Abstract
Hypericum perforatum and related species (Hypericaceae) are a reservoir of pharmacologically important secondary metabolites, including the well-known naphthodianthrone hypericin. However, the exact biosynthetic steps in the hypericin biosynthetic pathway, vis-à-vis the essential precursors and their localization in plants, remain unestablished. Recently, we proposed a novel biosynthetic pathway of hypericin, not through emodin and emodin anthrone, but skyrin. However, the localization of skyrin and its precursors in Hypericum plants, as well as the correlation between their spatial distribution with the hypericin pathway intermediates and the produced naphthodianthrones, are not known. Herein, we report the spatial distribution of skyrin and its precursors in leaves of five in vitro cultivated Hypericum plant species concomitant to hypericin, its analogs, as well as its previously proposed precursors emodin and emodin anthrone, using MALDI-HRMS imaging. Firstly, we employed HPLC-HRMS to confirm the presence of skyrin in all analyzed species, namely H. humifusum, H. bupleuroides, H. annulatum, H. tetrapterum, and H. rumeliacum. Thereafter, MALDI-HRMS imaging of the skyrin-containing leaves revealed a species-specific distribution and localization pattern of skyrin. Skyrin is localized in the dark glands in H. humifusum and H. tetrapterum leaves together with hypericin but remains scattered throughout the leaves in H. annulatum, H. bupleuroides, and H. rumeliacum. The distribution and localization of related compounds were also mapped and are discussed concomitant to the incidence of skyrin. Taken together, our study establishes and correlates for the first time, the high spatial distribution of skyrin and its precursors, as well as of hypericin, its analogs, and previously proposed precursors emodin and emodin anthrone in the leaves of Hypericum plants.
Highlights
Secondary metabolites actively participate in a plethora of physiological activities in plants, which includes imparting stress tolerance and accessory functions, unlike primary metabolites [1].these plant-derived specialized metabolites exhibit a wide array of pharmacological activities, which has opened gates to explore plant communities for novel compounds [2]
Hypericin (6) is well-studied for its occurrence and spatial distribution; previous reports have established that hypericin (6) and its protoforms accumulate in the dark glands owing to their photosensitizing properties [10]
We recently reported that skyrin (7) and its precursors serve as intermediates to hypericin production through another pathway, not involving emodin
Summary
Secondary metabolites actively participate in a plethora of physiological activities in plants, which includes imparting stress tolerance and accessory functions, unlike primary metabolites [1]. These plant-derived specialized metabolites exhibit a wide array of pharmacological activities, which has opened gates to explore plant communities for novel compounds [2]. Both Hölscher et al (2009) and Rizzo et al (2019) reported the accumulation of hypericin (6) in the dark glands, which plants develop during the placental stage [8,9]. The reason behind the evolutionarily-evolved accumulation of hypericin (6) in dark glands is due to its photosensitizing activities [10]
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