# The Chemistry of Stress: Understanding the 'Cry for Help' of Plant Roots
## 1 Introduction  
Over the last decade, studies on plant metabolites have increased significantly due to substantial technological advances in platforms for metabolomic analyses. These studies have revealed new exciting insights into the chemical diversity of plant metabolites. Plants synthesize more than 200,000 primary and secondary metabolites, including volatile and soluble compounds [1]. While primary metabolism mainly involves compounds important for plant growth, development, and reproduction, specialized metabolism encompasses compounds needed to successfully cope with fluctuating abiotic and biotic stresses. The great diversity of secondary metabolites in plants stems from a limited number of building blocks. These scaffolds are ubiquitous in the majority of plants but differ in a species-specific manner, in enzymatic permutation, and in decoration of their basic structures. For example, one of the most highly diverse and biologically intriguing group of plant secondary metabolites is the terpen, whose the biosynthesis is governed by terpen synthase genes that can generate volatile, semi-volatile and non-volatile derivatives [2]. A single terpene synthase gene can generate many different terpen depending on the linear precursor: geranyl diphosphate (GPP, C\({}_{10}\) monoterpenes), farnesyl diphosphate (FPP, C\({}_{15}\) sesquiterpenes, and C\({}_{30}\) triterpenes) and geranylgeranyl diphosphate (GGPP, C\({}_{20}\) diterpenes, and C\({}_{40}\) tetraterpenes) [3]. Plant secondary metabolites not only have important physiological functions but also a significant impact on plant ecology. By producing particular secondary metabolites, plants can provide detailed information about their physiological state. They can also in such way influence and manipulate the physiologyof neighboring plants and the behaviour of other (micro)organisms [4; 5; 6; 7]. For example, plant terpen such as 1,8-cineole, (\(E\))-\(\beta\)-ocimene, linalool, (\(E\))-\(\beta\)-caryophyllene all play important roles in plant-insect, plant-pathogen and plant-plant interactions [8; 9; 10]. In their natural environment, plants are often exposed to a variety of biotic and abiotic stresses such as drought, salinity, nutrient limitations, pests and pathogens. To withstand these different stresses, plants have developed sophisticated adaptive mechanisms including the production of bioactive secondary metabolites. Some of these metabolites can have direct defensive effects while others can warn neighboring plants to mount their own defenses or lead to the recruitment of beneficial (micro)organisms that minimize the intensity of plant stresses both above-and belowground. It is well-known that, in response to attacks by aboveground herbivores, plants have evolved a \"cry-for-help\" strategy where they recruit beneficial organisms to help to overcome the imposed stresses [11]. For example, the enhanced emission of terpen results in attraction of the natural enemies of herbivores [12]. In most cases, the function of plant metabolites is best studied for aboveground plant responses including defense against biotic and abiotic stresses, as signaling molecules attracting pollinating insects and as plant phytohormones [13]. Recent studies have indicated that plants roots have also evolved a \"cry-for-help\" strategy to recruit beneficial soil (micro)organisms to minimize the damage caused by these stresses [14; 15; 16; 17; 18]. As beneficial soil microbes can help plants to overcome different stresses and improve plant growth, it is crucial for plants to recruit, activate and assemble protective microbiomes. However, studying plant metabolites produced belowground is a challenging task as soils are complex consisting of a heterogeneous matrix of water- and air-filled pores that are recalcitrant to chemical analysis [19; 20]. Nevertheless, chemical communication is likely the most prevalent means in belowground interaction.