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Abstract:
Beneficial soil microbes can promote plant growth and induce systemic resistance (ISR) in aboveground tissues
against pathogens and herbivorous insects. Despite the increasing
interest in microbial-ISR against herbivores, the underlying
molecular and chemical mechanisms of this phenomenon
remain elusive. Using Arabidopsis thaliana and the
rhizobacterium Pseudomonas simiae WCS417r (formerly
known as P. fluorescens WCS417r), we here evaluate the role
of the JA-regulated MYC2-branch and the JA/ET-regulated
ORA59-branch in modulating rhizobacteria-ISR to
Mamestra brassicae by combining gene transcriptional, phytochemical,
and herbivore performance assays. Our data show
a consistent negative effect of rhizobacteria-mediated ISR on
the performance of M. brassicae. Functional JA- and ETsignaling
pathways are required for this effect, as shown by
investigating the knock-out mutants dde2-2 and ein2-1.
Additionally, whereas herbivory mainly induces the MYC2-
branch, rhizobacterial colonization alone or in combination
with herbivore infestation induces the ORA59-branch of the
JA signaling pathway. Rhizobacterial colonization enhances
the synthesis of camalexin and aliphatic glucosinolates (GLS)
compared to the control, while it suppresses the herbivoreinduced
levels of indole GLS. These changes are associated
with modulation of the JA-/ET-signaling pathways. Our data
show that the colonization of plant roots by rhizobacteria
modulates plant-insect interactions by prioritizing the JA/ETregulated
ORA59-branch over the JA-regulated MYC2-
branch. This study elucidates how microbial plant symbionts
can modulate the plant immune system to mount an effective
defense response against herbivorous plant attackers.
