Spatial Distribution of Astins in <i>Aster tataricus</i> and Their Production by <i>Cyanodermella asteris</i>

<i>Aster tataricus</i> is a plant used in Traditional Chinese Medicine for the treatment of cough, phlegm, and asthma. Phytochemical studies of <i>A. tataricus</i> have resulted in the isolation of 23 peptides, among which the astins are recognized for their potential application as anticancer drugs. However, it was found that some of the astins, namely, astins C, F and G, are in fact produced by an endophytic fungus, <i>Cyanodermella asteris</i>, isolated from the inflorescences of the plant, while the remainder were suggested to be transformation products of these astins by <i>A. tataricus</i>. Using mass spectrometry imaging and microscopy, we demonstrate that astins exhibit a nonhomogeneous distribution, vary in relative abundance in different plant tissue sections, and are likely colocalized with fungal cells. To gain further insights into the diversity and composition of astins produced by <i>C. asteris</i>, we applied HPLC-MS/MS and mass spectrometry-based molecular networking after <i>in vitro</i> cultivation of the fungus in media with increased salinity. We found that the fungus produced a higher variety of astins than previously known, including several yet undescribed astins, suggesting that the fungus alone is indeed able to produce the complete astin diversity and that cross-species biosynthesis is unlikely.