The antitumor astins come from the fungal endophyte Cyanodermella asteris, which lives in the medicinal plant Aster tataricus.

Astins are natural products that bind to a crucial human regulatory protein, the stimulator of interferon genes (STING), which is a promising new therapeutic target for cancer and immune disorders. Astins have long been considered phytochemicals of a Chinese medicinal plant. Here we show that they are produced by the newly identified fungal endophyte Cyanodermella asteris via a nonribosomal biosynthetic pathway. Furthermore, we provide evidence that key astin variants are produced during symbiosis with the aster plant. The production of specific phytochemical variants during symbiotic interactions is poorly studied and may be more widespread than expected. These results pave the way for cost-effective biotechnological production of astin.

Medicinal plants are a prolific source of natural products with remarkable chemical and biological properties, many of which have considerable curative effects. Many medicinal plants suffer from uncontrolled exploitation, so there is an urgent need to develop biotechnological processes for the production of their natural products. The plant Aster tataricus is widely used in traditional Chinese medicine and contains unique active ingredients called astins. These are macrocyclic peptides with promising antitumor activities and usually contain the highly unusual 3,4-dichloroproline moiety. The biosynthetic origins of astins are unknown although they have been studied for decades. Here we show that astins are produced by the recently discovered endophytic fungus Cyanodermella asteris. We were able to produce astins in reasonable and reproducible amounts using axenic cultures of the endophyte. We have identified the biosynthetic gene cluster responsible for astin biosynthesis in the C. asteris genome and propose a production pathway that relies on a nonribosomal peptide synthetase. The striking differences in endophyte and host plant production profiles imply a cross-species symbiotic biosynthetic pathway for astin C derivatives, in which plant enzymes or signals are required to trigger the synthesis of plant-exclusive variants such as astin A. Our results lay the foundation for sustainable biotechnological production of astins independent of asteris plants.

The production of astins by A. tataricus and C. asteris. (A) Chemical structures of the main astin variants known from A. tataricus. (B–E) HPLC-MS chromatograms of extracts prepared from fresh plant roots and fungal cultures (each one is a representative chromatogram out of at least 3 biological replicates). The chromatograms show astin peaks at characteristic retention times (indicated in capital letters). Extracted ion chromatograms (EIC) depict the respective [M+H]+ ions of astins. TIC+ denotes total ion chromatogram, positive mode. Chromatograms are shown for a representative A. tataricus plant either (B) containing astins or (C) being devoid of astins, (D) for extracts prepared from a C. asteris culture and (E) for extracts prepared from an A. tataricus plant (initially being devoid of astins like in C), 3 mo after reinfection with the endophyte C. asteris. Intens., ion intensity in arbitrary units.

Infection of Astin-free plants by C. asteris.

Astin-free A. tataricus plants were grown in a 2-compartment system in autoclaved soil, where a gauze (pore size 20 µm) separated the plant compartment from the fungal compartment. The soil in the fungal compartment was infected with 150 mg of freshly homogenized C. asteris tissue. Roots are not able to grow through the gauze, whereas fungal hyphae can. The 2-compartment system with infected plant and soil was grown in a climate chamber under long day conditions (16 h light at 23 °C, 8 h dark at 18 °C). Astin content was studied in plant tissues after 3 months of growth as described above. Three plants were infected with C. asteris; one uninfected plant served as a control.