Benefits of silicon-enhanced root nodulation in a model legume are contingent upon rhizobial efficacy

Abstract

Aims Silicon (Si) uptake and accumulation improves plant resilience to environmental stresses, but most studies examining this functional role of Si have focussed on grasses (Poaceae) and neglected other important plant groups, such as legumes (Fabaceae). Legumes have evolved a symbiotic relationship with nitrogen-fixing bacteria (rhizobia) housed in root nodules. 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’). Methods We examined how Si and rhizobial efficacy influence nodule and plant functional traits, including their chemical aspects. These combinations were supplied with or without Si in a glasshouse experiment, where we quantified nodule flavonoids and foliar chemistry (free amino acids, soluble protein, elemental C, N and Si). Results Si supply increased nodule number per plant, specific nodule flavonoid concentrations, 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 certain 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. Conclusions Collectively, our study indicates that Si 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.

Silicon supplementation and jasmonate activation synergistically increase phenolic defences against a legume herbivore

“…Root nodule abundance increased in response to Si supplementation, which has previously been demonstrated in M. sativa (Johnson et al 2017;Johnson et al 2018b) and M. truncatula (Putra et al 2022). This may have been a compensatory response to higher plant growth (and its associated increase in foliar C concentrations) to prevent the plant from becoming N-limited.…”

“…Moreover, root Si deposition may anatomically alter the structure of root nodules but does not necessarily promote rhizobial N-fixation (Putra et al 2021) and foliar N concentrations may be dictated by rhizobial efficacy. For example, when low-efficiency rhizobial strains were present in M. truncatula, foliar N was found to increase, whereas plants with high-efficiency strains had no effect on foliar N concentrations (Putra et al 2022).…”

“…Most studies on biotic responses to plant Si deposition have used species of Poaceae (mostly cereal crops) as high Si-accumulating model plants (Cooke & Leishman 2016;Johnson et al 2021), in which Si is assumed to be more functionally important. Foliar Si concentrations in legumes have been shown to increase (Johnson, Rowe & Hall 2020), decrease (Johnson et al 2018b) or remain unchanged (Johnson et al 2017) in response to Si supplementation and the outcome may depend on the diversion of resources towards plant growth and nodule development (Putra et al 2022).…”

Nanoparticulate Silica Internalization and Its Effect on the Growth and Yield of Groundnut (Arachis hypogaea L.)

“…20 Further, the application of Si increased the nodule numbers in legumes. 21,22 The foliar application of nutrients proved to be beneficial than the soil application because nutrient deficiencies can be controlled efficiently, thereby increasing plant growth. 23 Lack of moisture on topsoil makes nutrients unavailable to plants during drought.…”

Plasticity of plant silicon and nitrogen concentrations in response to water regimes varies across temperate grassland species

“…In contrast, low Si concentrations under drought and flooding might lead to higher herbivory damage. Furthermore, Si accumulation itself can increase plant N concentrations(Putra et al, 2022), reduce secondary metabolites in plants(Schaller et al, 2012) and improve tissue hydration under drought(Thorne et al, 2020). The species-specific plant Si responses to water regimes we observed, combined with the known defensive effect of Si, and its interaction with other plant traits which also affect herbivory imply that Si plasticity should influence insect herbivory across water regimes in temperate grasslands.5 | CON CLUS IONSWe showed strong plasticity in plant Si and N concentration of temperate grassland plants in response to drought, waterlogging and flooding.…”