Excretion of triacylglycerol as a matrix lipid facilitating apoplastic accumulation of a lipophilic metabolite shikonin.

Plants produce a large variety of lipophilic metabolites, many of which are secreted by cells and accumulated in apoplasts. These compounds often play a role to protect plants from environmental stresses. However, little is known how these lipophilic compounds are secreted into apoplastic spaces. In this study, we used shikonin-producing cultured cells of Lithospermum erythrorhizon as an experimental model system to analyze the secretion of lipophilic metabolites taking its advantages, such as the high production rate and the clear inducibility using culture conditions. Shikonin derivatives are lipophilic red naphthoquinone compounds that accumulate exclusively in apoplastic spaces of these cells and also in the root epidermis of intact plant. Microscopic analysis showed that shikonin is accumulated in a vast number of particles on the cell wall. Lipidomic analysis showed that L. erythrorhizon cultured cells secrete an appreciable portion of triacylglycerol (24 to 38% of total TAG), composed predominantly of saturated fatty acids. Moreover, in vitro reconstitution assay showed that TAG encapsulates shikonin derivatives with phospholipids to form lipid droplet-like structures. These findings suggest a novel role for TAG as a matrix lipid, a molecular component involved in the secretion of specialized lipophilic metabolites.