Secondary metabolic signals

In some scientific circles it is something of a sport to theorize about function, often with the intent of finding one overriding axiom true for all secondary metabolism. Speculations range from the notion that they are waste products or laboratory artefacts, to the concept that they are neutral participants in an evolutionary game, to ideas of chemical weaponry and signalling. 

Essential oil, also defined as essence, volatile oil, etheric oil or aetheroleum, is a complex mixture of volatile constituents biosynthesised by living organisms. Essential oils can be liberated from their matrix by water, steam and dry distillation, or expression in the case of citrus fruits [1-5]. Their occurrence and function in nature is still a question and the subject of ongoing research. However, there is evidence that organisms produce essential oils for defence, signalling or as part of their secondary metabolism. As a consequence essential oils comprise an important bio resource for renewable natural products [1-25]. 

This section reviews the current understanding of interspecific signal transfer mediated by small molecules that stimulate the initiation of secondary metabolism and cell differentiation in Streptomyces and related bacteria. In contrast to the deep understanding of intraspecific hormonal sensing, this research field is yet under development however, several new approaches have started to unveil the unseen nature of cross-talking in this group of bacteria. 

HorinouchiS (1999) 67-Butyrolactones that control secondary metabolism and cell differentiation in Streptomyces. In Cell-cell signaling in bacteria. American Society for Microbiology, Washington DC, p 193... 

Fig. 19. Effect of various signals on secondary metabolism in Catharanthus roseus cells.

For C. roseus suspension-cultured cells, elicitation with fungal elicitors results in the induction of TDC activity (99,186,202,203,284,329-331). This is due to the induction of expression of the Tdc gene. Similarly, SSS activity is induced (202,203,284,329,330). The induction by the Pythium aphanider-matum or yeast elicitor of the transcription of both genes is not affected by cycloheximide that is, the induction is independent of de novo protein biosynthesis, and thus follows an already available signal-transduction chain. The response is quite fast, for the enhanced transcription can already be measured 15 min after elicitation (202,203). Also, the NADPH cytochrome P-450 reductase mRNA level is induced by elicitation with fungal elicitors (113). Moreno et al (99,151) measured activities of a number of enzymes involved in secondary metabolism in C. roseus before and after elicitation with a P. aphanidermatum preparation. GlOH activity was found to be slightly decreased by elicitation and IPP-isomerase showed similar behavior. The pattern of terpenoids formed by the crude enzyme extracts from elicited and nonelicited cells was different. The total incorporation decreased, that is, the activities of the enzymes of the terpenoid pathway were lower. The relative incorporation decreased particularly for squalene. 

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