Pre-flowering UV-C treatment influences yield, bioactive compounds and sensory attributes by altering phenylpropanoid biosynthesis gene expression in tomato

The response of plants to Ultraviolet-C (UV-C), an emerging plant health and growth treatment, is dose-dependent, ranging from stimulation to inhibition. Previous studies on UV-C treatment in tomatoes primarily focused on green fruits and one-month-old seedlings, and has been successful in delaying ripening and reducing disease incidence. However, the UV-C-mediated alteration of desirable traits for the consumer, including fruit quality and nutritional content, sensory attributes such as flavour, and changes in bioactive compounds at the biochemical and molecular levels, have not been reported. Therefore, in this study, we systematically evaluated effects of a range of UV-C dosages at the pre-flowering stage (determinant stage for flower number) using a custom-made 'UV plant treatment apparatus' for two tomato cultivars (Red Rock and Super Star) of varying susceptibility level. We identified the optimal UV-C cumulative doses of 930 J/m2 for Red Rock (susceptible) and 1395 J/m2 for Super Star (comparatively tolerant), to significantly enhance flower number (1.65–1.67-fold), flower size (1.08–1.11-fold), fruit number (1.69–1.76-fold), fruit colour (1.08–1.20-fold) fruit weight (1.85–1.95-fold), and bioactive compounds, including total phenolic content (1.32–1.34-fold), total flavonoid content (1.34–1.44-fold), total anthocyanin (1.31–1.44-fold), ascorbic acid (1.29–1.40-fold), lycopene (2.54–2.62-fold), and total sugar (1.13–1.17-fold). The cytotoxicity analysis of UV-C treated plant fruit extract confirmed safety by revealing no negative effects on the normal human bronchial epithelial cell line (BEAS-2B) and inhibitory effect on the proliferation of lung cancer cell line (A549) by 1.29–1.35-fold because of enhanced bioactive compounds and lycopene content. Furthermore, UV-C treatment improved overall taste of fruit specifically acidic odour (1.31–1.37-fold), sweetness (1.78–1.80-fold), sourness (1.36–1.39-fold), and juiciness (1.43–1.59-fold). Analysis of genes associated with anthocyanin, phenolic and flavonoid compound production showed that treatment with the optimal UV-C dosage, resulted in the highest expression of 14 R2R3-SlMYB genes (SlMYB75, 114, 90, 113, 82, 23, 5, 66, 11, 7, 89, 12, 32, 111) and structural genes (SlFLS, SlCHS2, SlANS, SlPAL, SlCHS1); and lowest expression of 4 repressor R2R3-SlMYB genes (SlMYB3, 4, 6, 8). This study is the first approach to evaluate the effect of UV-C at a pre-flowering stage on alteration of bioactive compounds, sensory attributes and associated molecular changes in tomato flowers and fruit.