Rebecca Vandegeer: plant silicon defences against herbivores under drought

In this post Rebecca Vandegeer presents her study ‘Leaf silicification provides herbivore defence regardless of the extensive impacts of water stress’, discusses the importance of Silicon for plant growing and the joy of working with plants and insects 

My name is Rebecca Vandegeer and I recently completed a postdoctoral research fellowship with the Hawkesbury Institute for the Environment at Western Sydney University.

Rebecca at work in the glasshouse in Western Sydney
Rebecca at work in the glasshouse in Western Sydney

There is growing research interest in the role of elemental silicon (Si) in plant ecology. Si is found in all soils and is taken up by plants via the roots, where it is transported throughout the plant and deposition as silicon dioxide (SiO2). These SiO2 crystals are hard compounds within the leaf that make them unpalatable for hungry chewing herbivores like caterpillars. Since drought stress is known to influence plant Si uptake and impact plant physiology, we were interested to study how drought may affect the Si-herbivore relationship.

To investigate these interactions, we grew tall fescue grass hydroponically with and without Si added to the solution. Plants were exposed to osmotic stress as a proxy for drought by adding polyethylene glycol. We measured a range of plant responses including leaf water content and specific leaf mass. In addition, we placed caterpillars on the plants for a week and measured their growth. Our main finding was that leaf Si was a potent physical defence against the caterpillar herbivore, even when plants were drought-stressed. Our paper is only the third to study the interactive effect of drought and the addition of Si on herbivore performance, and the first to also present scanning electron microscopy (SEM) images and x-ray mapping (XRM) of Si on the leaf surface.

Tall fescue grass plants growing in nutrient solution
Tall fescue grass plants growing in nutrient solution

This research ties into a broader question of how Si influences plant physiology. Many studies have shown that Si and drought can have an interactive effect on plants, but there has been little work to try to find the underlying physiological mechanisms. This study was part of a larger effort to investigate these mechanisms. For example, another part of our work found that Si is deposited at the leaf cuticle where it influences water exchange, and also on the stomata where it influences responsiveness to the drought hormone, abscisic acid (Vandegeer et al. 2020*). We hope that by better understanding how Si influences plant water relations, we might better inform management of agro-ecosystems in the face of drought and the pressure of herbivore pests. For example, the addition of Si fertilisers or selection of high Si-accumulating cultivars may serve as additional tools that land managers could use to tackle drought and pests.

Leaf surface images taken with a Scanning Electron Microscope (SEM) showing silicon crystals in plants treated with silicon (bottom)
Leaf surface images taken with a Scanning Electron Microscope (SEM) showing silicon crystals in plants treated with silicon (bottom)

As a scientist, I enjoy multi-disciplinary studies and the challenges of working with both plants and insects. However, I am more of a plant scientist than an entomologist, which was evident with the challenge of wrangling the army of caterpillars used in this study. I had to regularly check the cages to make sure that none of them escaped in the dead of night! I have to credit the group that I worked in at the Hawkesbury Institute for the Environment, where we had post-docs, students and senior academics with experience in many different fields (plant physiology, entomology, chemistry), which meant that we could all share advice and knowledge.

Weighing day for this hefty caterpillar
Weighing day for this hefty caterpillar

My decision to study science at university started with a common story: I grew up going on regular camping and hiking holidays with my family, where I was fascinated by the beauty of the Australian bush. I first developed an interest in plant science and ecophysiology during my undergraduate degree where I completed an honours project working with a major global food staple, cassava, and the influence of drought stress. My interest in plant-insect-interactions began during a position with Agriculture Victoria where I worked with wheat and aphid herbivores. My recent post-doc position allowed me to combine my interest in plant ecophysiology, plant-insect interactions and plant-soil interactions. My initial fascination with plant science was learning the ways that plants can adapt to changes in their environment, including all the tiny changes that happen on a microscopic level, such as stomatal closure. It’s always interesting to observe how small changes can have a cascading effect and influence other trophic levels such as the insect herbivores that feed on the plants.

Read the article in full here.

*Vandegeer, R.K., Zhao, C., Cibils-Stewart, X., Wuhrer, R., Hall, C.R., Hartley, S.E., Tissue, D.T. & Johnson, S.N. (2020) Silicon deposition on guard cells increases stomatal sensitivity as mediated by K+ efflux and consequently reduces stomatal conductance. Physiologia Plantarum, 171, 358-370.

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