Ultrastructural study of lipid accumulation in tapetal cells of Brassica napus L. cv. Westar during microsporogenesis

Ultrastructural features of Brassica napus tapetal cells during microsporogenesis from early microspore development through late maturation are described. Emphasis is placed on the two major lipid-containing components (plastids and lipid bodies) of the tapetal cells, particularly the little-studied lipid bodies. By the early microspore stage, the walls of the tapetal cells are mostly dissolved, and a lipoid layer has been deposited on the tapetal side of the middle lamella of the outer tangential wall between the tapetal cells and the anther wall cells. An electron-dense layer of presumed sporopollenin is subsequently deposited on the tapetal side of the lipoid layer, thus forming a continuous peritapetal layer, occluding the plasmodesmata, and isolating the anther locule. A prominent feature of the young tapetal cells is an abundance of ribosomes and endoplasmic reticulum (ER); the vacuoles are small; plastids are undifferentiated; and only a few, small cytoplasmic lipid droplets are present. As maturation continues, the ER becomes associated with the developing lipid bodies; the plastids enlarge and accumulate plastoglobuli, forming elaioplasts. The lipid bodies differentiate into complex structures composed of a mixture of lipid and apparent membranous components. We propose a structural model for the biogenesis of the lipid bodies. As the microspores reach maturity, the lipid bodies, plastids, and other tapetal organelles are released from lysed tapetal cells, and the remnants of these organelles are deposited on the surface of the maturing pollen, forming the tryphine.