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When Plants Need to Defend Themselves, They Tend to Reach for the Same Weapon

Aphids feeding on a plant stem Aphids feeding on a plant stem Aphids feeding on a plant stem
There is new evidence that different types of plants use similar defense strategies against sap-feeding insects, like the aphids pictured here (Photo credit: Christophe Blouin, Pixabay).

A new meta-analysis led by Dr. Daniel Leybourne, a postdoctoral researcher at the Institute of Geobotany at Leibniz University of Hannover, suggests that plants resistant to certain insect pests deploy common defense strategies that could be co-opted to develop insect-resistant crop varieties.

Insect pests present a major production challenge for farmers around the globe by inflicting crop damage and reducing crop yields. These negative effects are expected to increase in response to climate change; some climate predictions suggest insect-related yield losses could increase by 10% for every 1˚C rise in temperature. Pesticides are the most common means of controlling herbivorous insect pests, but they have significant negative environmental and human health consequences, and are contributing to the global decline in beneficial insects. The broad use of pesticides has also fueled the evolution of pesticide-resistant insect populations, making crops even more vulnerable to pest infestations. 

Some plants, though, have developed special traits to defend themselves against insects. The defensive traits of these so-called resistant plants offer an alternative to controlling insect pests with pesticides. By comparing how insect pests feed on resistant versus more susceptible plants, researchers can determine what physical, nutritional, chemical, and molecular traits plants use to protect themselves. Identifying these traits can then lead to the development of pest-resistant crop varieties that are less reliant on pesticides to protect them.

There are numerous studies on plant defense mechanisms against insects, but it remains unclear whether different groups of plants use the same defensive strategies and whether these strategies are effective against different groups of insect pests. Leybourne and co-author Dr. Guðbjörg Inga Aradóttir, an entomologist at the National Institute of Agricultural Botany in the UK, addressed this question by synthesizing results from 129 studies evaluating how more than 30 species of plants defend themselves against dozens of different insect species.

As Leybourne explains, “After several discussions with Dr Aradóttir about how different plants defend themselves against different kinds of insects, we noticed we were making similar observations, indicating similar defensive mechanisms seemed to be present in different plant groups. We wanted to explore this further by stepping back and looking at the bigger picture. We decided the best way to do this was to conduct a meta-analysis, which is a statistical technique used to identify common trends and patterns across multiple studies. Our goal was to answer a fairly simple question: are the defense mechanisms used by resistant plants similar across different plant groups and are these defense mechanisms common across multiple plant-insect combinations?”

Leybourne and Aradóttir focused on plant defenses against sap-feeding insects—such as aphids, whiteflies, and leafhoppers—which are one of the most economically damaging groups of insect pests and for which there is an extensive body of research. Sap-feeding insects use specialized feeding structures (stylets) to pierce the plant’s flesh and siphon away the plant’s nutritional resources, which can directly harm the plant as well as facilitate the transmission of plant pathogens.

The meta-analysis revealed for the first time that multiple plant families use the same defensive traits against specific groups of insects. For example, cole crops and grasses use similar strategies to protect themselves from aphids. The authors found that insect-resistant plants tend to have a higher abundance of certain organic chemicals as well as a higher density of hair-like structures, or trichomes, on their leaves. These defensive strategies appear to make it harder for insects to puncture or probe the plant and reach the plant sap, and tend to be effective against many kinds of sap-feeding insects. These commonalities suggest there could be potential to develop crop varieties that have broad resistance against insects. However, more in-depth studies at the plant-insect species level are needed to determine whether different insect species within the same family or group have similar responses to plant defenses.

“We were really encouraged to see our initial observation borne out; that the plant defensive strategies used to defend against one aphid species in one plant type were the same as those used in other plant-aphid combinations. The most surprising and exciting finding was observing similar trends in other sap-feeding insects, not just aphids. This indicates plants use similar mechanisms to protect themselves regardless of the insect species they are protecting themselves from. This is a really important discovery that can be used to develop plant varieties that are resistant against multiple insect pests, and we hope it will be used as the basis for future crop breeding research,” explains Leybourne.

This open access manuscript published in Scientific Reports was supported by an Alexander von Humboldt Postdoctoral Research Fellowship.

Original Publication: Leybourne, D.J., Aradottir, G.I. Common resistance mechanisms are deployed by plants against sap-feeding herbivorous insects: insights from a meta-analysis and systematic review. Sci Rep 12, 17836 (2022).