Characterization of seed oil bodies and their surface oleosin isoforms from rice embryos

Plant seeds store triacylglycerols in discrete organelles called oil bodies. An oil body stores a matrix of triacylglycerols surrounded by phospholipids and alkaline proteins termed oleosins. Oil bodies in rice seeds are present in embryos and aleurone layers. They do not coalesce in crowded environments, as observed on electron microscopy. The detected isoelectric point of purified rice oil bodies is pH 6.2. This implies that rice oil bodies possess a negatively charged surface at neutral pH. The suspension of rice oil bodies in pH 6.5 buffer induces aggregation. Presumably, the negatively charged surface causes electrostatic repulsion that maintains rice oil bodies as discrete organelles. Rice oil bodies lose their integrity on trypsin treatment. Undoubtedly, oleosins play an important role in the stability of oil bodies. There are two oleosin isoforms in rice oil bodies. Antibodies raised against these two homologous isoforms do not cross-recognize each other. Both isoforms are restricted to oil bodies, as detected on immuno-assaying. Partial amino acid sequences of these two isoforms were obtained, and compared with the deduced sequences of two maize and two rice oleosin genes. The comparison confirmed that the two major proteins in rice oil bodies are the two oleosin isoforms.