Introduction And Brief Bibliographic Review

Cells store lipids in specialized structures whose name varies depending on considered species. In plants they are called oil bodies (OB), or oleosomes. They consist of a monolayer of phospholipids in which different proteins are embedded [1], surrounding a core of neutral lipids. For a long time they have been considered as balls of fat [2]. Depending on the species, plant seed OB may contain highly variable number of proteins. Recent proteomics studies that will be detailed in this chapter showed that the protein complement of plant seed OB contained from three in castor bean [3] to at least 24 different proteins in B. napus [4]. Plant OB proteins fall within two major categories: structural proteins and enzymes, with the function of other minor proteins still being unknown.

Oleosins are the most abundant structural proteins located within plant OB, and their presence is not restricted to seeds resistant to dessication [5]. They exhibit a conserved architecture among species. One of their functions may be to stabilize OB during seed imbibition prior to germination [6]. In addition to their structural function, oleosins may also serve as recognition signals on the surface of plant OB for the binding of newly synthesized lipase during germination [7]. In Arabidopsis, 16 different oleosins, with molecular mass between 11 and 53 kDa, are encoded by a multigene family [8]. Their mRNAs have been found in seeds, tapetum, and microspore, which lead to the classification of these proteins in four subfamilies corresponding to the location of their mRNAs [8]. Oleosins have also been located in root tips [9]. Interest for oleosins has considerably risen upon their identification outside of seeds. Roberts et al. [10] have sorted oleosins into gametophyte (pollen)-specific and sporophyte (seed)-specific members, with the two gene families either distantly related or of independent origin. They have suggested an analogy between pollen and seeds, both being a dehydrated propagule carrying amounts of stored lipids with a requirement for rapid mobilization of these reserves upon germination. Both pollen and seeds have acquired oleosins as a mean of stabilizing storage material and facilitating its mobilization. One oleosin reported in pollen coat proteome is required for rapid initiation of Arabidopsis pollinization [11, 12]; and its C-terminus, highly divergent between homologues and orthologues, makes it a good candidate for species-specific signal.

Another protein, named caleosin, is also found on the surface of OB. It was initially discovered associated with cell-membrane fractions in developing and germinating rice caryopses, in response to ABA or to osmotic stress [13]. The first caleosin described in OB was from sesame seeds [14]. In the case of Arabidopsis, caleosin belongs to a multigene family [9, 15]. AtClo1 expression is specific to embryo and mature seeds, and its promoter is active in root tips, where the protein was also immunodetected [9]. Expression of this protein, capable to bind calcium was induced by drought and also by high salinity [16].

The list of enzymes found within plant OB is growing with time. To date, all characterized enzyme activities found associated with OBs use lipids as substrates. The presence of triacylglycerol hydrolase initiating storage oil breakdown in germinating seeds and of enzymes belonging to the short-chain dehydrogenase/reductase family exhibiting steroid dehydrogenase activity will be described later. Some OB proteins are also described by authors under the generic term of "contaminants." However, their presence may reflect on one hand the vicinity of OB with different cellular compartments and on the other hand the impossibility to obtain pure organelle [4, 17].

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