# Origin and Function of Structural Diversity in the Plant Specialized Metabolome
## 1 Introduction  
With the development of new metabolomics techniques, the improvement of detection methods and, in parallel, the ever-expanding number of metabolomics profiling studies performed on different plant species or plants subjected to different treatments or mutations, the number of known specialized plant metabolites is continuously increasing, hence leading to the impression that the number of different structures is infinite [1; 2; 3; 4; 5; 6; 7; 8]. An aspect of plant biology that is not understood yet is the benefit of this huge structural diversity for plants. It is widely accepted today that specialized metabolites contribute to the chemical defense of plants, as sessile organisms without an adaptive immune system, against different forms of biotic and abiotic stress [9; 10]. In that context, it could be tempting to pursue a specific biological function for each distinct metabolite. Certain metabolites seem, indeed, to be the result of a co-evolution with an acceptor, and others play an obvious role as UV-screen or antioxidant [11; 12; 13; 14]. The principles of evolution tell us that wasteful biosynthetic pathways will be selected against and are prone to disappear, whereas eachaddition to a biosynthetic pathway can only be maintained by selection if it enhances the fitness of the plant [15]. Logically, such constraints should lead to a rather limited number of optimized defense compounds. The plethora of structurally similar yet distinct compounds found in each plant species' specialized metabolome seems, in that context, to form an evolutionary inconsistency. The \"Screening Hypothesis\", which was first described in 1991, assumes that most specialized metabolites produced by a plant do not possess biological activity [16]. According to this theory, natural selection favored plants that produce the largest possible chemical diversity, at the lowest possible cost. The greater the chemical diversity of the specialized metabolite pool, the greater the chance that it contains a useful, biologically active compound that can protect the plant. The main tool to obtain maximal structural diversity at minimal cost consists of matrix grid biosynthetic pathways in which a limited number of enzymes with a broad substrate tolerance participate in the production of a high diversity of compounds [17]. Well-known examples of such grid biosynthetic pathways are those leading to the production of carotenoids, anthocyanins, monolignols and glucosinolates. The \"Screening Hypothesis\" sees the specialized metabolome mainly as a plant defense system against different types of stress [18]. In contrast, another hypothesis, which was formulated twenty years later, states that the first and major role of the specialized metabolome lies in its participation in the redox chemistry, particularly the reduction in reactive oxygen species (ROS) and, in the absence of ROS, the reduction in molecular oxygen to H\({}_{2}\)O\({}_{2}\) by the exact same compounds [18]. This hormite effect (antagonistic effects of low and high doses) in the redox chemistry is seen as an essential characteristic of the specialized metabolome, making it involved in both regulatory and destructive processes: low levels of ROS stimulate growth, cell differentiation and stress resistance, whereas high levels induce programmed cell death [18]. Mainly phenolics, but also terpenoids and alkaloids, function as electron donors or acceptors in electron transfer reactions, and often possess both pro- and antioxidant activity, depending on their concentration and the redox environment. The high structural diversity among metabolites undergoing similar redox reactions suggests that the three-dimensional structure of the carbon skeleton does not determine the redox activity, but that these reactions are enabled by the presence of specific functional groups.