Glucosylation of the peptide leucinostatin A, produced by an endophytic fungus of European yew, may protect the host from leucinostatin toxicity.

BACKGROUND

Yew species (Taxus spp.) throughout the world are hosts to hundreds, or perhaps thousands, of endophytic organisms. Most commonly, these organisms are fungi, living in a commensal or a symbiotic relationship with their host plant, so the plants exhibit little or no outward evidence that they are supporting these microorganisms. Little is known about any of the biochemical mechanisms that mediate the interactions between the yew host and its associated microbes. We feel that such information may not only contribute to our understanding of endophyte-tree biology, but also may provide novel pharmaceutical leads, because some of the compounds produced by these endophytes have demonstrated pharmacological activities.

RESULTS

Acremonium sp. was isolated as an endophytic fungus of the European yew, Taxus baccata. Entry of Acremonium sp. into the plant may proceed via invasion of natural openings such as stomata. The relationship between Acremonium sp. and T. baccata may be a symbiotic one, because no symptoms are seen when Taxus media p.v. Hicksii is inoculated with this fungus. In culture, the fungus makes leucinostatin A, a peptide with phytotoxic, anticancer and antifungal properties. Although this peptide causes necrotic symptoms in many non-host plants and other cell types, it causes no visible symptoms in the host plant. T. baccata and several other plants have a UDP glucose; leucinostatin A glucosyl transferase that catalyzes the production of leucinostatin A beta di-O-glucoside from leucinostatin A. This glucoside, also made by the fungus, has a lower bioactivity against plants, fungi and a breast cancer cell line, BT-20, than leucinostatin A.

CONCLUSIONS

Leucinostatin A may be one of several potentially toxic peptides produced by Acremonium sp. that contribute to the defense of the host, thereby preserving the fungus' own biological niche. The host plant is relatively immune to leucinostatin A because it has an enzyme which transfers two glycosyl residues to leucinostatin A, markedly reducing the peptide's bioactivity. Our results suggest that glucosylation reactions may play a more general role in plant defenses, especially against toxin-mediated disease development.