Does the capacity to emit isoprene provide protection to plants in fending off invasives releasing allelopathic toxins into the soil?

Background/Question/Methods

: Invasive plant species are a threat to biodiversity and of concern for species conservation. The competitive success of many invasives is attributed to the synthesis of allelopathic metabolites, toxic to native species. Our previous findings evidenced that phytotoxins released from decomposing Amur honeysuckle (Lonicera maackii) leaves inhibit the germination and root growth in native species’ seedlings. Nevertheless, some native species have been shown to be more resistant to invasive species’ phytotoxins. These intricate native-invasive species interactions are scarcely understood. This work’s aim was to determine whether species with the ability to emit isoprene, a small volatile molecule responsible for many protective plant mechanisms, are less susceptible to allelopathic inhibition by honeysuckle phytochemicals than other, sensitive species. We used a transgenic Arabidopsis model, transformed with the kudzu isoprene synthase to emit isoprene. Isoprene emitting and non-emitting seedlings were grown in growth chambers at 18/16°C and 16h/8h day/night cycles, at 200 µmolm-2s-1 light intensity in an agar-containing medium, supplied with nutrients and vitamins. Honeysuckle leaf extract at 1 g/mL concentration was applied at the dry seed stage and seedling stage. Germination and survival rates, growth, and morphology were assessed twice a week, for four weeks, in treated and control plants.

Results/Conclusions

: Plants transformed to emit isoprene were less vulnerable to honeysuckle phytotoxins than non-emitters. Of the treated seeds, 10% of the non-emitter line and 72% of the transformed seed-line germinated, compared to the 95% germination of respective untreated controls. Among plantlets treated at the seedling stage, non-emitters’ root and shoot growth decreased by 53% and 70%, respectively, compared to controls. Isoprene emitters’ root and shoot growth decreased by only 12% and 15%, respectively. Roots of plants treated with honeysuckle extract developed a denser and 25% higher root hair-covered surface area than untreated controls, with a more pronounced effect in isoprene emitters. Prior data suggest isoprene to have a complex role in plants, beyond its stress-protective function, potentially acting similar to phytohormones, regulating gene expression and developmental patterns in emitters. We attribute the detected enhanced allelochemical resistance in isoprene emitters to be a consequence of such developmental control. Our data support the hypothesis that species with the capacity to emit isoprene may be more resistant to negative allelochemical control by invasives. Overall, a better understanding of chemical interactions between plants is expected to be the basis of developing approaches for long-term biological control of invasive species and plant conservation.