Metabolite signatures of Brassicaceae in effective and
ineffective partnerships
The metabolome changes induced by the different rhizobacteria in the
shoots of Arabidopsis and Broccoli were subjected to a comparative
analysis as both plant species belong to the Brassicaceae family.
Glucosinolates (GLS) are among the metabolites that showed alteration by
rhizobacteria treatment. In Arabidopsis, the effective partnership withPf SS101 enhanced the relative levels of aliphatic long-chain GLS
such as glucohirsutin and 8-methylthiooctyl glucosinolate, whereas the
ineffective partnership between Pf SS101 and Broccoli was
characterized by an increase in the levels of indolic GLS (i.e.,
glucobrassicin, desulfoglucobrassicin, 4-methoxyglucobrassicin and
neoglucobrassicin) and short-chain aliphatic GLS such as glucoibervirin.
In contrast to the effective Pf SS101-Arabidopsis partnership,
the ineffective Pbg -Arabidopsis partnership caused a drastic
reduction of all detected aliphatic GLS, (i.e., glucohirsutin
(8-methylsulfinyloctyl glucosinolate), glucolesquerellin
(6-methylthiohexyl glucosinolate), 7-methylthioheptyl glucosinolate and
8-methylthiooctyl glucosinolate. In Broccoli, both the ineffective
partnership with Pf SS101 as well as the effective partnership
with Pbg caused significant increases of the indolic GLS
glucobrassicin. Meanwhile, the effective MB-Arabidopsis partnership led
to upregulation of glucohirsutin while this rhizobacterial strain barely
influenced the level of GLS in Broccoli (Supplementary Material,Tables S4 and S7 ). Considering their inconsistent
accumulation patterns in effective and ineffective partnerships, GLS
seem not to be appropriate plant metabolic markers for discriminating
between effective and ineffective partnerships.
On the other hand, alteration of a particular branch of the
phenylpropanoid pathway showed co-occurrence with growth promotion
(effective partnership) or lack of growth promotion/ growth reduction
(ineffective partnership) in the different plant-rhizobacteria
combinations tested. Higher accumulation of flavonoids was evident in
the ineffective partnerships between Arabidopsis-Pbg and
Broccoli-Pf SS101, while flavonoids showed no significant change
in their abundance in the effective partnership between
Arabidopsis-Pf SS101 (Figure 3 and Supplementary
Material, Tables S4 and S7 ). Furthermore, in the
ineffective partnerships between Arabidopsis-Pbg, indole-derived
defensive compounds such as camalexin, β-d-glucopyranosyl
indole-3-carboxylic acid and metabolites from the phenylpropanoid
pathway (including anthocyanin and salicylic acid) showed higher
accumulation (Figure 2a2 , Supplementary Material, Table
S4 and Figure 3b ). Similarly, the ineffective
partnership between Broccoli and Pf SS101 resulted in a
significant accumulation of flavonoids and other defensive metabolites
(Figure 2c2 , Supplementary Material, Table S7 ). The
accumulation of these stress/defense-related metabolites in the
aforementioned ineffective partnerships suggests that both Pbgand Pf SS101 may be perceived as a biotic stressor by Arabidopsis
and Broccoli, respectively (Figure 2a2 and 3 ).