# The Chemistry of Stress: Understanding the 'Cry for Help' of Plant Roots
## 4 Chemistry of \"Volatile Affairs\" on Plant Roots
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Stress Type** & **Plant Species** & **Type of Exudation** & **Role of Exudate in Plant Defence** & **Reference** \\ \hline \multirow{6}{*}{Iron deficiency} & \multirow{6}{*}{_Arabidopsis thaliana_} & \multirow{6}{*}{Coumarin scopoletin} & Indirect via & \multirow{6}{*}{[53]} \\  & & & recruitment of & \\  & & & several bacterial & \\ \cline{1-1}  & & & genera having plant & \\ \cline{1-1}  & & & growth promoting & \\ \cline{1-1}  & & & properties & \\ \cline{1-1}  & & & Indirect via recruitment of _Bacillus_ & \\ \cline{1-1}  & & & _flexus_ KLBPM 491 able to enhance & \\ \cline{1-1}  & & & plant growth under salinity stress & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: \(Cont.\)approximately 12 cm. Furthermore, these recruited bacteria were able to inhibit the growth of _F. culmorum_[58]. Recently, a unique olfactometer-choice assay was applied to assess the migration of selected beneficial bacteria towards the roots of healthy and infected tomato plants in a soil system [59]. The study revealed that the infection of tomato plants with the fungal pathogen _Fusarium oxysporum_ alters the root VOCs profile. The infected plant roots emitted VOCs such as benzonitrile, benzothiazazole, dimethyl trisulfide, formic acid and a terpenelike compound, which are well-known for their antifungal activities. Interestingly, the infected and healthy plant roots did not show significant difference in the attraction of bacteria with biocontrol properties. However, these results were obtained only based on few selected bacteria; therefore, follow-up studies should be performed using total microbial communities to reveal whether the attraction of bacteria is significantly different between healthy and infected plant roots. Together, these findings suggest a novel strategy by which stressed plants can recruit and activate from distance soil microbes into the rhizosphere facilitating their adaptation. However, the role and mechanisms of root VOCs in the assemble and function of rhizosphere microbiome remain largely unexplored. Interestingly, both plant-beneficial and plant-pathogenic microbes can modify the plant VOCs profile. For example, the pretreatment of faba bean (_Vicia faba_ L.) plants with symbiotic arbuscular mycorrhizal fungi (AMF) reduced the release of sesquiterpenes [70]. The AMF suppressed emission of the sesquiterpenes (_E_)-caryophyllene and (_E_)-\(\beta\)-farresene, and aphid attractiveness to VOCs was negatively associated with the proportion of sesquiterpenes in the sample. Hence, the AMF have a key bottom-up role in insect host location by increasing the attractiveness of aphids to plant VOCs. Recent study revealed that the plant-beneficial _Pseudomonas putida_ induced the production of indole and \(\beta\)-caryophyllene VOCs in maize plants and triggered ISR against the maize anthracnose fungus _Colletotrichum graminicola_[50]. Furthermore, microbe induced plant VOCs can interfere with plant-to-plant communication. Recently, the effect of volatiles on microbial communities and neighboring plants was investigated in tomato plants (_S. lycopersicum_ L.) inoculated with plant growth-promoting rhizobacterium _Bacillus amyloliquefaciens_[71]. The study revealed that tomato plants treated with bacteria released \(\beta\)-caryophyllene, which elicited the release of a large amount of salicylic acid in the root exudates of neighboring tomato plants affecting their rhizosphere microbiome. Hence, plants are able via microbe-induced plant VOC production to effect the rhizosphere microbiome of neighboring plants from a distance. Considering the importance of root-emitted VOCs (next to the water-soluble exudates) on the assembly of protective-microbione under stress conditions, future studies should elucidate more on the specificity or generality of root-emitted VOCs under different stresses, and investigate whether such volatile emissions can mediate the recruitment of beneficial microbes and activation of their beneficial traits.