1  INTRODUCTION
Plants adapt to stress via sensor elicitation, signaling cascade activation, gene expression, and phenotype modification (Glazebrook, 2005, Jung, Tschaplinski, Wang, Glazebrook & Greenberg, 2009). Signal processing time is critical for success or failure in stress responses. Plants acquired the ability to anticipate and respond to imminent dangers, which conferred ecological competence in highly dynamic ecosystems. This system is known as defense priming (Conrath, Beckers, Langenbach & Jaskiewicz, 2015, Jung et al. , 2009).
Plants can acquire early warning information through their microbiome and plant–plant signals (Gilbert & Johnson, 2017, Vahabi, Reichelt, Scholz, Furch, Matsuo, Johnson, Sherameti, Gershenzon & Oelmuller, 2018, Yi, Heil, Adame-Alvarez, Ballhorn & Ryu, 2009). The plant can exploit the unique capabilities of its symbionts including the mycorrhizal network, dodder (Cuscuta spp.), and endophytic fungi, which directly prime plant defense responses or transfer inter-plant signals (da Trindade, Almeida, Xavier, Lins, Andrade, Maia, Mello, Setzer, Ramos & da Silva, 2019, Hettenhausen, Li, Zhuang, Sun, Xu, Qi, Zhang, Lei, Qin, Sun, Wang, Baldwin & Wu, 2017, Vahabi et al. , 2018). The plant microbiome and macrobiomecan modify plant-derived inter-plant signals such as volatiles and root exudates (Figure 1) (Sharifi, Lee & Ryu, 2018, Song, Sim, Kim & Ryu, 2016). Herbivore-induced plant volatiles (HIPVs) and microbe-induced plant volatiles (MIPVs) are good examples of inter-plant signals (Heil & Bueno, 2014, Sharifi et al. , 2018).