Benefits of silicon-enhanced root nodulation in a model legume are
contingent upon rhizobial efficacy
Abstract
Our study determined the impacts of silicon (Si) supplementation on
Medicago truncatula inoculated with Ensifer meliloti
rhizobial strains that differed in their capacity for nitrogen fixation:
Sm1021 (‘low-efficiency’) or Sm1022 (‘high-efficiency’). We then
examined how Si and rhizobial efficacy influence plant resistance to a
polyphagous insect, Helicoverpa armigera. These combinations were
supplied with Si or untreated in a glasshouse experiment, where we
quantified nodule flavonoids and foliar chemistry (free amino acids,
soluble protein, elemental C, N and Si). Si supply increased nodule
number per plant, specific nodule flavonoids, contents of foliar
nitrogenous compounds and foliar C, but not foliar Si. We also
demonstrated that rhizobial efficacy altered the magnitude of Si effects
on various traits. For example, Si significantly promoted concentrations
of foliar N and soluble protein in the plants associated with the
‘low-efficiency’ strain only and this was not the case with the
‘high-efficiency’ one. Additionally, increases in foliar free amino
acids in response to Si addition did not increase susceptibility to
H. armigera. Collectively, our study indicates that Si enrichment
generates positive effects on M. truncatula, particularly when
the association with rhizobia is relatively inefficient, and may play a
more prominent role in rhizobial functionality than previously thought.