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 abundant elaioplasts, which had few thylakoid membranes but were packed with globuli of neutral esters. Of the neutral esters, the major ester group possessed mainly 24-methylenecholesterol, 31-norcycloartenol, 24-dehydropollinastanol, and pollinastanol esterified to 18:3 and other unsaturated and saturated 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 proportion (16% w/w of total lipids) of monogalactosyldiacylglycerols (MGDG). This is the first report of plastids having steryl esters as the predominant lipids. We propose that the globuli contain steryl esters and are stabilized by surface MGDG and structural proteins. The tapetosomes, the other abundant 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 TAG of the tapetosomes, were degraded. Steryl esters similar to those of the elaioplasts were recovered from the pollen surface and were the major lipids of the pollen coat. The pollen coat steryl esters and proteins could be extracted with moderately polar or nonpolar solvents. These proteins, which were mostly fragments of oleosins derived from the tapetosomes, had a high proportion of lysine (13 mol %). The possible functions of the steryl esters and the proteins on the pollen surface are discussed.