METABOLIC COSTS OF TERPENOID ACCUMULATION IN HIGHER-PLANTS

The net value of any plant trait can be assessed by measuring the cost s and benefits associated with that trait. While the other contributor s to this issue examine the possible benefits of terpenoids to plants, this article explores the metabolic costs of terpenoid accumulation i n plants in the light of recent advances in terpenoid biochemistry. Te rpenoids are more expensive to manufacture per gram than most other pr imary and secondary metabolites due to their extensive chemical reduct ion, The enzyme costs of making terpenoids are also high since terpeno id biosynthetic enzymes are apparently not shared with other metabolic pathways. In fact, plant cells may even possess mom than one set of e nzymes for catalyzing the basic steps of terpenoid formation. Terpenoi ds are usually sequestered in complex, multicellular secretory structu res, and so storage costs for these substances are also likely to be s ubstantial. However, not all of the processes involved in terpenoid ac cumulation require large investments of resources. For instance, the m aintenance of terpenoid pools is probably inexpensive because there is no evidence that substantial quantities of terpenes are lost as a res ult of metabolic turnover, volatilization, or leaching. Moreover, plan ts may reduce their net terpenoid costs by employing individual compou nds in more than one role or by catabolizing substances that are no lo nger needed, although it is still unclear if such practices are widesp read. These findings (and other facets of terpenoid biochemistry and p hysiology) are discussed in relation to the assumptions and prediction s of several current theories of plant defense, including the carbon-n utrient balance hypothesis, the growth-differentiation balance hypothe sis, and the resource availability hypothesis.