Background: Melatonin (MEL) is a signaling molecule in plants that affects developmental processes during vegetative and reproductive growth. Investigations have proved that exogenously applied MEL also has the potential to improve seed germination and plant development.Methods: In the present study, seeds of stevia, a species with a very low germination rate, were germinated on an agar gel (AG) containing MEL at various concentrations (5, 20, 100, and 500 µM) in light. Seeds germinated on AG without MEL were used as controls. For the first 24 or 48 h of germination, the seeds were maintained in darkness as a pre-incubation step. Some seeds were not exposed to this pre-incubation step.Results: At concentrations of 20 and 5 µM, MEL significantly improved germination, but only in seeds pre-incubated in darkness for 24 h (p < 0.001). At concentrations of 100 and 500 µM, MEL had an inhibitory effect on germination, regardless of the pre-incubation time. Melatonin also affected plantlet properties. At a concentration of 20 µM, MEL increased plantlet fresh weight and leaf numbers. At a concentration of 5 µM, it promoted plantlet height. Regarding root development, the most favorable MEL concentration was 500 µM. Biochemical analysis revealed that MEL promoted higher pigment concentrations but hampered superoxide dismutase activity. On the other hand, the concentrations of sugars and phenolics, as well as the activities of catalase and peroxidase, increased at a MEL concentration of 500 µM.Discussion: The results suggest that MEL can improve germination of positively photoblastic stevia seeds and that it can play a role in plantlet development. However, the effects observed in the present study depended on the quantity of MEL that was applied.

The ester of caffeic acid with α-hydroxydihydrocaffeic acid, named rosmarinic acid (α-o-caffeoyl-3,4-dihydroxyphenyllactic acid; RA) can occur as oligomeric molecules, or in free, esterified, and glycosidic forms. Although it is commonly found among the members of the plants from the Lamiaceae (mints) and Boraginaceae (borages) families, only certain plant species produce a comparatively high concentration of RA. This valuable bioactive compound exhibits anti-cancer, anti-angiogenic, antioxidant, anti-inflammatory, antiviral, and antimicrobial properties, among others. As it is difficult to obtain high quantities of RA from natural sources, and since chemical manufacturing is costly and challenging, various biotechnological methods have recently been investigated to boost RA production. Plant cell tissue culture has been used to promote RA production in various plant species, particularly medicinal ones, with elicitation being the most commonly used technique. This review explores the main steps involved in RA biosynthesis in plants, including the molecular mechanisms and physiological alterations underlying its function, along with the primary mechanisms of RA accumulation in response to elicitation. Recent progress in synthetic biology-based RA synthesis, as well as metabolic engineering techniques to enhance the industrial production of this valuable bioactive constituent, are also discussed.

Functionality of the photosynthetic system under water stress is of major importance in drought tolerance. Oat (Avena sativa L.) doubled haploid (DH) lines obtained by pollination of F₁ oat crosses with maize were used to assess the differences in plant genotypic response to soil drought. The investigations were based on the measurements of gas exchange and chlorophyll a fluorescence kinetics. Drought was applied to 17-day-old seedlings by withholding water for 14 days and subsequent plant recovery. Non-stressed optimally watered plants served as controls. Yield components were determined when plants reached full maturity. It was shown differences among the oat lines with respect to drought stress susceptibility (SI) and stress tolerance index mean productivity and drought susceptibility index. Sensitivity to drought of individual DH lines was significantly different, as demonstrated by the correlation between drought susceptibility index and yield components, such as dry weight (GW) or grain number (GN) of the harvested plants. GW and GN were lower in drought-sensitive genotypes exposed to drought stress compared to those resistant to drought. The principal component analysis allow to separate three groups of lines differing in their sensitivity to drought stress and indicated that tolerance to drought in oat has a common genetic background.