Phenolic Compounds as Biomarkers of Interactions between the Endophyte Klebsiella oxytoca and the Common Duckweed, Lemna minor L. Belgrade

The common duckweed (Lemna minor L.) is a cosmopolitan, miniature, fast-reproducing higher aquatic plant. In the last decade, the common duckweed as a model organism is experiencing a form of a renaissance: it is being re-discovered as an optimal model organism due to its simplified morphology, small genome, dominant vegetative reproduction, and minimal requirements for in vitro growth. One of the most striking characteristics of L. minor is its ability to co-exist with various microorganisms. Rhizosphere was recognized as a natural source of various beneficial microorganisms that can be utilized in different areas, e.g. agriculture and bioremediation. In this study, our focus was on the interactions between duckweeds and a rhizosphere-associated bacterial strain Klebsiella oxytoca (Access. No. MK212915). Our aim was to identify different phenolic compounds that might reflect complex interactions between the plant and this endophytic bacterium. Five distinct phenolic compounds were isolated by liquid chromatography - mass spectrometry: luteolin 6,8-di-C-hexoside, p-hydroxybenzoic acid, caffeic acid, apigenin 6-C-(2"-pentosyl)hexoside and p-coumaric acid. These compounds were apparently unique to this type of interaction between two distinct species. All of the identified compounds might reflect the invasion and colonization of the plant by K. oxytoca. Bibliographic analysis of key terms discovered that this area of research is vastly underrepresented in scientific literature. This paper is another call for all plant physiologists to focus their research on L. minor as a versatile model organism and to analyze different aspects of complex plant/bacterium interactions.