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.