Seed disinfection effect of atmospheric pressure plasma and low pressure plasma on Rhizoctonia solani

Inactivation efficacy of low-pressure plasma treatment against seed-borne tomato pathogen Clavibacter michiganensis and effect of seed setting position and mesh sheet usage

Clavibacter michiganensis, a gram-positive actinomycete, is a major seed-borne tomato pathogen. We investigated the inactivation efficacy of low-pressure plasma treatment against C. michiganensis inoculated on tomato seeds by placing them on a mesh sheet above the bottom dielectric glass plate. The 2- and 5-minute plasma treatment reduced C. michiganensis populations on the tomato seeds by 0.8 and 1.8 log cfu/seed, respectively. The reduction rates were similar to those of C. michiganensis on shirona (cruciferous) seeds, which have different shapes and surface structures. In contrast, the inactivation of C. michiganensis cells using plasma was more difficult than that of X. campestris cells. Additionally, it was found that placing seeds on a mesh sheet laid on the dielectric glass plate was remarkably effective in inactivating the pathogens on tomato seeds. Since the tomato seeds were susceptible to damage from plasma treatment, methods to reduce its damage need to be investigated.

Applications of Plasma Produced with Electrical Discharges in Gases for Agriculture and Biomedicine

The use of thermal and non-thermal atmospheric pressure plasma to solve problems related to agriculture and biomedicine is the focus of this paper. Plasma in thermal equilibrium is used where heat is required. In agriculture, it is used to treat soil and land contaminated by the products of biomass, plastics, post-hospital and pharmaceutical waste combustion, and also by ecological phenomena that have recently been observed, such as droughts, floods and storms, leading to environmental pollution. In biomedical applications, thermal plasma is used in so-called indirect living tissue treatment. The sources of thermal plasma are arcs, plasma torches and microwave plasma reactors. In turn, atmospheric pressure cold (non-thermal) plasma is applied in agriculture and biomedicine where heat adversely affects technological processes. The thermodynamic imbalance of cold plasma makes it suitable for organic syntheses due its low power requirements and the possibility of conducting chemical reactions in gas at relatively low and close to ambient temperatures. It is also suitable in the treatment of living tissues and sterilisation of medical instruments made of materials that are non-resistant to high temperatures. Non-thermal and non-equilibrium discharges at atmospheric pressure that include dielectric barrier discharges (DBDs) and atmospheric pressure plasma jets (APPJs), as well as gliding arc (GAD), can be the source of cold plasma. This paper presents an overview of agriculture and soil protection problems and biomedical and health protection problems that can be solved with the aid of plasma produced with electrical discharges. In particular, agricultural processes related to water, sewage purification with ozone and with advanced oxidation processes, as well as those related to contaminated soil treatment and pest control, are presented. Among the biomedical applications of cold plasma, its antibacterial activity, wound healing, cancer treatment and dental problems are briefly discussed.

Biochemical and Physiological Plant Processes Affected by Seed Treatment with Non-Thermal Plasma

Among the innovative technologies being elaborated for sustainable agriculture, one of the most rapidly developing fields relies on the positive effects of non-thermal plasma (NTP) treatment on the agronomic performance of plants. A large number of recent publications have indicated that NTP effects are far more persistent and complex than it was supposed before. Knowledge of the molecular basis and the resulting outcomes of seed treatment with NTP is rapidly accumulating and requires to be analyzed and presented in a systematic way. This review focuses on the biochemical and physiological processes in seeds and plants affected by seed treatment with NTP and the resulting impact on plant metabolism, growth, adaptability and productivity. Wide-scale changes evolving at the epigenomic, transcriptomic, proteomic and metabolic levels are triggered by seed irradiation with NTP and contribute to changes in germination, early seedling growth, phytohormone amounts, metabolic and defense enzyme activity, secondary metabolism, photosynthesis, adaptability to biotic and abiotic stress, microbiome composition, and increased plant fitness, productivity and growth on a longer time scale. This review highlights the importance of these novel findings, as well as unresolved issues that remain to be investigated.

Application of Non-Thermal Plasma to Fungal Resources

In addition to being key pathogens in plants, animals, and humans, fungi are also valuable resources in agriculture, food, medicine, industry, and the environment. The elimination of pathogenic fungi and the functional enhancement of beneficial fungi have been the major topics investigated by researchers. Non-thermal plasma (NTP) is a potential tool to inactivate pathogenic and food-spoiling fungi and functionally enhance beneficial fungi. In this review, we summarize and discuss research performed over the last decade on the use of NTP to treat both harmful and beneficial yeast- and filamentous-type fungi. NTP can efficiently inactivate fungal spores and eliminate fungal contaminants from seeds, fresh agricultural produce, food, and human skin. Studies have also demonstrated that NTP can improve the production of valuable enzymes and metabolites in fungi. Further studies are still needed to establish NTP as a method that can be used as an alternative to the conventional methods of fungal inactivation and activation.

Implementation of a Non-Thermal Atmospheric Pressure Plasma for Eradication of Plant Pathogens from a Surface of Economically Important Seeds

Plant pathogenic bacteria cause significant economic losses in the global food production sector. To secure an adequate amount of high-quality nutrition for the growing human population, novel approaches need to be undertaken to combat plant disease-causing agents. As the currently available methods to eliminate bacterial phytopathogens are scarce, we evaluated the effectiveness and mechanism of action of a non-thermal atmospheric pressure plasma (NTAPP). It was ignited from a dielectric barrier discharge (DBD) operation in a plasma pencil, and applied for the first time for eradication of Dickeya and Pectobacterium spp., inoculated either on glass spheres or mung bean seeds. Furthermore, the impact of the DBD exposure on mung bean seeds germination and seedlings growth was estimated. The observed bacterial inactivation rates exceeded 3.07 logs. The two-minute DBD exposure stimulated by 3-4% the germination rate of mung bean seeds and by 13.4% subsequent early growth of the seedlings. On the contrary, a detrimental action of the four-minute DBD subjection on seed germination and early growth of the sprouts was noted shortly after the treatment. However, this effect was no longer observed or reduced to 9.7% after the 96 h incubation period. Due to the application of optical emission spectrometry (OES), transmission electron microscopy (TEM), and confocal laser scanning microscopy (CLSM), we found that the generated reactive oxygen and nitrogen species (RONS), i.e., N2, N2+, NO, OH, NH, and O, probably led to the denaturation and aggregation of DNA, proteins, and ribosomes. Furthermore, the cellular membrane disrupted, leading to an outflow of the cytoplasm from the DBD-exposed cells. This study suggests the potential applicability of NTAPPs as eco-friendly and innovative plant protection methods.

Hyperosmotic stress-induced somatic embryogenesis and its continuous culture in Japanese honewort (Cryptotaenia japonica)

Japanese honewort (Cryptotaenia japonica) is consumed as a traditional vegetable and has medicinal applications. In Japan, C. japonica is mainly produced using hydroponic culture systems; however, damping-off is often caused by the adherence of pathogens to its seeds. Therefore, the use of sterile artificial seeds in hydroponic culture is likely to be effective for preventing disease. In this study, we established methods for stress-induced somatic embryogenesis and artificial seed production in Japanese honewort. Shoot apex explants from seedlings were treated with 0.7 M sucrose as a hyperosmotic stress for 3 or 6 weeks, and then transferred to stress-free conditions. Somatic embryos were formed after culture in stress-free conditions for 7 weeks. Stress-treated shoot apex explants that formed somatic embryos were cultured in Murashige and Skoog liquid medium with shaking. After 2 weeks of culture, approximately 800 somatic embryos were formed from each explant. Somatic embryos were formed continuously during 37 weeks under the same culture conditions. Thus, somatic embryogenesis was effectively induced in Japanese honewort via hyperosmotic stress, and embryogenic competence was maintained under stress- and phytohormone-free conditions. The somatic embryos produced by liquid culture were used to produce artificial seeds by enveloping the embryos in whipped alginate gel to avoid hypoxic conditions. The artificial seeds had a high germination rate (72%). This system is suitable for the sterile, highly productive hydroponic culture of Japanese honewort.

Effects of Atmospheric Plasma Corona Discharges on Soil Bacteria Viability

Crop contamination by soil-borne pathogenic microorganisms often leads to serious infection outbreaks. Plant protection requires disinfection of agricultural lands. The chemical and the physical disinfection procedures have several disadvantages, including an irreversible change in the soil ecosystem. Plasma, the "fourth state of matter" is defined as an ionized gas containing an equal number of negatively and positively charged particles. Cold-plasma technology with air or oxygen as the working gas generates reactive oxygen species, which are found to efficiently eradicate bacteria. In this study, we examined the effect of atmospheric plasma corona discharges on soil bacteria viability. Soil that was exposed to plasma for 60 s resulted in bacterial reduction by two orders of magnitude, from 1.1 × 10⁵ to 2.3 × 10³ cells g^-1 soil. Exposure for a longer period of 5 min did not lead to further significant reduction in bacterial concentration (a final reduction of only 2.5 orders of magnitude). The bacterial viability was evaluated using a colorimetric assay based on the bacterial hydrogenases immediately after exposure and at selected times during 24 h. The result showed no recovery in the bacterial viability. Plasma discharged directly on bacteria that were isolated from the soil resulted in a reduction by four orders of magnitude in the bacterial concentration compared to untreated isolated bacteria: 2.6 × 10^-3 and 1.7 × 10^-7, respectively. The plasma-resistant bacteria were found to be related to the taxonomic phylum Firmicutes (98.5%) and comprised the taxonomic orders Bacillales (95%) and Clostridiales (2%). To our knowledge, this is the first study of soil bacteria eradication using plasma corona discharges.

Plant Disease Control by Non-Thermal Atmospheric-Pressure Plasma

Disease stresses caused by pathogenic microorganisms are increasing, probably because of global warming. Conventional technologies for plant disease control have often revealed their limitations in efficiency, environmental safety, and economic costs. There is high demand for improvements in efficiency and safety. Non-thermal atmospheric-pressure plasma has demonstrated its potential as an alternative tool for efficient and environmentally safe control of plant pathogenic microorganisms in many studies, which are overviewed in this review. Efficient inactivation of phytopathogenic bacterial and fungal cells by various plasma sources under laboratory conditions has been frequently reported. In addition, plasma-treated water shows antimicrobial activity. Plasma and plasma-treated water exhibit a broad spectrum of efficiency in the decontamination and disinfection of plants, fruits, and seeds, indicating that the outcomes of plasma treatment can be significantly influenced by the microenvironments between plasma and plant tissues, such as the surface structures and properties, antioxidant systems, and surface chemistry of plants. More intense studies are required on the efficiency of decontamination and disinfection and underlying mechanisms. Recently, the induction of plant tolerance or resistance to pathogens by plasma (so-called "plasma vaccination") is emerging as a new area of study, with active research ongoing in this field.

Atmospheric Pressure Pulsed Plasma Induces Cell Death in Photosynthetic Organs via Intracellularly Generated ROS

The toxicity of atmospheric-pressure pulsed plasma on plant leaf tissues is studied. A nanosecond-pulsed plasma jet is applied to Arabidopsis thaliana leaves. In case of cotyledon, cell death is induced by treatment of only a few seconds. Cell death is also induced in the adult leaf by only 5 seconds of plasma treatment. Plasma induced reactive oxygen species (ROS) accumulation across the tissues within plasma-treated area. Plasma also induced direct physical damage to epidermis tissue of treated area but merely no damage to mesophyll. Thus, we propose direct physical damage in epidermis and ROS accumulation across the treated area induced cell death by plasma treatment. Plasma treatment with same duration in different organ also induced ROS accumulation but not plant death, suggests damage on photosynthetic organ by oxidative stress might be direct reason to induce cell death. We could also observe similar plasma induced death in Solanum esculentum, Petunia axillaris, and Nicotiana benthamiana but death is induced only in treated area. Thus, we propose atmospheric plasma induce oxidative stress in photosynthetic organ to induce cell death in plants.

Bacterial and Fungal Counts of Dried and Semi-Dried Foods Collected from Dhaka, Bangladesh, and Their Reduction Methods

　Food is a basic necessity for human survival, but it is still the vehicle for the transmission of food borne disease. Various studies have examined the roles of spices, herbs, nuts, and semi-dried fruits, making the need for safe and convenient methods of decontamination a necessity. The current study determined the bacterial and fungal loads of 26 spices and herbs, 5 nuts, 10 semi-dried fruits and 5 other foods. Spices, herbs and semi-dried foods demonstrated the highest bacterial and fungal loads with the majority showing over 10⁴ CFU/mL. Nuts and other foods showed growths ranging from 10² to 10⁶ CFU/mL. The current study also attempted to determine the effects of heat and plasma treatment. The log reduction of bacterial growth after heat treatment (maximum: 120 min for 60℃) was between 0.08 to 4.47, and the log reduction after plasma treatment (maximum: 40 min) ranged from 2.37 to 5.75. Spices showed the lowest rates of reduction, whereas the semi-dried and other foods showed moderate to high levels of decrease after heat treatment. The log reduction of fungal growth after heat treatment ranged from 0.27 to 4.40, and log reduction after plasma treatment ranged from 2.15 to 5.91．Furthermore, we validated the sterilization effect of plasma treatment against Bacillus spp. and Staphylococcus spp. by using scanning electron microscopy. Both treatment methods could prove to be advantageous in the agriculture related fields, enhancing the quality of the foods.

Low-Pressure Plasma Application for the Inactivation of the Seed-borne Pathogen Xanthomonas campestris

The aim of this study was to investigate the effect of low-pressure plasma treatment on seed disinfection and the possible mechanisms underlying this effect. Seed-borne disease refers to plant diseases that are transmitted by seeds; seed disinfection is an important technique for prevention of such diseases. In this study, the effectiveness of low-pressure plasma treatment in the inactivation of the seed-borne plant pathogenic bacterium, Xanthomonas campestris, inoculated on cruciferous seeds, was evaluated. The highest inactivation effect was observed when the treatment voltage and argon gas flow rate were 5.5 kV and 0.5 L/min, respectively. The viable cell number of X. campestris was 6.6 log cfu/seed before plasma treatment, and decreased by 3.9 log after 5 min of treatment and by 6.6 log after 40 min. Ethidium monoazide treatment and quantitative real-time PCR results indicated that both the cell membrane and target DNA region were damaged following 5 min of plasma treatment. Although both heat and ozone were generated during the plasma treatment, the contribution of both factors to the inactivation of X. campestris was small by itself in our low-pressure plasma system. Overall, we have shown that our low-pressure plasma system has great applicability to controlling plant pathogenic bacterium contamination of seeds.