Casey Hall is a postdoctoral researcher at The Hawkesbury Institute for the Environment, Western Sydney University looking at plant defence against insects in a changing climate. In this Insight, she talks about her erecent paper Elevated atmospheric CO2 suppresses jasmonate and silicon‐based defences without affecting herbivores.

About the paper

Casey Hall in the glasshouse, with the grass Brachypodium distachyon growing in hydroponics
Casey Hall in the glasshouse, with the grass Brachypodium distachyon growing in hydroponics

Dissolved silicon is taken up from the soil and embedded in plant tissues as solid silicon dioxide. Grasses, in particular, can accumulate high concentrations of silicon. These tiny sand-like deposits in the plant are a very effective defence against insect herbivores. We had evidence that silicon uptake in grasses is stimulated by the jasmonic acid (JA) defence pathway and that silicon uptake was reduced in plants grown under elevated CO2 (eCO2). However, it was unknown how eCO2 would affect the interaction between JA and silicon.  We hypothesised that eCO2 would suppress silicon uptake via a reduction of JA.

Helicoverpa armigera caterpillar eating an experiment plant. Photo by Ximena Cibils-Stewart
Helicoverpa armigera caterpillar eating an experiment plant. Photo by Ximena Cibils-Stewart

Our experiment tested this by growing plants with and without silicon and in ambient and elevated CO2, we then sprayed half with methyl jasmonate. This compound tricks the plant into thinking that it’s under attack so we can then measure the JA produced by the plant as a defence response. We found that eCO2 reduced JA in the plants which also reduced silicon concentrations. However, when we fed these plants to a pest caterpillar species, Helicoverpa armigera, they showed no difference in growth rate, i.e. they starved equally on all silicon containing plants, and conversely grew equally fat on all non-silicon plants.  This result surprised us, demonstrating that silicon remains a potent physical defence against a global mega pest species under climate change, despite the predicted reduction in silicon.

About the research

We have focused on investigating these mechanisms in grasses, as six of the ten most productive crops worldwide are high silicon accumulating grasses, and grasslands store approximately a third of global carbon stocks. Understanding how climate change alters silicon based defence is important for global food security. Our results suggest that silicon may be able to mitigate some of the negative effects of climate change on plant defences.

We found throughout several experiments that when hungry, Helicoverpa armigera caterpillars will eat through almost anything, including mesh bags and a Styrofoam box.

Surprisingly, one of the most challenging aspects of the experimental work was the handling of the caterpillars. We found throughout several experiments that when hungry, Helicoverpa armigera caterpillars will eat through almost anything, including mesh bags and a Styrofoam box. It’s therefore not surprising that they are a global mega pest species, yet they would rather starve than eat silicon supplemented plants.

Based on the interactions we’ve seen between JA and plant silicon, the next exciting questions for us are: how does silicon alter other JA-based chemical defences? What are the effects of silicon versus chemical defence on different insect herbivores? And how are these relationships altered under climate change?

About the author

I originally fell in love with entomology during my undergraduate degree. I was also lucky enough to attend an undergraduate field trip to Borneo which introduced me to some of the most spectacular rainforest in the world. From there I went on to complete my PhD in plant-insect interactions within Australian rainforests. I have always had a strong interest in chemistry, particularly the amazing diversity of compounds produced by plants, and have been lucky to be able to incorporate plant chemistry into my research.

Future field experimental set up using open top chambers to investigate the effects of elevated CO2. Photo by Casey Hall.
Future field experimental set up using open top chambers to investigate the effects of elevated CO2. Photo by Casey Hall.

I am currently in the last six months of my postdoc which has been focused on investigating the effect of eCO2 on silicon-based defence against insect herbivores. I am working on several papers where we looked at the effects of silicon and eCO2 on the production of neurotoxic alkaloids from the pasture grass Phalaris aquatica. We are also about to start an exciting new experiment using open top chambers to look at how CO2 and silicon alter plant responses under field conditions.

Read the paper in full here or the free plain language summary here.