Development and disintegration of tapetum-specific lipid-accumulating organelles, elaioplasts and tapetosomes, in Arabidopsis thaliana and Brassica napus

The pollen coat covering the surface of pollen grains has many important roles for pollination. In Brassicaceae plants, the pollen coat components are synthesized and temporarily accumulated in two tapetum-specific organelles, the elaioplast and the tapetosome. Although many biochemical and electron microscopic analyses have been attempted, the structure and biogenesis of these organelles have not been fully elucidated. To resolve this problem, we performed live imaging of these organelles using two markers, FIB1a-GFP and GRP17-GFP. FIB1a is an Arabidopsis fibrillin, a structural protein of elaioplast plastoglobules. In transgenic Arabidopsis, fluorescence of FIB1a-GFP appeared in young elaioplasts, in which small plastoglobules were developing. However, the fluorescence disappeared in later stages, while enlargement of plastoglobules continued. GRP17 is an Arabidopsis oleopollenin, an oleosin-like protein in tapetosomes. Fluorescence microscopy of GRP17-GFP expressed in Arabidopsis and Brassica napus revealed that tapetosomes do not contain oleopollenin-coated vesicles but have an outer envelope, indicating that the tapetosome structure is distinct from seed oil bodies. Visualization of GRP17-GFP also demonstrated that the tapetal cells become protoplasts and migrate into locules before pollen coat formation, and provided live imaging of the foot formation between pollen grains and stigmatic papilla cells.

► Early development of elaioplasts in living tapetal cells was imaged by FIB1a-GFP. ► Development and disintegration of tapetosomes were traced by GRP17-GFP. ► New insights into tapetosome structure distinct from seed oil bodies were obtained. ► Brassica tapetum becomes protoplasts and migrates into locules to form pollen coat. ► Pollen coat movement toward stigma surface at pollen landing was imaged by GRP17-GFP.