INTRODUCTION
Plants are continually strengthening their mechanical and chemical defense processes against insects (Hahn et al., 2019; Willsey et al., 2017). Meanwhile, invertebrate herbivores have advanced their mechanisms of tolerance and resistance to overcome plant defense strategies (Huang et al., 2016; Kessler & Baldwin, 2002; Ryan, 1990;). This evolutionary interaction between plants and herbivores is an evidence of their interdependence and close mutualistic relationship (Mello & Silva-Filho, 2002; Sugio et al., 2015). However, the effect of plant-insect interactions on rhizosphere microbes is not fully understood.
Plants respond to insect attack by inducing internal resistance (IR) (Blaazer et al., 2018). Internal resistance can be divided into two categories: induced systemic resistance (ISR), and systemic acquired resistance (SAR) (Vallad et al., 2004). Both types of resistance entail a metabolic cost for the plant (Krattinger & Keller, 2016) where survival is achieved at the cost of growth (Morris et al., 2006).
ISR is potentiated by plant growth-promoting rhizobacteria (PGPR) such as species of Pseudomonas that cause no visible damage to the plant’s root system (van Loon et al., 1998). ISR pathways can be regulated by jasmonate and ethylene (Knoester et al., 1999; Pieterse et al., 1998; Yan et al., 2002). Additionally, there is demonstrated specificity for plant genotypes to express ISR (van Wees et al., 1997; Yan et al., 2002).
SAR also serves as systemic plant protection against subsequent invasion (Hammerschmidt, 2001; Sticher et al., 1997), which generally leads to the development of broad-spectrum and long-lasting responses against predators (Hammerschmidt, 2001). SAR is not restricted to one plant and can also be transmitted to neighboring plants to be used as an indirect form of defense through the use of organic compounds (Heil and Karban, 2010). Further, after wounding by the herbivore, specific plant tissues produce volatile organic compounds (VOCs) which play essential roles as alarm signals for undamaged neighbors (Franco et al., 2017). Root–root interactions, by transferring defense-related signaling compounds, also help boost defensive enzyme activities and defense-related gene expression in neighboring plants (Heil and Karban, 2010). At the root level, modulation effects of root-derived compounds, such as primary and secondary metabolites, in response to insect injury have been reported (Badri et al., 2013; De-la-Peña et al., 2010; Hubbard et al., 2019).
Microbial mediation of plant-herbivory interactions alters plant physiology, nutrition, and defensive chemistry (Barbosa et al, 1991). The systemic responses on aboveground organs following insect attack are well understood, but knowledge of the impacts on the associated microbial communities and specific microbial taxa after herbivory damage belowground remains scarce (Yi et al, 2011). Fewer reports have discussed the biological functions of the rhizosphere microbiota modulated after insect pest attack and how this relates to plant inducible resistance (Li-Li et al., 2018; Morgane et al., 2018). In a wide survey across different plant species, our results showed a significant relative abundance shift in the rhizosphere beneficial bacterial community after the aboveground attack of a specific insect. In addition, we suggest that bacterial recruitment initiated by the plant after aboveground insect attack can be outweighed by the specific cost in induced resistance in the subsequent generation.