3.4  Acoustic and electric signals
Plants utilize acoustic and electric signals as internal and inter-plant signals. A sound vibration signal can be generated by a herbivore walking on the plant, breaking trichomes, chewing the plant, and even by water stress (Caicedo-Lopez, Contreras-Medina, Guevara-Gonzaleza, Perez-Matzumotob & Ruiz-Ruedab, 2020, Kollasch, Abdul‑Kaf, Body, Pinto, Appel & Cocroft, 2020). Pest species might be discriminated based on the vibration frequency they produce (Kollasch et al. , 2020).
Perception of a sound vibration signal modifies plant epigenetics, transcriptome, proteome, and metabolome (Ghosh, Mishra, Choi, Kwon, Won Bae, Park & Bae, 2016, Jung, Kim, Jung, Jeong & Ryu, 2020). Sound vibration signal perception modulates defense hormones such as SA, leading to activation of MAPKs, MYBs, and transcription factors (Body, Neer, C., Lin, Vu & Cocroft, 2019, Ghosh et al. , 2016). This upregulation of key enzymes and secondary metabolites, including catalase and PAL, increases the biosynthesis of phenols, alkaloids, terpenes, and oxylipin-derivative volatile organic compounds (VOCs) (Body et al. , 2019, Ghosh et al. , 2016, Kollasch et al. , 2020). These metabolites enhance plant resistance or act as signals in plant–plant communication. Jung et al. (2020) reported that sound vibration induces resistance against the root pathogen Ralstonia solanacearum by modulating cytokinin signaling, increasing aliphatic glucosinolate biosynthesis through epigenetic DNA methylation by H3K27me3, and improving cell wall reinforcement by downregulating miR397b suppression of lignin accumulation–related transcripts. Vibration sensing is an evolutionarily ancient system that arose before vascular plants emerged, as microalga also have mechanosensory proteins that respond to vibration (Paika, Jinb, Simc & N.L., 2018).
Electric signals, primarily Ca2+ signaling, have important roles in intra- and inter-plant signaling (Choi, Miller, Wallace, Harper, Mittler & Gilroy, 2017, de Toledo, Parise, Simmi, Costa, Senko, Debono & Souza, 2019, Simmi, Dallagnol, Ferreira, Pereira & Souza, 2020). The oomycete pathogen Pythium aphanidermatumexploits electric signals to target host roots (Van West, Morris, Reid, Appiah, Osborne, Campbell, Shepherd & Gow, 2002). Both acoustic and electric signals have high transmission speed and good potential for use in precision agriculture (Choi et al. , 2017, Kollasch et al. , 2020).