On the effectiveness of ozone treatments: a silver bullet for plant health?

The development of innovative and eco-friendly strategies to protect plant health is one of the main challenges for the agricultural sector to respond to the increasing global food demand. In this contest, ozone (O3) could be a promising sustainable alternative to current pesticides, since it is a powerful oxidizing agent and does not leave residues in the environment. However, the molecular mechanisms involved in its potential bioactivity as a plant defense inducer are still poorly known. Thus, this study aimed to understand the role of O3 in plant defenses, as well as in plant growth, development and physiology, by a multidisciplinary approach. Here, O3 was applied as ozonated water to the soil in field experiments or to the nutrient solution of hydroponically grown plants. Experiments were carried out on different plant species, including the model plant Nicotiana tabacum and agronomically important crops such as lettuce, bean, and tomato. The analysis of several physiological parameters, such as plant weight, chlorophyll content, and stomatal conductance, indicated that O3 effects are species-specific. Moreover, the expression analysis of specific defense-related genes showed that O3 induced substantial changes in key hormonal and defense signaling pathways. Overall, O3 was demonstrated to trigger plant defenses, mainly mediated by pathogenesis-related proteins, mimicking a pathogen attack.