The regulation of isoprenoid synthesis
In plants, isoprenoids are synthesized in different organs and tissues according to the specific demand. Large amounts of hydrophobic isopre-noids are synthesized in specialized tissues such as the glandular and epidermis cells of leaves and the osmophores of flowers. The enzymes for synthesis of isoprenoids are present in the plastids, the cytosol, and the mitochondria. Each of these cellular compartments is essentially self-sufficient with respect to its isoprenoid content. Some isoprenoids, such as the phytohormone gibberellic acid, are synthesized in the plastids and then supplied to the cytosol of the cell. The various prenyl pyrophosphates, from which all the other isoprenoids are derived, are synthesized by different enzymes.
This spatial distribution of the synthetic pathways makes it possible that, despite their very large diversity, the different isoprenoids synthesized by basically similar processes, can be efficiently controlled in their rate of synthesis via regulation of the corresponding enzyme activities (e.g., terpene synthases) in the various compartments. Results so far indicate that the syn-thesis of the different isoprenoids is regulated primarily at the level of gene expression. This is especially obvious when, after infections or wounding, the isoprenoid metabolism is very rapidly activated by elicitor-controlled gene expression. Competition may occur between isoprenoid synthesis for maintenance and for defense. In tobacco, for instance, the fun-gal elicitor induced phytoalexin synthesis blocks steroid synthesis. In such a case, the cell focuses its capacity for isoprenoid synthesis on defense.