Hexacyclopeptides secreted by an endophytic fungus Fusarium solani N06 act as crosstalk molecules in Narcissus tazetta

Wen-Xuan Wang, Souvik Kusari, Selahaddin Sezgin, Marc Lamshöft, Parijat Kusari, Oliver Kayser, Michael Spiteller

The basis of chemical crosstalk in plants and associated endophytes lies in certain so-called communication molecules that are responsible for plant-microbe and microbe-microbe interactions. Consequently, elucidating the factors that affect the nature, distribution, and amount of these molecules and how they impact the interaction among endophytes and associated organisms is essential to understand the true potential of endophytes. In the present study, we report the discovery of nine hexacyclopeptides from an endophytic fungus, Fusarium solani, isolated from the bulb of Narcissus tazetta, and their selective accumulation by an endophytic bacterium, Achromobacter xylosoxidans isolated from the same tissue. We used matrix-assisted laser desorption ionization imaging high-resolution mass spectrometry (MALDI-imaging-HRMS) to firstly identify and visualize the spatial distribution of the hexacyclopeptides produced by endophytic F. solani. After culture condition optimization, their sequence was identified to be cyclo((Hyp or Dhp)-Xle-Xle-(Ala or Val)-Thr-Xle) (Dhp: dehydroproline) by the characteristic a, b, or y ions using liquid chromatography tandem mass spectrometry (LC-HRMS(n)). These hexacyclopeptides were confirmed to be fungal biosynthetic products by deuterium labeling experiments. Finally, in order to understand the plausible ecological relevance of one or more of the discovered hexacyclopeptides within the contexts of microbial "neighbor communication," we devised a dual-culture setup to visualize using MALDI-imaging-HRMS how the hexacyclopeptides released by the endophytic fungus are accumulated by another endophytic bacterium, A. xylosoxidans, isolated from the same bulb tissue. This work exemplifies the relevance of cyclopeptides in endophyte-endophyte interspecies neighbor communication occurring in nature. Such communication strategies are evolved by coexisting endophytes to survive and function in their distinct ecological niches.