Plants modulate their defence signalling networks in response to various biotic stresses via inter-organ communication. Song et al. evaluate the root-mediated transmission of systemic acquired resistance (SAR) in response to soil-borne and air-borne plant pathogens.

The SAR-triggering chemical benzothiadiazole (BTH) induces a SAR signal which is transmitted through the root-to-root system from SAR-induced plants to neighbouring plants subjected to local chemical and pathogenic bacteria. Salicylic acid (SA) is shown to be a major signal molecule in this SAR transmission through the root system.
This paper is part of the Root Biology Special Issue.
Reference
Geun Cheol Song, Hee-Jung Sim, Sang-Gyu Kim, Choong-Min Ryu, 2016, 'Root-mediated signal transmission of systemic acquired resistance against above-ground and below-ground pathogens', Annals of Botany, vol. 118, no. 4, pp. 821-831 http://dx.doi.org/10.1093/aob/mcw152