Endoplasmic Reticulum, Oleosins, and Oils in Seeds and Tapetum Cells

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Abstract
Diverse organisms contain neutral lipids in sub- cellular particles for food reserves and other purposes. These lipid particles are present in seeds, flowers, pollen and fruit of higher plants, the vegetative and reproductive organs of primitive plants, algae, fungi, nematodes, mammalian glands and brown adipose tissue of mammals, and bacteria. Of all these lipid particles, the oil bodies (OBs) in seeds are the most prominent and best studied. Seeds of most plant species store oils (triacylglyc- erols (TAGs)) as a food reserve for germination and postgerminative growth. TAGs are present in small subcellular spherical OBs of approximately 1 m mi n diameter. Each OB has a matrix of TAGs surrounded by a layer of phospholipids (PLs) and structural pro- teins termed oleosins. The small size of OBs provides a large surface area per unit TAG, which would facilitate lipase binding and lipolysis during germina- tion. OBs inside the cells of mature seeds or in isolated preparations are remarkably stable and do not aggre- gate or coalesce. This stability is in contrast to the instability of artificial liposomes made from amphi- pathic and neutral lipids; the liposomes gradually coalesce after formation. Seed OBs are stable because their surface is shielded by a layer of oleosins. In maturing seeds, TAGs, PLs, and oleosins are synthe- sized in the endoplasmic reticulum (ER), from which budding OBs are released. Research on seed OBs and oleosins has been re- viewed by Huang (1992, and an earlier Updatearticle in 1996), Napier et al. (1996), Galili et al. (1998), Frandsen et al. (2001), and Murphy (2001). This Update provides brief reviews of earlier work and emphasizes recent major discoveries. Detailed information and references to earlier work can be found in the previous reviews.