Microbe-derived bioactive metabolites
Microbe-derived compounds that trigger enhanced plant growth and defense
are therefore attractive and promising alternatives to agricultural
chemicals. In particular, rhizosphere bacteria produce a portfolio of
bioactive secondary metabolites (BSMs) and inhibitory allelochemicals
that are responsible for Induced Systemic Resistance (IR), biocontrol,
and ecological fitness (Fig. 2A). For example, the typical
plant-associated species,Bacillus velezensis has
considerable biocontrol potential, based on its ability to trigger IR
leading to rapid and robust defense responses, as well as direct
antagonistic actions on pathogens (Ongena et al., 2007). Over 12% of
the genome in bacteria such as B. velezensis is devoted to the
synthesis of compounds that contribute to ecological competence and
biocontrol activity. They produce a wide range of non-ribosomally
synthesized compounds such as oligopeptides, cyclic lipopeptides (CLPs),
and polyketides (PKs), in addition to post-translationally-modified
lanthipeptides and bacteriocins. These molecules play a key role in
multitrophic interactions because they can be antagonistic with other
microbes and/or beneficial to the host via the stimulation of innate
immune responses leading to IR. BSMs are not only produced in
vitro conditions but also in planta by bacterial cells evolving
as biofilm-structured microcolonies on root tissues (Andrić et al.,
2021; Cawoy et al., 2015; Zihalirwa Kulimushi, Argüelles Arias, Franzil,
Steels, & Ongena, 2017).
Plant-associated bacteria have evolved a polymer-sensing system to
perceive the host. They increase BSM synthesis in response to the
perception of root exudates. For example, the production of the cyclic
lipopeptide surfactin by B. velezensis is stimulated by pectin,
which is recognized as a cell surface molecular pattern in root
exudates. Surfactin, which is synthesized non-ribosomally by a complex
enzyme system, is formed as a mix of naturally coproduced homologs with
fatty acid chains of various lengths (Hoff et al., 2021). It optimizes
biofilm formation, motility, and early root colonization by the
bacterium. It also reinforces the defensive capacity of the host. The IR
phenotypes triggered by surfactin and other PBBs are an attractive
addition to the crop protection toolbox, as bio-sourced alternatives to
chemicals. However, the underlying biochemical and molecular basis for
PBB action remains poorly understood.