Figure 3.

Overview of long distance signals in plants. Long-distance signals include electrical, hydraulic, and chemical signals. Electrical signals found in plants include: slow wave potentials (SWPs) propagated in the functional xylem; action potentials (APs) initiated in the phloem; system potentials (SPs) propagated in the apoplast following mechanical perturbations or wounding; and wound potentials (WPs) through changes in cell turgor leading to plasma membrane depolarisation. Hydraulic signals involve changes in turgor pressure, mass flow, and pressure waves. SWPs are closely linked to hydraulic signals as a result of cavitation events or changes in turgor [154,155,156]. Chemical signals can be classified as: (i) secondary messengers including reactive oxygen species (ROS), inositol triphosphate (IP), Ca²⁺, K⁺, and anion fluxes; (ii) signalling cascade chemicals including mitogen-activated protein kinases (MAPKs); and (iii) chemical response signals including phytohormones and volatile organic compounds (VOCs). Herbivore-induced plant volatiles (HIPVs) have been well-documented involving both above- and below-ground biocontrol of herbivores by insect predators and parasitoids, such as the yellow jacket wasp and the parasitic wasp, respectively [157,158]. Recent advanced analyses have elucidated the role of various mobile molecules including small peptides [155,159,160] and small RNAs (small interfering RNA and micro RNAs) in long-distance systemic signalling [159,161,162,163]. Stomatal closure is one of the initial responses to osmotic stress to prevent hydraulic failure [164] and is regulated by abscisic acid (ABA) [165,166]. During water stress, the root-to-shoot communication is mediated by a small mobile root-derived peptide, CLAVATA3/EMBRYO-SURROUNDING REGION-RELATED 25 (CLE25), which then triggers ABA accumulation in the leaves via BARELY ANY MERISTEM1 (BAM1) and BAM2 receptors in the leaf vascular bundles [159]. Sound vibrations (SVs) or acoustic signals produced by both biotic and abiotic stresses act as stimuli capable of priming plants for future stress challenges and as long-range signals that activate plant-signalling pathways [148]. Leaf vibrations caused by herbivore chewing [148,149] and the “clicking” sound produced by the collapsing water column (cavitation) in the xylem [167] have been demonstrated to trigger systemic responses in distal regions of the plant. The perturbation of the plasma membrane by SVs is characterised by a sequence of molecular episodes including cell wall modification and microfilament rearrangement in plant cells [147,149,168].