Oleosins and Oil Bodies in Seeds and Other Organs

Abstract

Diverse organisms store lipids in subcellular particles as food reserves, which will be mobilized during a forthcoming period of active metabolism.These lipid particles can be found in the seeds, pollens, flowers, roots, and stems of flowering plants, the spores and vegetative organs of nonflowering plants, and algae.They are also present in some animal cells, fungi, and Euglena.Of a11 these subcellular storage lipid particles, those from seeds have been studied most extensively.Seeds of most plant species store TAGs as food reserves for germination and postgerminative growth.The TAGs are present in small subcellular spherical oil bodies of approximately 1 pm in diameter.Each oil body has a matrix of TAGs surrounded by a layer of PLs.The small entities provide a large surface area per unit TAG, which would facilitate lipase binding and lipolysis during germination.Oil bodies inside the cells of mature seeds or in isolated preparations are remarkably stable and do not aggregate or coalesce.This stability is in contrast to the instability of artificial liposomes made from amphipathic and neutra1 lipids; the liposomes slowly coalesce after formation.Seed oil bodies are stable because their surface is shielded by a layer of unique proteins, termed oleosins.Oleosins, in addition to being present in the storage oil bodies of seeds and pollens, have recently been found in the nonstorage tissue of floral tapetum.The preliminary findings suggest that the tapetum oleosins are similar to seed oleosins in structure and function.They may prevent subcellular lipid droplets from coalescing; these lipid droplets are to be deposited onto the surface of the maturing pollen. OLEOSINS AND 011 BODIES IN SEEDS Oleosins and Their mRNAs Are Abundant in SeedsOleosins in the seeds of diverse species are small proteins of about 15 to 26 kD (Huang, 1992; Murphy, 1993; Herman, 1995).They completely cover the surface of the small subcellular oil bodies.When a seed contains a high percentage of oils and its oil bodies are small (i.e.having a large surface area per unit TAG), the amount of oleosins is high and can account for several percent of the total seed proteins (Tzen et al., 1993).For example, '