Effects of (?)-dunnione and quinone-containing extracts from in vitro-cultured plantlets of Streptocarpus dunnii Hook. f. and a hybrid ?Ruby? on seed germination

Mature plants of S. dunnii Hook. f. produce naphthoquinones in aerial structures and anthraquinones in roots. Immature in vitro-cultured plantlets of Streptocarpus dunnii and S.× hybridus ‘Ruby’ (Gesneriaceae) excrete orange-red metabolites into their growth media (‘artificial’ rhizosphere). Our objectives were to: (i) extract and identify excreted compounds; (ii) determine the phytochemical effects of prolonged micropropagation and (iii) examine if excreted compounds affected seed germination. Results demonstrated that both Streptocarpus plantlets excreted complex mixtures of naphtho- and anthraquinones from around leaflet bases, petiolodes and fibrous roots. Chemical analyses showed (±)-dunnione as the main naphthoquinone, even after prolonged micropropagation. A TLC-based bioassay and a replicated, multifactorial, concentration-dependent experiment, found radish seed germination was inhibited in a concentration-dependent manner. ‘Low’ (±)-dunnione (25 μg) concentrations delayed germination for 2 days, with higher (125 μg) concentrations producing longer, but still transient, delays, i.e. not high enough to inhibit germination. (±)-Dunnione exerts its effects through redox cycling and ROS production. Since ROS production is essential for triggering germination, low concentrations were considered antioxidant reducing ROS production in seed coats and embryos thereby delaying germination, while higher concentrations were pro-oxidant, creating oxidative stress and poor seedling growth. Our results suggest that effects on seeds are biphasic and concentration-dependent. The significantly greater inhibitions in germination produced with whole medium extracts were attributed to higher (±)-dunnione levels in combination with other bioactive quinones. Since (±)-dunnione is also fungitoxic, we conclude that it acts an allelochemical protecting developing Streptocarpus plantlets from fungal attack and together with other excreted quinones can inhibit germination of seeds from competitor plants. © 2020 SAAB