In this post, Adam Frew discusses his paper ‘Increased root herbivory under elevated atmospheric carbon dioxide concentrations is reversed by silicon-based plant defences‘
As the global climate changes the global population continues to rise, we are faced with the daunting challenge of achieving sustainable crop production to meet the increasing demand for food. Professor John Beddington in 2009, UK chief scientist at the time, highlighted this potential ‘perfect storm’ of global events and the urgent need to address these challenges.
Insect herbivores are one of the main contributors to crop losses globally, which have traditionally been dealt with, with the application of pesticides. In fact, over the last 40 years, there has been a seven-fold increase in insecticide usage (Tilman et al. 2001). However the application of insecticides, often prophylactically, is environmentally damaging, unsustainable and costly. This has led to increased restrictions on insecticide application, and highlights the need for new methods of crop protection to ensure global food security under climate change. While climate change incorporates several factors including changes in global temperature and rainfall, we were interested in the impacts of elevated atmospheric carbon dioxide concentrations (eCO2) on insect herbivory.
Impacts of elevated CO2 on plants and insects
Indeed, eCO2 is known to alter plant defence mechanisms, their chemistry and physiology. Typically, as concentrations of CO2 increase, the net carbon uptake of plants increases which causes the carbon to nitrogen ratio (C:N) of plant tissue to increase. This C:N is an indicator of the nutritional value of plant material to insect herbivores, as nitrogen is typically a limiting factor in their diets. Therefore, many insects have exhibited a compensatory feeding response to an increase in C:N, as they attempt to meet their nitrogen dietary requirements. This increase in consumption suggests a possible exacerbation of damage from insect herbivores under eCO2. Of course, it is difficult to truly replicate the gradual increases in global atmospheric CO2 concentrations, and possible acclimation responses to eCO2 by plants is difficult to test experimentally, although may not be so relevant to semi-perennial/annual plant systems and agricultural crops that are regularly harvested.
There are relatively few studies that focus on climate impacts on root-feeding insects (compared to aboveground insects), yet these belowground herbivores significantly reduce yield in many agricultural systems. It is therefore important to understand how plant-insect interactions belowground will be altered by eCO2, especially in the context of novel control strategies that remediate any possible adverse effects of climate change in crop plant susceptibility to root-feeding insects.
Plant silicon defences
One promising avenue of research is plant silicon and the ecological significant of silicon in plants is only now being realised (see the Functional Ecology special issue ‘The Functional Role of Silicon in Plant Biology’ – Cooke, DeGabriel & Hartley 2016). Silicon plays a role in plant defences against pathogens and herbivores, and is known to promote growth in many plants, particularly those among the Poaceae, although this is a gross generalisation (see Katz 2014, 2015). Silicon is also known to alleviate abiotic plant stress including heavy metal toxicity, drought and heat stress (see meta-analysis by Cooke & Leishman 2016). Silicon in plants has been shown to have considerable negative effects on insect herbivores in aboveground systems (Reynolds, Keeping & Meyer 2009), yet only one study has investigated the impact of plant silicon on a root-feeding insect (Frew et al. 2016).
Considering that eCO2 dramatically alters plant-insect relationships and the importance of plant silicon defences and the effects on insect herbivores, we saw the need to investigate the efficacy of plant silicon-based defences against root feeding insects under eCO2. Sugarcane (Saccharum spp. hybrids) is a grass crop that is grown across Queensland and northern New South Wales, Australia. Larvae of the greyback cane beetle (Dermolepida albohirtum), known as canegrubs, feed on sugarcane roots and cause the sugar industry losses up to AU$40 million (~£24.4 million) when outbreaks occur. The economic significance of this pest provided a good model to investigate the impacts of plant silicon defences on a root-feeding insect under eCO2.
Our study found that eCO2 increased plant photosynthesis and biomass. While at the same time we observed an increase in root C:N, indicative of a decrease in plant nutritional value. We found that the canegrubs increased their consumption of sugarcane roots under eCO2 in response to this decrease in nutritional value, which has been observed in many other insect herbivores. Interestingly, we also found that the canegrub growth rates increased. These responses suggest that in the future, damage inflicted by root-feeding insects to agricultural crops could be exacerbated by increases in CO2 concentrations.
However, our study also demonstrated that increasing plant silicon, by applying soil silicon fertiliser, dramatically decreases both root consumption and performance of the canegrub, under both ambient and elevated CO2 conditions. In fact, increasing soil silicon almost entirely masked the impacts of eCO2 on canegrub root consumption and growth rates. The negative effects of plant silicon on insect herbivores is largely attributed to an increase in the physical toughness of the plant tissue, reducing digestibility through mechanical protection of the parenchyma cells, where insects retrieve much of their starch and protein.
Implications for agriculture
So what are the implications of our findings for crop production and agriculture? As current control strategies, such as the application of pesticides, are becoming more restricted and are often environmentally unsustainable, novel pest management alternatives are continually being researched. Plant silicon defences should play a central role in climate change remediation regarding pest management. While much work on the role of silicon in plants has focussed on high silicon accumulating Poaceae crops, silicon has been shown to have significant impacts in many non-grass species including horticultural crop species (Katz 2014).
Indeed, it is common for plant-available silicon to be depleted in agricultural systems, and as silicon is still not considered an essential nutrient for crops, it is not often considered when growers attempt to optimise growth and yield. Our study suggests it would be beneficial to characterise bioavailable silicon in field soils which would facilitate targeted application of silicon fertilisers, which are already commercially available to agriculture. Silicon uptake by crops can vary markedly between different varieties (Soininen et al. 2013), which highlights the potential for crop breeding programs to select for varieties with higher silicon accumulation traits. In taking these steps, the potential crop pest exacerbation by climate change could be remediated by exploiting a previously undervalued, natural plant defence.