Pisum sativum (garden pea) plants communicate stress cues via their roots to allow neighboring unstressed plants to anticipate an abiotic stressor. Pea plants are commonly grown in temperate regions throughout the world. However, this adaptation allows plants to anticipate abiotic stresses such as drought. In 2011, Falik et al. tested the ability of unstressed pea plants to sense and respond to stress cues by inducing osmotic stress on a neighboring plant. Falik et al. subjected the root of an externally-induced plant to mannitol in order to inflict osmotic stress and drought-like conditions. Five unstressed plants neighbored both sides of this stressed plant. On one side, the unstressed plants shared their root system with their neighbors to allow for root communication. On the other side, the unstressed plants did not share root systems with their neighbors.
Falik et al. found that unstressed plants demonstrated the ability to sense and respond to stress cues emitted from the roots of the osmotically stressed plant. Furthermore, the unstressed plants were able to send additional stress cues to other neighboring unstressed plants in order to relay the signal. A cascade effect of stomatal closure was observed in neighboring unstressed plants that shared their rooting system but was not observed in the unstressed plants that did not share their rooting system. Therefore, neighboring plants demonstrate the ability to sense, integrate, and respond to stress cues transmitted through roots. Although Falik et al. did not identify the chemical responsible for perceiving stress cues, research conducted in 2016 by Delory et al. suggests several possibilities. They found that plant roots synthesize and release a wide array of organic compounds including solutes and volatiles (i.e. terpenes). They cited additional research demonstrating that root-emitted molecules have the potential to induce physiological responses in neighboring plants either directly or indirectly by modifying the soil chemistry. Moreover, Kegge et al. demonstrated that plants perceive the presence of neighbors through changes in water/nutrient availability, root exudates, and soil microorganisms.
Although the underlying mechanism behind stress cues emitted by roots remains largely unknown, Falik et al. suggested that the plant hormone abscisic acid (ABA) may be responsible for integrating the observed phenotypic response (stomatal closure). Further research is needed to identify a well-defined mechanism and the potential adaptive implications for priming neighbors in preparation for forthcoming abiotic stresses; however, a literature review by Robbins et al. published in 2014 characterized the root endodermis as a signaling control center in response to abiotic environmental stresses including drought. They found that the plant hormone ABA regulates the root endodermal response under certain environmental conditions. In 2016 Rowe et al. experimentally validated this claim by showing that ABA regulated root growth under osmotic stress conditions. Additionally, changes in cytosolic calcium concentrations act as signals to close stomata in response to drought stress cues. Therefore, the flux of solutes, volatiles, hormones, and ions are likely involved in the integration of the response to stress cues emitted by roots.