OLEOSINS PREVENT OIL-BODY COALESCENCE DURING SEED IMBIBITION AS SUGGESTED BY A LOW-TEMPERATURE SCANNING ELECTRON-MICROSCOPE STUDY OF DESICCATION-TOLERANT AND DESICCATION-SENSITIVE OILSEEDS

In order to clarify further the physiological role of oleosins in seed development, we characterized the oil-body proteins of several oilsee ds exhibiting a range of desiccation sensitivities from the recalcitra nt (Theobroma cacao L., Quercus rubra L.), intermediate (Coffea arabic a L., Azadirachta indica A. Juss.) and orthodox categories (Sterculia setigera Del., Brassica napus L.). The estimated ratio of putative ole osins to lipid in oil bodies of e. rubra was less than 5% of the equiv alent values for rapeseed oil bodies. No oleosin was detected in T. ca cao oil bodies. In A. indica cotyledons, oil bodies contained very low amounts of putative oleosins. Oil bodies both from C. arabica and S. setigera exhibited a similar ratio of putative oleosins to lipid as fo und in rapeseed. In C. arabica seeds, the central domain of an oleosin was partially sequenced. Using a low temperature field-emission scann ing electron microscope, the structural stability of oil bodies was in vestigated in seeds after drying, storage in cold conditions and rehyd ration. Despite the absence or relative dearth of oleo sins in desicca tion-sensitive, recalcitrant oilseeds, oil bodies remained relatively stable after slow or fast drying. In A. indica seeds exposed to a leth al cold storage treatment, no significant change in oil-body sizes was observed. In contrast, during imbibition of artificially dried seeds containing low amounts of putative oleosins, the oil bodies fused to f orm large droplets, resulting in the loss of cellular integrity. No da mage to the oil bodies occurred in imbibed seeds of Q. rubra, C. arabi ca and S. setigera. Thus the rehydration phase appears to be detriment al to the stability of oil bodies when these are present in large amou nts and are lacking oleosins. We therefore suggest that one of the fun ctions of oleosins in oilseed development may be to stabilize oil bodi es during seed imbibition prior to germination.