Abstract: [Objective] The laticifer cells in rubber tree (Hevea brasliensis) are characterized by the presence of abundant rubber particles, a kind of organelles specific for rubber biosynthesis and storage. However, the origination of rubber particles still remains controversial. Ultrastructural observation was performed to ascertain rubber particle origination and lay a foundation for further understanding of the function of rubber particles.[Method] By using transmission electron microscope (TEM) technique, the ultrastructure of laticifer cells at different development stages in the bark of both tender shoot and mature rubber tree trunk was investigated.[Result] In primary laticifers of the tender shoot, the recognizable original rubber particles appeared as small electron-dense spherical particles in the dense cytoplasm. The particles mainly distributed in the regions where abundant endoplasmic reticula (ER) and ribosomes were present. In the trunk of mature rubber tree, the young secondary laticifer cells near the vascular cambia contained abundant endoplasmic reticula and scattered rubber particles as well as various organelles with legible outlines, which is suitable for exploring the origination of rubber particles. At higher magnification, small electron-dense spherical particles with a diameter of about 50 nm were found to connect to the ERs while larger spherical particles with clear membrane and electron-lucency dispersed in the surrounding. The changes in diameter and interior of the spherical particles were consistent with the rubber particle development. There were small spherical particles near Golgi body. These particles were different from the original rubber particles in that their inclusion appeared to be granular other than homogenous electron-dense. The matured and ageing secondary laticifer cells were not suitable for investigating organelle origination since the matured secondary laticifer cells filled with rubber particles while rubber particle coagulation occurred in the ageing secondary laticifer cells.[Conclusion] The immature secondary laticifer cells near the vascular cambia are suitable for studying the origination of rubber particles. At this developmental stage, the connection of original rubber particles to ERs was clearly observed in the secondary laticifer cells, supporting the viewpoint that the rubber particle was originated from ERs. The results not only provide the cytological evidence for origination of rubber particles, but also provide a basis for studying the derivation of other organelles. It is necessary to investigate the origination of small and large rubber particles by using techniques as colloid gold-labeled immunocytological localization, considering the difference in composition of membrane proteins and function between the two kinds of rubber particles.

Key words: Hevea brasiliensis, laticifer cell, ultrastructure, rubber particle, endoplasmic reticulum