resulted in better growth morphology, oxidation and defence gene expression [12; 59; 60]. In terms of pest control [22], PAW was found effective against _Planococcus citri_[61] revealing a high mortality rate in lab conditions. The mechanism leading to the killing of pathogenic bacteria on the plants' surface [62] can be related to RNS and ROS, which can enter bacterial cells through the immediate pores in the active transport cell membrane. The RNS and ROS can oxidize DNA, proteins and lipids in the cell, breaking DNA, degrading proteins and inducing lipid peroxidation thereby causing the contents to flow out of the bacteria and die [62], with ozone and peroxynitrite being the dominant species in the sterilization process [63; 64; 55; 65; 66]. Many researchers also speculate that acidity and active compounds are related to PAW. In an acidic environment, the RNS and ROS react with the lipids and carbohydrates of the DNA proteins in the cells, lowering the pH of the cells and causing physiological dysfunction and cell death [66]. In addition, the effects of PAW on biofilms have also been explored [67; 68; 69; 70]. The importance of biochemically reactive species formed in PAW for the destruction and degradation of the biofilm matrix and the release of resident microbial cells have been highlighted [71]. Moreover, as for Plasma, the PAW can serve as a resistance inducer for plants, triggering their defences against pests and diseases, strengthening the plant pathogen defence pathway through ROS mediated channels [72; 73]. In the present manuscript, LTP and PAW were used to evaluate their anti-microbial activities against the \"De Donno\" strain of _Xylella fastidiosa_ subspecies _pauca_ haplotype ST53 (XfDD). Due to the difficulties in experimenting in a not confined laboratory, our approach was to create a portable device to produce plasma through a Surface Dielectric Barrier Discharge (SDBD), using ambient air, with a measured relative humidity of about 40%. All the treatments were carried out in a laminar flow hood.