Jamie Waterman: silicon and simulated herbivory modify plant defences

Jamie Waterman is a PhD Candidate at the Hawkesbury Institute for the Environment, Western Sydney University studying the mechanisms behind plant defences against insect herbivores. In this insight, he discusses his recent paper Short‐term resistance that persists: Rapidly induced silicon anti‐herbivore defence affects carbon‐based plant defences.

About the paper

Jamie enjoying the weather from his balcony in Sydney.
Jamie enjoying the weather from his balcony in Sydney.

            Plants uptake dissolved silicon through their roots and it is deposited throughout plant tissues as solid silicon dioxide. These deposits are essentially microscopic grains of sand that make the tissue very unpalatable to many insect species. Grasses, in particular, are known to accumulate relatively high concentrations of silicon compared to other plant taxa. Previous studies from our lab group (see Hall et al 2019 and Hall et al 2020) have demonstrated that silicon uptake can be induced by insect herbivory and, more specifically, activation of the jasmonic acid (JA) pathway. However, it was unclear how quickly this activation occurs, and additionally, how silicon affects other plant defences when herbivore signals are present.

Scanning Electron Microcope (SEM) image of Brachypodium distachyon leaves devoid of Si (-Si, left) and Si-supplemented (Si+, right).
Scanning Electron Microcope (SEM) image of Brachypodium distachyon leaves devoid of Si (-Si, left) and Si-supplemented (Si+, right).

Our experiment tested how quickly silicon accumulation is induced following stimulation of the JA pathway, using methyl jasmonate (MeJA) as a form of simulated herbivory (allows for more control over dosage and timing than live insects). We found that the model grass, Brachypodium distachyon, induced silicon accumulation by as much as 20% in as little as 6 hours following MeJA treatment. Additionally, MeJA disrupted the known trade-off between silicon- and carbon-based defences (not to be confused with silicon-based lifeforms; see Star Trek: The Original Series). This suggests that under herbivory stress, plants can utilize both silicon and carbon defences, but in the absence of stress, plants supplemented with silicon might opt for silicon-based defences, which are known to be the energetically cheaper alternative.

About the research

Brachypodium distachyon grown in the hydroponic setup used in this study.
Brachypodium distachyon grown in the hydroponic setup used in this study.

Understanding the ecology of grasses is vital as they not only cover a large portion of terrestrial ecosystems but are also among the most important agricultural crops worldwide; we (humans) obtain 70% of our calories from them, either directly (e.g. through cereal crops) or indirectly (e.g. through livestock). Understanding how grasses can be best equipped to defend against insects is therefore critical for global food security, sustainable agriculture and conservation of vulnerable habitats. This study and several others from our lab group aim to understand the ecological implications of silicon accumulation in grasses in the context of plant-herbivore interactions. In other words, how silicon is utilised by plants as a defence and how it affects alternative defence mechanisms. Our hope is that this work will inform not only ecological and evolutionary research regarding silicon accumulation but also how silicon may be used in various landscapes to help mitigate the impacts of herbivory and potentially reduce the need for pesticide application.

About the Author

I first became interested in research early on in my undergrad, while I was studying forestry and biochemistry. This combination enabled me to become involved in research projects in both chemistry and forest ecology labs; my primary interests were in trying to explain ecological phenomena through plant chemistry. Since moving to Australia (from the US), and beginning my PhD, in 2018 I have transitioned from forests to grasslands, researching the mechanistic basis of silicon as a defence against insect herbivory. In addition to investigating silicon accumulation and its ecological implications, I also work on the applications of simulated herbivory in ecology to answer questions not afforded with live insects; after all, they are living creatures that often behave unpredictably, making certain research questions quite difficult to answer. I am currently working on a meta-analysis on this topic and have published a review paper on the usefulness of simulated herbivory in an ecological context in Trends in Ecology and Evolution.

When I’m not in the lab or in front of a computer screen I can be found playing table tennis, completing only 50% of a jigsaw puzzle, or watching my favourite Australian rugby league team destroy the competition (Go the Eels!).

Read the paper in full here.

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