Submitted to: Lipids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 19, 1999
Publication Date: N/A
Interpretive Summary: In plants, pollen grains serve as the equivalent of sperm cells in animals for sexual reproduction. Anthers are the male reproductive structures of flowers in which pollen is produced. A specialized layer of cells in the anthers contribute nutrients and structural components to the developing pollen grains. In this study it was shown that much of the material passed on to the pollen surface comes from a unique subcellular body (organelle) called the elaioplast. Furthermore, the major protein and lipid (fat) compounds deposited as a coating on the pollen grains were chemically characterized and found to be quite unusual. It is thought that these compounds in the pollen coat may be involved in fulfilling the nutritional requirements of insect pollinators and/or in species recognition which allows pollen germination and fertilization to occur. This kind of information is needed to achieve an understanding of the complex ecology of plant/insect interdependence and the biochemical basis of sexual compatability/incompatibility in plants.
Tapetum cells in developing anthers of Brassica napus contained abundant elaioplasts. These organelles had few thylakoid membranes but were packed with globuli of neutral esters. Two groups of neutral esters were separated by HPLC. Of these, the major group (E1) included 24-methylenecholesterol, 31-norcycloartenol, 24-dehydropollinastanol and pollinastanol as the main sterol moieties esterified to 18:3 and other fatty acids. The minor ester fraction (E2) had one predominant alcohol, tentatively identified as 12-dehydrolupeol, esterified mainly to 18:0, 16:0 and 20:0 fatty acids. Elaioplasts also contained a high proportion of monogalactosyldiacylglycerols (MGDGs). This is the first report of plastids having steryl esters as the most abundant lipids. We propose that the elaioplast globuli consist of steryl esters stabilized by surface MGDG and structural proteins. Tapetosomes, the other abundant lipid-containing organelles in the tapetum, included triacylglycerols (TAGs) as the predominant lipids. At a late stage of anther development, E2 and MGDGs in elaioplasts, and TAGs in tapetosomes, were degraded. E1 steryl esters from the elaioplasts were transferred from lyzed tapetum cells to the pollen surface, thus becoming the main lipid component of the pollen coat. Proteins of the pollen coat consisted mostly of fragments of oleosins derived from the tapetosomes, and were shown to be rich in lysine (13 mole percent).