Steryl esters in the elaioplasts of the tapetum in developing Brassica anthers and their recovery on the pollen surface

The tapetum cells in the developing anthers of Brassica napus contained-abu ndant elaioplasts, which had few thylakoid membranes but were packed with g lobuli of neutral esters. Of the neutral esters, the major ester group: pos sessed mainly 24-methylenecholesterol, 31-norcycloartenol, 24-dehydropollin astanol, and pollinastanol esterified to 18:3 and other unsaturated and sat urated fatty-acyl moieties. The minor ester group had a dominant component tentatively identified as 12-dehydrolupeol esterified to mostly 18:0, 16:0, and 20:0 fatty-acyl moieties. The elaioplasts also contained a high propor tion (16% w/w of total lipids) of monogalactosyldiacylglycerols (MGDG). Thi s is the first report of plastids having steryl esters as the predominant l ipids. We propose that the globuli contain steryl esters and are stabilized by surface MGDG and structural proteins. The tapetosomes, the other abunda nt lipid-containing organelles in the tapetum, possessed triacylglycerols ( TAG) as the predominant lipids. At a late stage of anther development, the minor group of neutral esters and MGDG of the elaioplasts, as well as the T AC of the tapetosomes, were degraded. Steryl esters similar to those of the elaioplasts were recovered from the pollen surface and were the major lipi ds of the pollen coat. The pollen coat steryl esters and proteins could be extracted with :moderately polar or nonpolar solvents. These proteins, whic h were mostly fragments of oleosins derived from the tapetosomes, had a hig h proportion-of lysine(13 mol %). The possible functions of the steryl este rs and the proteins on the pollen surface are discussed.