The Chemistry of Stress: Understanding the 'Cry for Help' of Plant Roots

Abstract

Plants are faced with various biotic and abiotic stresses during their life cycle. To withstand these stresses, plants have evolved adaptive strategies including the production of a wide array of primary and secondary metabolites. Some of these metabolites can have direct defensive effects, while others act as chemical cues attracting beneficial (micro)organisms for protection. Similar to aboveground plant tissues, plant roots also appear to have evolved "a cry for help" response upon exposure to stress, leading to the recruitment of beneficial microorganisms to help minimize the damage caused by the stress. Furthermore, emerging evidence indicates that microbial recruitment to the plant roots is, at least in part, mediated by quantitative and/or qualitative changes in root exudate composition. Both volatile and water-soluble compounds have been implicated as important signals for the recruitment and activation of beneficial root-associated microbes. Here we provide an overview of our current understanding of belowground chemical communication, particularly how stressed plants shape its protective root microbiome.

Keywords: abiotic and biotic stresses; cry-for-help; plant-microbe interactions; root exudates; volatiles.

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 terpenes 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, accessed on 18 April 2021).

Figure 2

Schematic overview of how plants under siege can attract via exudation of specialized volatile and water-soluble root exudates, beneficial microorganisms which in turn can enhance plant fitness (right panel). At the same time, the exudates can also be used by scavengers such as belowground pathogens and pests as chemical information to locate and benefit from the stressed plants, leading to induced susceptibility to stresses (left panel). Furthermore, the biotic and abiotic stresses such as drought, salinity, nutrient limitations and pathogens, pests are stress factors not only for the plants but also for the root-associated microbiome (right panel) which can provide early warning and protection to the plant. This figure was designed with Biorender (https://www.biorender.com, accessed on 18 April 2021).