Summary by Sarah Thorne, a post-doc at the University of Sheffield, UK researching changes in plant silicon in response to stress.
Silicon (Si) is an important antiherbivore defence in many grass species, including crops such as wheat. Numerous studies have reported increased Si accumulation in response to herbivory, although the focus has been on systemic increases in Si occurring throughout the plant. By analysing damaged and undamaged leaves of damaged plants separately, Hartley et al. (2015) showed that mechanical damage increased Si specifically in damaged leaves in three grass species. It was hypothesised that this localised induction was the result of increased transpiration in damaged leaves. However, subsequent research showed that this was not the case, with localised Si induction occurring even when plants were bagged to reduce transpiration (McLarnon et al., 2017).
In this study, we continued to investigate the cause of localised increased Si in response to damage using previously characterised high and low Si-accumulating wheat genotypes (Thorne et al., 2021). Initially, ten genotypes were grown with supplemental Si to examine genotypic variation in the Si response to damage. A significant increase in Si accumulation specifically in damaged leaves was found in all genotypes, although the extent of this induction varied among genotypes. However, there was no systemic increase in Si for any genotype. A second experiment found that this localised increase in Si in damaged leaves occurs even when plants are grown without Si (as far as possible, as Si contamination is a common problem).
Previous studies have shown that once deposited as silica, Si cannot be remobilised (Samuels et al., 1991), suggesting that a continuous supply of Si would be required for an induction of Si defences to occur. Therefore, it was hypothesised that if plants were initially grown with supplemental Si and then moved to Si-free medium once damaged, no increase in Si in response to damage would be observed. In contrast to this hypothesis, we observed a significant increase in Si in damaged leaves even days after plants were moved to Si-free medium, albeit to a smaller extent than that observed in plants grown continuously with Si. No difference in the Si content of roots or stems between damaged and undamaged plants was observed, suggesting that localised Si induction was the result of the redirection of Si from undamaged to damaged leaves.
As silica cannot be remobilised, the Si being redirected must still be in a soluble form. By adapting a method commonly used to measure soluble Si in the soil, we showed that sufficient soluble Si was present in the leaves to account for the localised increase in Si. While we were unable to verify that changes in Si transporters were co-ordinating the observed redirection of Si, we found evidence of soluble Si in the phloem to support our proposed mechanism of Si being transported from undamaged to damaged leaves. It was suggested that the redirection of soluble Si to sites of damage might be a more energetically favourable way of increasing defences compared to increasing Si uptake.
Proposed model: The localised increase in Si in damaged leaves can be explained by the redirection of soluble Si from undamaged leaves of damaged plants. Figure created using BioRender.com.
Enjoyed the blogpost? Read the full paper at:
Thorne S.J., Maathuis F.J.M., Hartley S.E. 2023. Induction of silicon defences in wheat landraces is local not systemic and driven by mobilisation of soluble silicon to damaged leaves. Journal of Experimental Botany erad224. https://doi.org/10.1093/jxb/erad224
(https://eprints.whiterose.ac.uk/200434/ for open access author accepted manuscript)
Siliceous Plants 