Importance of the flavonoid pathway in plant growth promotion
In the ineffective rhizobacteria-plant partnerships, flavonoids showed
considerably higher accumulation while they were not significantly
changed or significantly reduced in effective and highly effective
partnerships, respectively. Considering our results and other reports on
the negative association between flavonoids and auxin transport, auxin
distribution and turnover (Brown et al. 2001; Buer et al.2013; Yin et al. 2014; Kuhn et al. 2017), we further
investigated the effects of Pbg on growth of 12 isogenic
homozygous Arabidopsis transparent testa (tt )-mutants
(Appelhagen et al. 2014) disrupted in biosynthetic genes of
different branches of the flavonoid pathway (Figure 4a ). First,
the inherent variation in shoot biomass of each of the 12 mutant lines
and wild type (WT) Col-0 was assessed in absence of Pbg . Second,
the effect of Pbg on growth of these 12 mutants and Col-0 was
assessed by calculating the percent change in shoot biomass relative to
the biomass of the respective non-treated mutant line or WT. Both
measures were subjected to analysis of variance (ANOVA) by considering
the ‘genotype’ as a factor and root biomass and percent change in shoot
biomass as dependent variables.
In absence of Pbg , we observed significant variation in shoot
biomass among the tt- mutants and between several of thett- mutants and WT Col-0 (Figures 4b and 4c and
Supplementary material, Table S5 ). Mutants disrupted in the
initial steps of the flavonoid pathway (tt- 4 and tt- 6)
showed similar shoot biomass as WT Col-0 and higher shoot biomass than
many of the mutants with genes disrupted in the downstream steps of the
pathway, especially mutants tds 4, aha 10, and tt 10
that are associated with the biosynthesis of flavan-3-ols (PA)
(Figure 4c ).
Root tip inoculation with Pbg led to significant changes in shoot
fresh biomass between the 12 mutants (Figure 4d and
Supplementary material, Table S6 ). Interestingly, Pbghad the strongest growth-promoting effect on mutants affected in the
downstream part of the flavonoid pathway, particularly on mutants in the
proanthocyanidin branch (tt 10-8, aha 10-6, tt 12-2,tds -4, tt 15-5 and tt 18-5) (Figure 4d ). Thett- 7 mutant that accumulates kaempferol-derived flavonols showed
inherently the lowest shoot biomass accumulation (Figure 4d )
and root inoculation of this mutant with Pbg had no significant
growth-promoting effect as compared to the WT (Figure 4d ).
Collectively, these results indicate that the proanthocyanidin (PA)
branch plays a key role in Pbg -mediated growth promotion.