Chemical warfare in the plant world: Does the synthesis and release of secondary negative allelopathic metabolites into the soil provide a competitive advantage to Amur honeysuckle (Lonicera maackii)?

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Csengele Barta, Alyka Zahnd, Lauren Tinoco, Amie Haddock, Jessica Poush, Harrison Meers, Mehreen Tai, Kaitlyn Atkins, Bryan Jenkins, Devon Lindstrom and Rachael Prawitz, Biology, Missouri Western State University, Saint Joseph, MO

Background/Question/Methods
Invasive species, employing a large array of strategies, through which they negatively impact the germination, growth, survival, or reproduction of native species, are a threat to biodiversity and a relevant concern for species conservation and restoration. Decomposing leaves of the invasive Amur honeysuckle (Lonicera maackii) leach a variety of allelopathic phenolic substances into the soil, with devastating inhibitory effects on native plant species in the region and affect multiple trophic levels. The seedling inhibitory mechanism of these phenolics has not yet been studied. We explored the inhibition mechanism triggered by honeysuckle leaf extracts in germinating standard (control, Rbr), gibberellin (GA) synthesis deficient (Rosette-Dwarf, ros) and GA overproducing (Tall, ein) mutants of field mustard (Brassica rapa L. var. rapa) in Petri dish seed and seedling development essays. The metabolite profile of the decomposing honeysuckle leaf extracts was analyzed using high performance liquid chromatography (HPLC).
Results/Conclusions
Extracts (0.01-0.2 g/mL) significantly decreased the germination of mustard seeds and inhibited seedling development in a concentration-dependent manner with most pronounced impacts on the ros mutants. The ein seeds were able to overcome inhibition, with only a delay in their germination, at the highest extract concentrations. We observed a similar relationship when control seeds were treated with exogenous GA (up to 100 µM), with the strength of inhibition decreasing with increasing GA amounts. We hypothesize, that the phenolic allelochemicals in the honeysuckle leaf extract alter the hormonal balance in germinating mustard seeds, effect that can be alleviated by higher than physiological GA concentrations. Metabolite analysis of the extract revealed a profile rich in phenolic molecules, dominated by apigenin and luteolin and their derivatives. Seed and seedling assays performed with synthetic pure apigenin and luteolin at physiologically relevant concentrations (0.002 mM for both) showed that while both cause inhibition in standard control seeds and their individual effects can be relieved by GA. Interestingly however, the molecules inhibit seed germination in a synergistic manner, with a 4-fold higher efficiency than expected from the sum of the individual compound’s effects. A better understanding of the inhibition mechanism by allelopathic agents is expected to contribute to the better understanding of plant-plant communication and competition and will aid the developing of efficient invasive species management approaches in the future.