Mysterious giants in the world of lipids: long linear isoprenoid functions in plant physiology and reproduction

Isoprenoids, sometimes also called terpenoids, form a large and diversified family of natural compounds. All isoprenoids are synthesized from a branched five-carbon isoprene unit, which is derived in plants on the cytoplasmic/ER/peroxisome mevalonic acid (MVA) pathway and plastidial MEP pathway. The isoprene unit modules in isoprenoid molecules may be arranged in various ways, the relatively simplest being iterative head-to-tail condensation. Long-chain linear polymers derived in this manner are present in plastidial membranes, where they play a role in photosynthesis. Long-chain linear isoprenoid polymers synthesized on the mevalonate pathway are also found in the endoplasmic reticulum membrane (dolichols) and mitochondrial inner membrane (all-trans(E)-nonaprenol trivial name solanesol or all-trans(E)-decaprenol trivial name spadicol). Both dolichol and solanesol play crucial roles in eukaryotic metabolism: dolichol serves as a cofactor in protein glycosylation and GPI anchor synthesis, while solanesol (or spadicol) acts as the side chain of ubiquinone in electron transport during cellular respiration. Some new roles of the long-chain linear isoprenoid compounds in plants may also be anticipated based on in vivo work on yeast and in vitro biophysical experiments. This review focuses on the known and emerging roles of these compounds in the context of plant physiology and sexual reproduction.