Specialized naphthoquinones present in Impatiens glandulifera nectaries inhibit the growth of fungal nectar microbes

Authors: Block, Anna; YAKUBOVA, ELENA - Purdue University; WIDHALM, JOSHUA - Purdue University

Submitted to: Plant Direct
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2019
Publication Date: 5/13/2019
Citation: Block, A.K., Yakubova, E., Widhalm, J.R. 2019. Specialized naphthoquinones present in Impatiens glandulifera nectaries inhibit the growth of fungal nectar microbes. Plant Direct. 3(5):1-7. https://doi.org/10.1002/pld3.132.
DOI: https://doi.org/10.1002/pld3.132

Interpretive Summary: The success of the invasive plant Himalaya balsam in certain parts of Europe and North America has been partially attributed to its ability to compete for bee pollinators with its rich nectar and to its production of specialized naphthoquinone compounds that act as natural herbicides to kill off its competitors. In this study Gainesville-ARS scientists and researchers from Purdue University investigate the antimicrobial activity and the sites of production of these specialized compounds. This study revealed that these compounds have antimicrobial activity against fungi but not bacteria found in nectar and therefore they may function in regulating the nectar microbiome to aid in plant health or pollinator attraction. Furthermore the researchers discovered that high levels of these compounds were found in nectaries on the stems of the plant than those in the flower. This leads to the hypothesis that release of these compounds onto the stems of the plant may aid their dispersal into the surrounding environment, i.e. by rain, where there herbicidal activities would function. Overall this research increases the knowledge about these versatile compounds that could potentially be commandeered for use in agricultural systems.

Technical Abstract: The invasion success of Impatiens glandulifera (Himalaya balsam) in certain parts of Europe and North America has been partially attributed to its ability to compete for bee pollinators with its rich nectar and to the production and release of allelopathic 1,4-naphthoquinones (1,4-NQs) from its roots and leaves. In this study, we investigated if these allelopathic 1,4-NQs, 2-methoxy-1,4-naphthoquinone (2-MNQ) and lawsone, are present in the nectaries of I. glandulifera. Both 2-MNQ and lawsone were detected in the floral nectaries of I. glandulifera at levels comparable to leaves and roots but were discovered to be at levels more than 50-fold higher in its extra-floral nectaries (EFNs). Though nectar microbe inhibition assays revealed that the common nectar bacteria Gluconobacter oxydans and Asaia prunellae are not inhibited by 2-MNQ or lawsone, both compounds were found to inhibit the growth of the common fungal nectar microbes Metschnikowia reukaufii and Aureobasidium pullulans. Taken together, these findings suggest that 2-MNQ and lawsone may serve an additional role in I. glandulifera to protect its rich nectar against fungal growth. The high abundance of 2-MNQ and lawsone in I. glandulifera EFNs may also point to a potential mechanism by which these allelopathic 1,4-NQs are leached from exuded nectar by rainfall into the soil where they exhibit inhibitory effects.