8. R.D. Firn & C.G.Jones (2004) The evolution of plant biochemistry and the implications for physiology. In "The Evolution of Plant Physiology". Ed.
Some biochemical pathways are common to most plants ("primary metabolism"). Other pathways are widely found but are not universal (e.g. some parts of the carotenoid or lipid biosynthetic pathways). There are also minor branches leading from these pathways which are found only in a very small number of species ("secondary metabolism"). Why is plant metabolism like this? The Jones-Firn model to explain why plants and microbes produce so many "secondary metabolites" is based on the fact that the chances of any molecule possessing potent biological activity is very low. This model has now been extended by considering what other properties new molecules could bring to their producer. Two new propery classes are defined and it is proposed that selection would shape metabolism depending on the type of property. Furthermore, because particular properties will be associated with individual molecules, and not with particular pathways, it is predictable that many pathways will be multifunctional. By considering the evolution of regulatory systems controlling such pathways it becomes clear that such regulatory systems will have been shaped by the underlying multifunctionality of some pathways¬ with the result that "cross talk" is inevitable. An appreciation of these constraints may help those seeking to understand any physiological process that involve biologically active molecules (for example plant hormones or compounds involved in a plant's response to insects, fungi or bacteria).