ULTRASTRUCTURAL-STUDY OF LIPID-ACCUMULATION IN TAPETAL CELLS OF BRASSICA-NAPUS L. CV WESTAR DURING MICROSPOROGENESIS

Ultrastructural features of Brassica napus tapetal cells during micros porogenesis from early microspore development through late maturation are described. Emphasis is placed on the two major lipid-containing co mponents (plastids and lipid bodies) of the tapetal cells, particularl y the little-studied lipid bodies. By the early microspore stage, the walls of the tapetal cells are mostly dissolved, and a lipoid layer ha s been deposited on the tapetal side of the middle lamella of the oute r tangential wall between the tapetal cells and the anther wall cells. An electron-dense layer of presumed sporopollenin is subsequently dep osited on the tapetal side of the lipoid layer, thus forming a continu ous peritapetal layer, occluding the plasmodesmata, and isolating the anther locule. A prominent feature of the young tapetal cells is an ab undance of ribosomes and endoplasmic reticulum (ER); the vacuoles are small; plastids are undifferentiated; and only a few, small cytoplasmi c 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 differ entiate into complex structures composed of a mixture of lipid and app arent membranous components. We propose a structural model for the bio genesis of the lipid bodies. As the microspores reach maturity, the li pid bodies, plastids, and other tapetal organelles are released from l ysed tapetal cells, and the remnants of these organelles are deposited on the surface of the maturing pollen, forming the tryphine.