New Synthesis—Systems Chemical Ecology

Abstract

Secondary metabolites are enigmatic in terms of costs and benefits for their producers and how their ontogenetic accumulation or timely induction affects the evolution of single species and co-evolution of interacting species. More than 50 years ago, Gottfried Fraenkel dispelled the notion of secondary metabolites as waste products: “Thus, the animal world which surrounds the plant is deeply influenced not only by their morphology, but also by their chemistry”. Today, in the post-genomic era, with huge amounts of data generated by functional genomic, metabolomic, and proteomic studies, the emerging picture of secondary metabolites is more complex. Also, the recognition has grown that one of the ultimate challenges in the life sciences is not just to understand component parts, but rather the systems comprised of these parts. This has led to the establishment of a new discipline - systems biology which advocates an integrative rather than reductionist approach. The relations between plants and the microbes and herbivores that colonize and eat them once were described as a “wobbling triangle” of interactions that occur both below- and above ground. In this scenario, a reliable and flexible signalling system is mandatory to coordinate gene expression in separate tissues accordingly. Present knowledge suggests that in plants, as in other organisms, co-ordinated redox chemical reactions between reactive oxygen species (ROS) and hormones represent the upstream part of this signalling system which, further downstream, is continued by MAP kinases, helping to maintain the homeodynamics of micro- and macromolecules required during ontogenesis (Mittler et al., 2011). One important recognized component is retrograde (organelle to nucleus) signalling. Abiotic and biotic stresses impair the functionality of electron transport chains in chloroplasts and mitochondria. As a consequence, instead of four electrons that are required to reduce molecular oxygen to water, incorrectly transferred single electrons lead more quicklytothe formation of superoxide anion radicals, O2 – , than upregulated enzyme expression, e.g. NADPH oxidase, which is involved in systemic ROS production (Kerchev et al., 2011). So far, metabolomic studies have revealed extensive reprogramming ofprimary metabolites in context with retrograde signalling. The fact that no single cytosolic component yet has beenidentified unequivocally as a retrograde signal induced Thomas Pfannschmidt (2010) to ask a heretical, albeit justified question: ”Maybe a single metabolite is not sufficient to work as a signal, but what about a metabolite signature?” .I n terms of a systems biological approach, we may also consider whether secondary metabolites function as ensemble components of a metabolite signature. Although GC–MS is far from ideal for detecting non-volatile