Plants are constantly subjected to environmental changes that deeply affect their metabolism, leading to the inhibition or synthesis of “specialized” compounds, small organic molecules that play a fundamental role in adaptative responses. In this work, Melissa officinalis L. (an aromatic plant broadly cultivated due to the large amounts of secondary metabolites) plants were exposed to realistic ozone (O₃) dosages (80 ppb, 5 h day⁻¹) for 35 consecutive days with the aim to evaluate its potential use as elicitor of specialized metabolite production. Ozone induced stomatal dysfunction throughout the whole experiment, associated with a low photosynthetic performance, a decrease in the potential energy conversion activity of PSII, and an alteration in the total chlorophyll content (−35, −36, −10, and −17% as average compared to the controls, respectively). The production of hydrogen peroxide at 7 days from the beginning of exposure (+47%) resulted in lipid peroxidation and visible injuries. This result suggests metabolic disturbance within the cell and a concomitant alteration in cell homeostasis, probably due to a limited activation of antioxidative mechanisms. Moderate accumulated doses of O₃ triggered the accumulation of hydroxycinnamic acids and the up-regulation of the genes encoding enzymes involved in rosmarinic acid, phenylpropanoid, and flavonoid biosynthesis. While high accumulated doses of O₃ significantly enhanced the content of hydroxybenzoic acid and flavanone glycosides. Our study shows that the application of O₃ at the investigated concentration for a limited period (such as two/three weeks) may become a useful tool to stimulate bioactive compounds production in M. officinalis.

Plants constantly face the environment that surrounds them and fight for survival against biotic and abiotic stress factors. To deal with harmful conditions, plants have developed a multilayer defence system, making them capable of recognising threats and promptly recovering from them. This phenomenon, which takes advantage of the “memory effect”, is referred to as bio-priming and represents a new frontier in terms of crop protection. Here, we investigated the “indirect” protective mechanisms of a new yeast extract formulate in Vitis vinifera cv. Sangiovese plants at both the biochemical and genic levels. The formulate was applied once a week for three consecutive weeks, and grapevine leaves were sampled from the first to the fifth day after treatment (dat) at every week of the experiment. Increased levels of jasmonic acid (every week at 2 dat; +70% as average) and abscisic acid (at 1 dat of the first week, more than 1.7-fold higher than the control) and the underproduction of salicylic acid (from 2 dat; −18%) confirmed that these signalling molecules/”specialised compounds” are actively involved in the early activation of defence pathways in treated vines. In addition, pr2 and chit1b, two genes involved in regulating hormonal crosstalk, were significantly up-regulated (both in the first and second week of the trial) and were also found to underlie upstream molecular activation. The results obtained by this investigation confirm the use of this new product to prime and protect grapevines from a wide range of fungal and fungal-like plant pathogens through the induction of defence responses.

Plasmopara viticola (Pv), the causal agent of downy mildew, is one of the most damaging pathogens affecting grapevine. Current control strategies largely depend on copper-based fungicides and synthetic chemicals, raising increasing concerns related to environmental sustainability and pathogen resistance. This study evaluated the efficacy of a novel Saccharomyces cerevisiae extract (U) as an inducer of resistance in the grapevine–Pv interaction. Microscopic observations revealed the ability of U to inhibit Pv spread over the leaf. Additionally, biochemical and molecular responses were analyzed in grapevine leaves subjected to four treatments: plants treated only with water (U⁻/Pv⁻; i.e., control) or U (U⁺/Pv⁻), inoculated with Pv (U⁻/Pv⁺), or both treated with U and then inoculated with Pv (U⁺/Pv⁺). Fully expanded leaves were sampled at 2-, 5-, 24-, and 72-h post inoculation (hpi). In U⁺/Pv⁻ leaves, jasmonic, salicylic and abscisic acid (JA, SA, and ABA), as well as hydrogen peroxide (H₂O₂) increased at 2 hpi (+44, +33, +38%, and 3-fold, respectively), accompanied by upregulation of pr1 (2-fold higher than control, respectively), suggesting the capacity of U to trigger the plant alert system. In U⁻/Pv⁺ leaves, peaks of JA and H₂O₂ occurred at 24 hpi (+40% and 4-fold higher than control), followed by marked ethylene emissions and upregulation of pr1 and pr2 (i.e., genes associated with Pv defense; around 2-fold, averagely) at 72 hpi, confirming the progression of infection. In contrast, U⁺/Pv⁺ leaves showed stronger peaks of H₂O₂ at both 2 and 5 hpi (7-fold and +58%, respectively), together with SA accumulation and upregulation of pr1, pr2, eds1, and chit1b at 72 hpi (more than 2-fold), suggesting a priming effect of U. Overall, U effectively enhanced grapevine defense responses and limited Pv development, highlighting its potential as a sustainable disease management strategy.