Submitted to: Lipids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/2001
Publication Date: 6/29/2001
Interpretive Summary: Rapeseed oil is an important international crop that is obtained from the plant Brassica napus. Canola oil was developed from this species by conventional plant breeding and several companies are now trying to engineer "designer genes" into this fast-growing species to enable it to produce other types of vegetable oils with unique properties. Pollen development and germination are important stages of the plant life cycle. The current research was undertaken to investigate the role of lipids in the pollen of Brassica napus. It had previously been shown that pollen contains high levels of fat (triacylglycerols) that is used as a source of energy. This study revealed that the surface of pollen grains and the organelles in tapetum cells (where pollen grains are synthesized) contain high levels of sterols, specifically steryl esters. Evidence is presented that suggests that these compounds in the pollen surface may have important tphysiological properties, such as functioning as feeding stimulants for insect pollinators or serving as hormonal stimulants for pollen germination and growth.
Technical Abstract: 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. 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 taptum, possessed triacylglycerols (TAGs) 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 TAGs of the tapetosomes, were degraded. The elaioplast steryl esters were transferred from the lyzed tapetum cells to the pollen surface, becoming the predominant lipids of the pollen coat. The pollen-coat steryl esters could be extracted with both moderately and strongly hydrophobic solvents, whereas the proteins could be removed only with moderately hydrophobic solvents. These proteins, which were mostly fragments of oleosins derived from the tapetosomes, had a high proportion of lysine (13 mole %). The possible functions of the steryl esters and the proteins on the pollen surface are discussed.