Lipotubuloids in ovary epidermis of Ornithogalum umbellatum act as metabolons: suggestion of the name 'lipotubuloid metabolon'.

ABSTRACT

A metabolon is a temporary, structural-functional complex formed between sequential metabolic enzymes and cellular elements. Cytoplasmic domains called lipotubuloids are present in Ornithogalum umbellatum ovary epidermis. They consist of numerous lipid bodies entwined with microtubules, polysomes, rough endoplasmic reticulum (RER), and actin filaments connected to microtubules through myosin and kinesin. A few mitochondria, Golgi structures, and microbodies are also observed and also, at later development stages, autolytic vacuoles. Each lipotubuloid is surrounded by a tonoplast as it invaginates into a vacuole. These structures appear in young cells, which grow intensively reaching 30-fold enlargement but do not divide. They also become larger due to an increasing number of lipid bodies formed in the RER by the accumulation of lipids between leaflets of the phospholipid bilayer. When a cell ceases to grow, the lipotubuloids disintegrate into individual structures. Light and electron microscope studies using filming techniques, autoradiography with [(3)H]palmitic acid, immunogold labelling with antibodies against DGAT2, phospholipase D1 and lipase, and double immunogold labelling with antibodies against myosin and kinesin, as well as experiments with propyzamide, a microtubule activity inhibitor, have shown that lipotubuloids are functionally and structurally integrated metabolons [here termed lipotubuloid metabolons (LMs)] occurring temporarily in growing cells. They synthesize lipids in lipid bodies in cooperation with microtubules. Some of these lipids are metabolized and used by the cell as nutrients, and others are transformed into cuticle whose formation is mediated by cutinsomes. The latter were discovered in planta using specific anti-cutinsome antibodies visualized by gold labelling. Moreover, LMs are able to rotate autonomously due to the interaction of microtubules, actin filaments, and motor proteins, which influence microtubules by changing their diameter.