# The Chemistry of Stress: Understanding the 'Cry for Help' of Plant Roots
## 4 Chemistry of \"Volatile Affairs\" on Plant Roots  
Apart from soluble compounds, roots release various volatile organic compounds (VOCs) into the rhizosphere. It is estimated that VOCs account for approximately 1% of the total secondary metabolites released by roots [61]. Plant VOCs are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid and amino acid derivatives [9]. Although VOCs are considered as minor component of root exudates, root VOCs hold a significant role in plant stress resilience. In general, VOCs have low molecular weight, with a lipophilic character and low boiling points [62]. Due to these physicochemical properties, root-emitted VOCs can easily diffuse via both air- and water filled pores in the soil and therefore, can cover long-distance chemical interactions. In the last years, the realization that VOCs play integral part in the belowground interactions has increased research attention in analysis of belowground VOCs [63]. Similar to soluble compounds, belowground plant VOCs can serve as direct and indirect plant defenses. For example, VOCs emitted from glucosinolate or cyanogenic glycoside conversion (such as cyanides and isothiocyanates) may serve as direct plant defenses as they are toxic to a wide range of belowground herbivores and pathogens [64; 65]. Plant root VOCs can play important role as indirect plant defenses, e.g., attracting natural enemies or predators. One of the first studies on the indirect defenses via VOCs function belowground revealed that entomopathogenic nematodes were attracted to the roots of _Thuja occidentalis_ damaged by larvae of the black vine weevil (_Otiorhynchus sulatus_) when given a choice in a Y-tube olfactometer filled with sand [66]. In another study, maize roots damaged by the rootworm _Diabrotica virifera virifera_ emit the sesquiterpene (E)-\(\beta\)-caryophyllene which attracts entomopathogenic nematodes that infest and kill the root-feeding larvae [67]. Root VOCs can act as both signaling molecules and nutrient sources for soil microbes [68], and hence might influence the assembly and proliferation of root-associated microbiome. A microcosm experiment showed that VOC-derived carbon released (in the headspace) during decomposition of \({}^{13}\)C-labelled leaf litter accounted for fractions in microbial biomass (located separately from the decomposition site), suggesting that VOCs can be carbon sources for soil microbes [69]. In another study, gaseous ethylene produced by peanut roots (as a response to cyanide released by neighboring cassava plants) altered microbial composition of peanut rhizosphere by shifting the abundance of actinobacterial species, resulting in improved seed production [60]. This finding indicates that plants response to an environmental stimulus (cyanide-derived signal) via production of root-VOCs leading to the assembly of rhizosphere microbial community. Evidence is available that under a biotic stress situation, root VOCs are involved in the recruitment of beneficial bacteria into the rhizosphere. This phenomenon had been clearly shown in a recent study by [PERSON] et al. [58] who found that upon the infection by _Fusarium culmorum_, the root of sand sedge plant (_Carex arenaria_) emitted a specific blend of VOCs (i.e., including the monoterpene (\(Z\))-limonene oxide) which clearly differed from the healthy plants. Interestingly, this specific blend of VOCs was able to attract certain beneficial bacteria within a synthetic bacterial community from a distance of