When I took Plant Physiology as an undergraduate twenty years ago, we used Taiz and Zeiger's 1991 first edition text of the same name. The chapter on terpenoids was written by Jonathan Gershenzon, who at the time was a lab director for Prof. Rodney Croteau at the Institute for Biological Chemistry at Washington State University. Reading that chapter changed my life, as I became interested in this family of natural products as a result and later completed my PhD in Rod's lab. Later as apost-doc I went to work for Jonathan, who by then had become a Director at the Max Planck Institute for Chemical Ecology in Jena, Germany. It was a profoundly rewarding experience. In fact, I am still studying plant terpenoid biosynthesis as a struggling independent researcher in the harsh landscape of Spain in the age of austerity. It is a profoundly difficult experience. All of those experiences, both good and challenging, are indirectly the result of reading that influential chapter as an undergraduate and feeling a fascination for a subject that no other really compared to at that time.
|The 1991 edition of Taiz and Zeiger back when terpenoids got some respect|
Today I received in the mail, after months of waiting on a pre-order, the sixth edition of this textbook, now titled "Plant Physiology and Development" and featuring Licoln Taiz, Eduardo Zeiger, Ian Max MØller, and Angus Murphy as its authors. (Check out the publisher's page here for details or just go to Amazon).
|Taiz, Zeiger et al. 6th edition: more development and less metabolism|
Glancing through its contents, and comparing them to the contents of the 1991 edition (still sitting on my shelf), it seems that one subject matter is virtually unrecognizable when laid against the other. The field of plant physiology has changed a lot, and our understanding of how plants work now is many levels ahead of where we were twenty years ago. Still, lifetimes of fascinating questions remain.
Strangely, isoprenoids (another name for terpenoids) seem to have been eliminated completely from this new edition. Looking through the list of contributors, it is true that none of them seem to be connected to isoprenoid research. The plastidic pathway for isoprenoid biosynthesis, commonly known as the 2C-methylerythritol 4-phosphate (MEP) pathway, is never even mentioned directly. The closest they come is in chapter 15, where it is noted in a figure that abscisic acid is a carotenoid cleavage product that is derived from "the terpenoid pathway". Never mind that there are two pathways for making terpenoids in plants and neither is discussed. I guess carotenoids, giberellins, cytokinins, chlorophyll and plastoquinone membrane anchors and most of the plant volatiles released into the environment as hemi and monoterpenes just aren't that important (same for the origin of all cytosolic isoprenoids). To be fair, they do mention terpenes in chapter 23 (Biotic Interactions), but no mention is made of where they come from or why plants, in addition to the mevalonate pathway used by nearly all eukaryotes, inherited a biochemically distinct pathway for making the universal isoprenoid precursors isopentenyl and dimethylallyl diphosphate (IPP and DMAPP). But therein lies one of the greatest stories of evolution: the acquisition of cyanobacterial cells by protoplants and the emergence of plastids capable of photosynthesis within the protective environment of plant cells. But plastids do a lot more than photosynthesis. When plants acquired early plastids, they inherited a completely separate biochemical pathway for making IPP and DMAPP, a prokaryotic pathway, that ran parallel to a pathway in the cytosol that made the same end products (the eukaryotic or mevalonate pathway). In turns out the pathway in the plastid is much more efficient at cranking out huge quantities of isoprenoid precursors since it is directly connected to the output of the Calvin cycle (plus a bit a of pyruvate from the cytosol, but that gets complicated). The elucidation of the MEP pathway for isoprenoid biosynthesis in plants (and some bacteria) has taken place over the last twenty years and was hailed as a technical triumph ushered in by the age of proteomics and genomics. The study of its complex light and biotic stress related regulation now occupies scores of highly productive labs around the world.
The complete absence of isoprenoid research in this edition was a bit disappointing. At least in my research field, the new edition of Taiz and Zeiger does not appear to have kept up with the progress these labs have made. Or perhaps natural products like isoprenoids, even essential ones, are no longer sufficiently in vogue to merit space in a newer textbook next to all the other exciting things happening in plant science. And to be fair, there are a lot and space is limited. To the author's credit, they kept the price reasonable compared to similarly comprehensive undergraduate texts (<$100), but that means they have to be selective with content. I will take a closer look and see what else this new text has to offer.