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Bayer HealthCare Pharmaceuticals.

Reactive oxygen intermediates, i.e. the superoxide radical (O*-)(2), hydrogen peroxide (H2O2) and the hydroxyl radical (*OH), are generally regarded as harmful products of oxygenic metabolism causing cell damage in plants, animals and microorganisms. However, oxygen radical chemistry may also play a useful role in polymer breakdown leading to wall loosening during extension growth of plant cells controlled by the phytohormone auxin. Backbone cleavage of cell wall polysaccharides can be accomplished in vitro by (*OH) produced from H2O2 in a Fenton reaction or in a reaction catalyzed by peroxidase supplied with O2 and NADH. Here, we show that coleoptile growth of maize seedlings is accompanied by the release of reactive oxygen intermediates in the cell wall. Auxin promotes release of (O*-)(2) and subsequent generation of (*OH)when inducing elongation growth. Experimental generation of (*OH) in the wall causes an increase in wall extensibility in vitro and replaces auxin in inducing growth. Auxin-induced growth can be inhibited by scavengers of (O*-)(2), H2O2 or (*OH), or inhibitors interfering with the formation of these molecules in the cell wall. These results provide the experimental background for a novel hypothesis on the mechanism of plant cell growth in which (*OH), produced from (O*-)(2) and H2O2 by cell wall peroxidase, acts as a wall-loosening agent.

The two most prominent neutral lipids of the yeast Saccharomyces cerevisiae, triacylglycerols (TAG) and steryl esters (SE), are synthesized by the two TAG synthases Dga1p and Lro1p and the two SE synthases Are1p and Are2p. In this study, we made use of a set of triple mutants with only one of these acyltransferases active to elucidate the contribution of each single enzyme to lipid particle (LP)/droplet formation. Depending on the remaining acyltransferases, LP from triple mutants contained only TAG or SE, respectively, with specific patterns of fatty acids and sterols. Biophysical investigations, however, revealed that individual neutral lipids strongly affected the internal structure of LP. SE form several ordered shells below the surface phospholipid monolayer of LP, whereas TAG are more or less randomly packed in the center of the LP. We propose that this structural arrangement of neutral lipids in LP may be important for their physiological role especially with respect to mobilization of TAG and SE reserves.Fatty acids and sterols are important building blocks of biomembranes. To maintain balanced cellular levels of these components, a certain portion is put on hold in the form of complex neutral lipids. The yeast Saccharomyces cerevisiae similar to other eukaryotic cells stores fatty acids and sterols in the form of triacylglycerols (TAG) 3 and steryl esters (SE). Both TAG and SE are hydrophobic molecules that are not soluble in the cytosol but are also not typical bilayer membrane lipids. Consequently, they need to be stored in specific subcellular compartments that are known as lipid particles (LP), lipid droplets, lipid bodies, or oil bodies. Yeast LP are small spherical organelles with an approximate diameter of 400 nm consisting of 95% neutral lipids, TAG, and SE, and only small amounts of phospholipids and proteins (1). The highly hydrophobic core formed by TAG and SE is surrounded by a phospholipid monolayer containing a well defined set of proteins (2).Identification of LP proteins in various types of cells has changed the view of this organelle. The presence of certain enzymes on the surface of LP suggests that besides lipid storage LP are involved in various metabolic processes. In S. cerevisiae ϳ40 proteins are known to be present on the surface of LP. Strikingly, most of these proteins are enzymes involved in lipid metabolism. As prominent examples, LP from S. cerevisiae harbor enzymes of phosphatidic acid biosynthesis (3), fatty acid activation (4 -6), sterol biosynthesis (7), but also TAG biosynthesis and degradation (8 -11) and SE hydrolysis (12-14).The biogenesis of LP is still a matter of dispute and is under investigation in several laboratories. Whereas little is known about the cell biology of this process, enzymes involved in the formation of major LP components, TAG and SE, have been identified within the last few years (for recent review see Refs. 15 and 16). In the yeast as in other types of cells, the last step of TAG synthesis requires acylation of diacylglycerol. Two different ...