Flavonoids: a common signature of rhizobacteria-mediated plant
growth promotion
Our results showed that one of the most distinctive metabolic signatures
between effective and ineffective partnerships is the differential
regulation of the flavonoid pathway. In effective partnerships,
metabolites from this pathway were either suppressed or showed only a
slight accumulation. In ineffective partnerships, metabolites from this
pathway were highly induced in the plant shoot (Figure 2 and3 ). Of all rhizobacteria-plant combinations, thePbg -Artemisia combination can be considered as a highly-effective
partnership in terms of growth promotion and was hallmarked by a
substantial alteration of the host metabolome (Figure 1 andFigure 2c1 and c2 ). This ‘rewiring’ of the Artemisia
metabolome primarily involved accumulation of hydroxycinnamates and
suppression of other phenolic compounds such as flavonoids and benzoic
acid derivatives (Figure 3a2 and Supplementary Material,Table S8 ). This phenomenon might be the result of the
competition for the common precursor P -coumaroyl CoA between
hydroxycinnamoyl transferase (HCT) and chalcone synthase (CHS), key
enzymes involved in the biosynthesis of hydroxycinnamates (monolignols)
and flavonoids, respectively (Figure 3a2 and Supplementary
Material, Table S6 ).
Similar to the ineffective partnerships of Pbg -Arabidopsis orPf SS101-Broccoli, Besseau et al . (2007) showed that the
HCT-mutants displayed reduced growth and a concomitant accumulation of
flavonoids of mainly kaempferol derivatives. Accumulation of flavonoid
glycosides in Arabidopsis are reported to affect auxin transport
(Besseau et al. 2007), distribution and turnover (Kuhn et
al. 2016), thereby affecting plant growth. Besseau et al . 2007,
further showed that suppression of flavonoid production via CHS
silencing, restored auxin transport and normal development of
HCT-deficient plants. Another interesting Pbg -induced chemotype
we observed in Arabidopsis was the accumulation of metabolites derived
from the indole pathways such as IAA and camalexin. The accumulation of
auxin in the aerial part of the plant might suggest that rootward
transport of auxin is inhibited in Arabidopsis colonizaed by Pbg .
Flavonoids were implicated as endogenous metabolites that reduce the
basipetal transport of auxin (Peer et al. 2004; Santelia et
al. 2008). Interestingly, the ineffective partnership between PfSS101 and Broccoli was characterized by the accumulation of indolic
glucosinolates while IAA and camalexin were not detected in the samples.
In contrary to Besseau et al. (2007), Li et al. (2010)
suggested that growth reduction associated with lignin biosynthesis in
Arabidopsis is independent of flavonoids. Other studies also showed that
accumulation of flavonoids either increases plant biomass (Sharmaet al. 2016) or does not lead to growth penalty (Nakabayashiet al. 2014), making the association between growth and
flavonoids debatable. Elucidation of the role of a class of metabolites
in plant growth by targeting a gene at branching points of different
biosynthetic pathways or by targeting transcription factors that
potentially involve in multiple metabolic pathways will have spillover
effect, and could jeopardize our attempt to establish relationship
between a given metabolite class and plant growth phenotype. Hence, in
our study we used specific Arabidopsis mutants and focused on the role
of the flavonoid pathway and its different branches in
rhizobacteria-mediated plant growth promotion.