Evolutionary History of Plants - Evolution of Secondary Metabolism

Evolution of Secondary Metabolism

Secondary metabolites are essentially low molecular weight compounds, sometimes having complex structures. They function in processes as diverse as immunity, anti-herbivory, pollinator attraction, communication between plants, maintaining symbiotic associations with soil flora, enhancing the rate of fertilization etc., and hence are significant from the evo-devo perspective. The structural and functional diversity of these secondary metabolites across the plant kingdom is vast; it is estimated that hundreds of thousands of enzymes might be involved in this process in the entire of the plant kingdom, with about 15–25% of the genome coding for these enzymes, and every species having its unique arsenal of secondary metabolites. Many of these metabolites are of enormous medical significance to humans.

What is the purpose of having so many secondary metabolites being produced, with a significant chunk of the metabolome devoted to this activity? It is hypothesized that most of these chemicals help in generating immunity and, in consequence, the diversity of these metabolites is a result of a constant war between plants and their parasites. There is evidence that this may be true in many cases. The big question here is the reproductive cost involved in maintaining such an impressive inventory. Various models have been suggested that probe into this aspect of the question, but a consensus on the extent of the cost is lacking. We still cannot predict whether a plant with more secondary metabolites would be better off than other plants in its vicinity.

Secondary metabolite production seems to have arisen quite early during evolution - even bacteria possess the ability to make these compounds. But, they assume more significant roles in life from fungi onwards to plants. In plants, they seem to have spread out using different mechanisms, like gene duplications, evolution of novel genes etc. Furthermore, studies have shown that diversity in some of these compounds may be positively selected for.

Although the role of novel gene evolution in the evolution of secondary metabolism cannot be denied, there are several examples where new metabolites have been formed by small changes in the reaction. For example, cyanogen glycosides have been proposed to have evolved multiple times in different plant lineages. There are several such instances of convergent evolution. For example, enzymes for synthesis of limonene – a terpene – are more similar between angiosperms and gymnosperms than to their own terpene synthesis enzymes. This suggests independent evolution of the limonene biosynthetic pathway in these two lineages.