5 Sterol metabolism and functions in higher plants

Abstract

Higher plants synthesize a bewildering array of sterols, with sitosterol, stigmasterol, and 24-methylcholesterol as major compounds. All plant tissues contain free and conjugated sterols. The first steps of sterol biosynthesis occur via the classical acetate/mevalonate pathway, concomitantly with synthesis of many other isoprenoids. A crosstalk between cytosolic and plastid 2-C-methylerythritol phosphate pathways has been clearly demonstrated. Squalene synthase is the first enzyme of the sterol branch. All subsequent steps occur within the endoplasmic reticulum. In addition to their recognized role in regulating membrane properties, sitosterol, and 24-methylcholesterol modulate a variety of metabolic and ontogenetic events. The enzyme 3-hydroxy-3-methylglutaryl CoA reductase is a rate-limiting step for carbon flux toward sterols and also positively or negatively responds to a depletion of endogenous sterols or accumulation of intermediates. Sterol methyltransferases and acyltransferase participate in sterol homeostasis. A coordinated regulation of the biosynthetic pathways of sterols and some specific lipids may occur during membrane biogenesis.