H.K. WU and M.C. CHUNG
Institute of Botany, Academia Sinica. Taipei. ROC
Oleosin is a protein that encases an oil body. The gene encoding for oleosin and its products have been well characterized in many crops including rice, however, the transportation route and deposition site of the latter are obscure.
For immunocytochemistry studies, antibodies against two isoforms, 16kDa and 18kDa of rice oleosin were raised respectively in chickens. Immunoglobulins were purified from the egg yolks. Pre-immune eggs were saved for negative controls. Ultrathin sections (90nm) were prepared from rice embryos (of cultivar Tainung 67) sampled at 5, 10 and 30 days after that pollination (DAP). The sections were incubated with the primary antibody (chicken against oleosin) followed by secondary antibody (rabbit against chicken IgY, conjugated to 12nm gold particles). Negative controls were ultrathin sections incubated with the pre-immune serum instead of the primary antibody.
The observations disclosed that at 5 DAP, oleosin is translated on the free polysomes (Fig. 1A) in the scutellar cells of rice embryos, and transported via Golgi-derived vesicles into the vacuoles and the middle lamella (Fig. 1B). At 10 DAP, only a few oleosin-oil drop complexes exist in the vacuoles as well as in the cytoplasm of the scutellar cells (Fig. 2A). Only the oleosin parts are labeled (2A, inset). The oleosin residing in the middle lamella is transported back to cell wall but the oil drops along the inner wall are not labeled (Fig. 2B). At 30 DAP, the scutellar cells are fully filled with the oleosin-oil drop complexes that are disintegrating (Fig. 3A). A striking feature is that the oil drops released from the complex are surrounded by oil drops with oleosin on their surfaces (Fig. 3B). The oleosin that was in the scutellar cell walls is now covering surface of the oil drops that appear alongside the inner wall (Fig. 3C). The results observed here are not consistent with others that say oleosin is transported via the endoplasmic reticulum to the oil body.
Fig. 2. [A] A scutellar cell at 10 DAP accommodates a few oleosin-oil drop complexes (OL-O) either in the vacuole or in the cytoplasm. [Inset] An enlarged oleosin-oil drop complex from one denoted by an arrow in Fig. A. Note that the immuno-gold particles are located on the oleosin. [B] The gold particles are located on the wall (CW) but not the oil drop (O).
Fig. 3. [A] Scutellar cells at 30 DAP are filled with oleosin-oil drop complexes (OL-O) that are disintegrating. [B] An enlarged oleosin-oil drop complex (OL-O) from Fig.3A, showing the immunogold-labeled oil drops (O) surrounding the complex. [C] The oil drops (O) alongside the inner wall are labeled with immunogold particles.