Phosphorylation of Metabolites Involved in Salvage Pathways for Isoprenoid Biosynthesis in Plants

The recycling of metabolic products is a major way to reduce the energy cost of de novo biosynthesis. The salvage pathways involved not only regain a metabolic product but also generate additional molecules that might serve specific physiological, developmental and/or defensive functions. The isoprenoid pathway is a perfect example of a fine-regulated biosynthetic pathway, by virtue of the large number of molecules with different functions that must be synthesized simultaneously. Additionally, isoprenoid salvage pathways have been characterized. Thus, to produce isoprenoid precursors such as farnesyl diphosphate or phytyl diphosphate, short-chain isoprenols recovered from end-chain metabolites are phosphorylated. In the first instance, the so-called FPP-salvage machinery recycles farnesyl diphosphate from proteolyzed farnesylated proteins. In a second example, phytyl diphosphate is recycled from degraded chlorophyll, to be used for the biosynthesis of vitamin E. Both compounds are recovered as alcohols and require two phosphorylation events to be reactivated and reintegrated into the isoprenoid biosynthetic pathway. This review covers current knowledge of isoprenol biosynthesis, metabolism and function, as well as potential benefits of recycling pathways for plants, with a particular focus on stress responses.