How do plants identify the harmful insect egg?
New NWO/STW VIDI-research by Wageningen University & Research, tries to enhance the plant’s recognition of harmful insect eggs, in order to promote crop resistances and biological control of pests.
Various plants possess a unique first line-of-defence, that helps them get rid of enemies before they can become destructive: egg-killing. For example, some butterfly eggs can trigger a so-called hypersensitive response in the plant. This response leads to a programmed cell death of the plant tissue around the egg and, thus, leads to desiccation or dropping of the egg. This way, the plant prevents the eggs from hatching. Consequently, less caterpillars will damage the plant. On top of that, it appears that many of these plants make themselves more attractive to parasitic wasps that kill the harmful eggs. So, the first line-of-defence is actually a double line!
The purpose of our research is to find out how plants recognize herbivore insect eggs, and subsequently activate this double line-of-defence. Are there specific signals or components that elicit this egg-killing? If these signals are known, it may be possible to enhance the existing egg-killing capabilities of plants. By switching on relevant genes via application of so-called chemical elicitors, it has been shown that the natural resistances of plants against pathogens may be enhanced. Also, their attractiveness to biological control agents against feeding insects can be enlarged.
Act before harm
Exploiting the natural egg-killing capabilities of plants is a very promising route to reduce crop losses in the future. In general, biological control through natural enemies of the harmful herbivores, as well as exploitation of the genetic variation in resistance traits among wild relatives, are two promising sustainable strategies to reduce pests. However, such traits often allow the pest to continue feeding and act when damage has already occurred. Killing the insect egg before the pest can cause harm is therefore an extra attractive option. However, to effectively do this, we need to study their underlying mechanisms.
‘Smart’ moths and butterflies cause major damage
We study caterpillars of the so-called lepidopteran insect species: butterflies and moths. They are major pests in forests, stored grains, and fibre and food crops. Domestication of plants has, unintentionally, led to reduced natural defence against these animals. For example, the bitter taste due to mustard oils in several cabbage species (Brassicaceae), is unfavourable for human consumers. It is, however, also a natural defence against insects. By selecting cabbages that have less of this unfavourable taste, we are stuck with cabbage crops that are more susceptible to insects.
The classic way to combat these insects is through insecticides. But many pest insect species have appeared ‘smart’ enough to develop resistance to these insecticides. Most herbivore insects have developed enzymes that reduce the toxicity of the insecticide, or the toxicity of the plant itself and thus have adapted to their host’s resistances. Some insects have become invasive too, causing increasing problems.
In some plants, genes are identified that cause resistance against insects that feed on their bast. Similar genes that cause resistance against feeding caterpillars are not known to date. There is, therefore, an urgent need to explore this field. How do insects overcome the plant defence mechanisms? And how do plants prevent caterpillars to do harm and to act at the beginning of plant attack: during egg-laying?
The sooner the better
For many herbivorous insects, in particular lepidopteran species, the egg stage is the first phase of colonizing a new host plant. From the plant’s point of view, this is the first and therefore likely the best moment to defend itself. Egg deposition of various herbivorous insects has been described to induce responses in different plant species, ranging from direct egg-killing to specific defences against the feeding larvae, induced by the egg laying. Plants are also known to attract egg parasitoids (e.g. Trichogramma wasps) through the release of volatiles (see also the WUR programme BINGO).
To further this research in early defence against pests, our aim is to identify different mechanisms that will help us to understand the ‘race of arms’ between lepidopteran pests and crop plants.
Unravelling an insect egg-killing plant trait
Before our NWO/STW Vidi project Pest killing plants: Unravelling a programmed cell death response lethal to insect eggs, eggs of different lepidopteran species and also of the Colorado Potato beetle have already been observed to trigger a necrosis in various plant species. This so-called hypersensitive response is a form of programmed cell death, activated in plants and animals in response to pathogens25. When triggered by insect eggs, larvae do not hatch due to desiccation and dropping of the egg.
In previous studies we have shown, that eggs of cabbage white butterflies (Pieris spp.), trigger a similar hypersensitive response in black mustard plants (Brassica nigra), wild relatives of cabbage crops. We observed, however, a phenotypic variation in this response, both between and within plant populations. Different genotypes can express no, mild or strong necrosis when infested by eggs of the cabbage white butterflies. Moreover, we showed that black mustard plants expressing a hypersensitive response also become more attractive to egg-killing parasitic wasps, through the emission of plant volatiles.
This synergistic use of two egg-killing defence types was shown to lead to butterfly egg mortalities up to 80% in nature. This ‘double defence line’ is a rather unique way to combat pest insects and highly promising to be applied in crop protection.
The VIDI project aims to unravel the genetic basis and molecular mechanisms of the hypersensitive response necrosis induced by insect egg deposition. We will study the interaction between turnip rape (or Chinese cabbage, Brassica rapa) and specialist cabbage white butterflies (Pieris spp.). Of this cabbage species, many variations with accompanying genomic and genetic data are available. The egg-killing hypersensitive response through necrosis is a common trait in B. rapa. Together with its short generation time and high economic value, it makes this cabbage a highly suitable model plant. Because the necrosis resembles a resistance response to pathogens, we will also look at the role of insect symbionts, such as bacteria, transferred via the egg and their possible role in triggering egg-killing responses.
The project is supported by five breeding and seed companies: Hazera, Syngenta, Bayer Crop Science, Rijk Zwaan, and Keygene.
Prospects of pest-killing defenses for sustainable crop protection
Fatouros, N.E., Cusumano, A., Danchin, E.G.J., and Colazza, S. P. Ecology and Evolution, 6, 6906-18
Resisting the onset of herbivore attack : Plants perceive and respond to insect eggsCurrent Opinion in Plant Biology 32 (2016). - ISSN 1369-5266 - p. 9 - 16.
Attraction of egg-killing parasitoids toward induced plant volatiles in a multi-herbivore contextOecologia 179 (2015)1. - ISSN 0029-8549 - p. 163 - 174.
Early herbivore alert matters: plant-mediated effects of egg deposition on higher trophic levels benefit plant fitnessEcology Letters 18 (2015)9. - ISSN 1461-023X - p. 927 - 936.
Plant responses to insect egg depositionAnnual Review of Entomology 60 (2015). - ISSN 0066-4170 - p. 493 - 515.
To be in time: egg deposition enhances plant-mediated detection of young caterpillars by parasitoidsOecologia 177 (2015)2. - ISSN 0029-8549 - p. 477 - 486.
Synergistic effects of direct and indirect defences on herbivore egg survival in a wild cruciferProceedings of the Royal Society. B: Biological Sciences 281 (2014)1789. - ISSN 0962-8452 - 9 p.