# The Chemistry of Stress: Understanding the 'Cry for Help' of Plant Roots
## 3 Chemistry of Microbial Recruitment by Roots of Plants under Siege  
Root-associated microorganisms are essential for plant growth and health. Past and present plant microbiome studies have indicated that root microbiota are not merely passengers, but instead, they improve plant immune functions [42] and enhance plant resilience to biotic and abiotic stresses [43]. In this context, it has been postulated that upon pathogen or pest attacks, plants change their root chemistry to actively recruit beneficial microbiota to facilitate adaptation and/or protection to the stresses, a phenomenon referred to as \"cry for help\" [14]. This phenomenon has been elegantly depicted in an early study where the infection of _Arabidopsis_ leaves by _Pseudomonas syringae pv tomato_ (_Pst_) attracted the beneficial bacterium _Bacillus subtilis_ FB17 to the roots, that in turn triggered systemic resistance against subsequent infections by _Pst_[44]. Similarly, infection of _Arabidopsis_ leaves by the downey mildew pathogen _Hyalopernospora arabidopsis_ resulted in the enrichment of specific bacterial genera in the rhizosphere, in particular _Microbacterium_ sp., _Stentrophomonas_ sp., and _Xanthomonas_ sp. [45]. When applied to the soil individually or in mixture, these microbes were able to significantly reduce mildew incidence via induced systemic resistance (ISR) [45]. In a recent study, the belowground \"cry for help\" concept was also supported by results from a field experiment, where durum wheat (_Triticum turgidum_ L. var _durum_) naturally infected by the crown-rot pathogen _Fusarium gramimearum_  
Figure 1: Schematic representation of the direct role of root exudates in plant resistance against biotic and abiotic stresses; drought and salinity (A), nutrient deficiency (B), and belowground pathogens and pests (C). Some specialized metabolites such as prolines, coumarins and organic acids can promote plant growth under abiotic stress conditions (i.e., drought, salinity, and nutrient deficiency) either via improved nutrient/mineral acquisition or active root osmoregulation. Meanwhile, upon a particular biotic stress, specialized root exudates such as phenolic compounds, non-volatile terpenoids, volatile terpen and sulfurous compounds (i.e., dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS)) are released and can directly inhibit the growth of invading soil-borne pathogens and pests. This figure was designed with Biorender ([[https://www.biorender.com](https://www.biorender.com)]([https://www.biorender.com](https://www.biorender.com)), accessed on 18 April 2021). enriched for _Stentotrophomonas rhizophila_ (SR80) in the rhizosphere and root endosphere [16]. Upon re-introduction, strain SR80 was able to induce resistance against the crown-rot disease and enhance wheat growth [16]. Evidence is mounting that the recruitment of beneficial microbiota by plants under siege is, at least in part, driven by changes in the exudate profiles. These studies include the early work on malic acid secreted by _Arabidopsis_ roots upon _Pst_ infection [44]. This was further supported in the follow-up study showing that _Pst_ infection enhanced the expression of Aluminum-Activated Malate Transporter1 (MLT1) which became the key regulator for the recruitment of _Bacillus subtilis_ FB17 on _Arabidopsis_ roots following foliar _Pst_ infection [46]. In another study, local infection of cucumber roots by _Fusarium oxysporum_ f.sp. _cucumerinum_ increased tryptophan but reduced raffinose exudation; these changes enhanced root colonization by the beneficial bacterium _Bacillus amyloliquefaciens_ SQR9 [47]. Furthermore, increased exudation of the fatty acid oxylipin was shown in tomato roots exposed to various stresses (wounding, salt, pathogen attack), in which the compound acted as a chemoattractant for the biocontrol fungus _Trichoderma harzianum_[48]. Benzoxazi-noids, a class of defensive secondary metabolites commonly released by maize roots upon herbivory attacks can recruit the beneficial bacterium _Pseudomonas putida_ KT2240 [49].