Membrane systems for lipid secretion in the hairy roots of Lithospermum erythrorhizon observed by electron microscopy

Many plant-derived lipophilic compounds benefit human life. For example, shikonin derivatives--lipophilic red pigments from the medicinal plant Lithospermum erythrorhizon--exhibit wound-healing and antimicrobial effects. These compounds are excreted from root epidermal cells and accumulate in the root bark. Our understanding of how lipophilic compounds are transported in plant cells remains limited, and L. erythrorhizon hairy roots serve as a good model for studying the lipid secretion. To investigate this, the shikonin secretion was induced in cultured hairy roots, which were then fixed via chemical fixation or high-pressure freezing followed by freeze-substitution and observed by transmission electron microscopy. Electron micrographs from chemically fixed samples showed multivesicular bodies (MVBs) surrounding electron-dense bodies--presumed to contain shikonin derivatives--and fusing with the plasma membrane. Similarly, freeze-substituted samples revealed MVBs fused with the plasma membrane. Additionally, some MVBs contacted plastids containing enzymes for shikonin precursor biosynthesis. These findings suggest MVBs play key roles in both synthesis and secretion of shikonin derivatives. HighlightThis study reveals that multivesicular bodies mediate both synthesis and secretion of shikonin derivatives in the hairy roots of Lithospermum erythrorhizon.