# The Chemistry of Stress: Understanding the 'Cry for Help' of Plant Roots
## 2 Root Exudate Chemistry of Plants Exposed to Abiotic and Biotic Stresses  
Plants release a significant fraction of their photosynthetically fixed carbon belowground in the form of root exudates [21], consisting of a diverse array of volatile and non-volatile compounds [22; 23]. Shaped through a long evolutionary process, the exudation of root metabolites is among plants' sophisticated strategies to survive in changing environments. External stress stimuli such as pathogen and pest attacks, heavy metal contamination, as well as nutrient and/or water limitation can lead to modification of carbon allocation belowground. Both the quantity and composition of root exudates can change upon plant exposure to different external stress factors [15; 24]. Intriguingly, such stress-driven alteration in exudate profiles can directly enhance plants' survival. For instance, increased secretion of organic acids, particularly malate has been observed in soybean plants grown under phosphorus (P) starvation [25; 26]. Similarly, the response of roots of white lupine (_Lupinus albus_ L.) to soil P deprivation induces the production of carboxylate [27; 28]. Organic acids in the group of di-and tricarboxylic acids such as malate, carboxylate, and oxalate can replace inorganic phosphate (Pi) bound in insoluble P-metal complexes either via metal ion chelation or by anion exchange, leading to P mobilization and increased P uptake by roots [29; 30; 31]. Exudation of low molecular-weight organic acids (LMWOA) into the rhizosphere has also been reported as a survival strategy of plants to improve nutrient acquisition in metal-contaminated soils. For example, in acidic soils with high aluminum (Al) concentrations, roots of _Glycine max_ (soybean) and _Zea mays_ (maize) secrete citrate with a strong metal chelating capability, thereby reducing the uptake of harmful Al\({}^{3+}\) by the plants [26]. Furthermore, under moderate drought conditions, roots can produce mucilage, a polysaccharide acting as lubricant to facilitate root movement through dry soils [32]. In addition to this, increased levels of the osmolytes proline and pinitol were found in roots of soybean exposed to drought [33]. These compounds can maintain cell turgor via active osmoregulation, thereby increasing plants' survival amidst water scarcity [33]. Another class of compounds secreted by plants in response to different stresses are the phenolics. For instance, roots of _Arabidopsis thaliana_ grown in iron (Fe)-deficient soils (due to high pH) produce the coumarin scopoletin that reduces Fe\({}^{3+}\) to Fe\({}^{2+}\), thereby enhancing Fe-bioavailability to plants upon alkaline stress [34; 35]. Furthermore, the growth of _Arabidopsis_ mutants lacking 2-oxoglutarate-dependent dioxygenase Feruloyl-CoA 69-hydroxylase1 (F6\({}^{\prime}\)H1), an enzyme involved in scopoletin biosynthesis, was significantly impaired in growth on synthetic agar media supplemented with low Fe [36]. Increased secretion of phenolic compounds was also shown for roots of barley plants (_Hordeum vulgar_) infected with the soil-borne pathogenic fungus _Fusarium germinearum_[14]. Among these compounds was the antifungal \(t\)-cinnamic acid [14]. Similarly, the antifungal rosmarinic acid was secreted by roots of sweet basil (_Ocinum basilicum_) upon infection by the oomycete pathogen _Pythium ultimum_[15]. Increased accumulation of the non-volatile terpenoids zealexins and kauralexins was found for roots of maize following infection of soil-borne fungus _Fusarium verticillioides_ and the belowground herbivore _Diabrotica balteata_[37]. Interestingly, the study also showed that plants deficient in kauralexins production (_an2_ mutant) were more sensitive to drought. These results suggest that the terpenoids, apart from their role in direct defense against pests and pathogens, are associated with drought tolerance in maize plants. In another study, the semi-volatile diterpene rhizathalene was produced by _Arabidopsis_ roots exposed to the root-feeding insect (_Bradysia_) [38].