3.2  Wireless signal transfer
Plant infochemicals can diffuse around the elicited plant (producer), and are amplified by receiver plants to expand their effective zone. Infochemicals can diffuse distances of a few centimeters to several meters according to their chemical structure and vectors such as mycorrhiza and dodder. Wheat infected by Fusarium culmorumtransfers signals to neighbor plants located 1–3 meters distant. However, there is a negative correlation between distance and response intensity in the receiver plant. Several studies show that receiver plants also propagate signals and act as nodes in inter-plant signaling (Chen, Yang, Chen, Luo, Cui, Yan & Gerwick, 2019a, Piesik, Pańka, Jeske, Wenda-Piesik, Delaney & Weaver, 2013, Wenig et al. , 2019). Thus, neighboring plants amplify signals in clusters of receiver plants. However, the regulation of induced signaling pathways involved in infochemical synthesis in receiver plants requires time, which results in a wave of infochemical synthesis and metabolic responses in receiver plants. Falik, Mordoch, Ben-Natan, Vanunu, Goldstein and Novoplansky (2012) observed that osmotic stress in Pisum sativumactivated stomatal closure in the focal plant and the next three plants in the cluster within 1 h, whereas more distant plants responded after 1 h. In this study, focal plant stomata remained closed for 24 h, whereas receiver plants opened their stomatal apertures because they did not directly encounter osmotic stress. Receiver plants did prepare for imminent osmotic stress, with the ability to return to a naive state if the stress was resolved. Weed plants use stress signals from P. sativum neighbors to improve their acclimation (Falik et al. , 2012).
Wenig et al. (2019) showed that monoterpenes such as α- and β-pinene act as immunity-inducing signals between plants (Figure 3). Systemic acquired resistance (SAR) is an SA-mediated induced resistance in systemic plant tissues that is effective against a broad range of plant pathogens (Wenig et al. , 2019). LEGUME LECTIN-LIKE PROTEIN1 (LLP1) is a predicted lectin that acts in the recognition of SAR signals, which also regulate intra- and inter-plant monoterpene synthesis. In the non-vascular marine alga Pyropia haitanensis , the 1-octone-3-ol plant–plant defense signal can self-stimulate in receiver plants and activate SA hormones (Chen et al. , 2019a).