SEB Bulletin October 2005 - Plant-aphid Interactions
Aphids have gained a reputation as a notorious pest species, and are unwelcome visitors on a huge number of crop plants1. They belong to the Hemiptera (true bugs)2, the only group of insects that have evolved the ability to feed primarily on the plant sap3, through their piercing and sucking mouthparts4 . A long stylet, containing a food canal and a saliva canal, is inserted into the plant sieve element, the tissue specialised for carrying dissolved nutrients throughout the plant5. The sieve elements are long cells that have no nucleus and are connected end on end by sieve plates, which form the end walls of the cells6. Each sieve element is closely associated with a companion cell which regulates the flow of molecules through the phloem and keeps the sieve element alive7.
Feeding this way causes problems for the plant: aphids can transmit plant viruses, and the sticky honeydew they exude can attract sooty moulds to grow8, although only extreme infestations are fatal. Due to their unusual lifecycle, they can reproduce very quickly. In the summer, aphids reproduce asexually, with females giving birth to live young and populations can double every 3-4 days under ideal conditions. As winter approaches, sexual males and females are produced, and after mating the females lay overwintering eggs that hatch in the spring when the weather warms up again9. Not good news for the gardener lovingly tending their plants! The response of aphids to warmer weather conditions is one interesting question that emerged from the session on insect-plant interactions at the Annual Main meeting10, organised by Dr. Jeremy Pritchard of the University of Birmingham11. “We don't yet know if the response of sucking pests to global warming is predictable”, says Dr. Pritchard, “so we need to understand the mechanistic basis of the ecological interactions between aphids and plants”. The session brought together researchers working on all aspects of the interaction, from aphids' feeding habits to how plants defend themselves.
The first thing an aphid has to do is find a phloem from which to feed, but because it has to push its stylet through many layers of plant tissue, how does it know when it has reached its target? According to Dr. Freddy Tjallingii, of Wageningen University12, researchers don't yet know what cues the aphids are using to tell them they have reached phloem tissue. “Aphids puncture nearly every cell in the stylet path on the way to the phloem” he says “substances in the cell can be taken up the stylet to a gustatory organ which has hundreds of chemosensory receptors”. As the aphid gets closer to the phloem, it samples a greater proportion of the cells it punctures, but the criterion that allows them to start feeding is still a mystery. The aphids also spit out a gelling saliva that sets around the stylet, and this shields the stylet as it is pushed through plant tissue.
Researchers are able to investigate what happens as an aphid is piercing and then feeding from a plant by using a technique called the electrical penetration graph (EPG) technique13. “During an EPG recording, the aphid and the plant are connected to an electrical circuit, which is completed as soon as the aphid penetrates the plant”, Dr. Tjallingii explains. By looking at the waveforms generated due to voltage fluctuations in the circuit, researchers can begin to decode what they mean. For example, a fluctuating voltage can be caused by conductivity changes in the circuit, which may indicate valves opening or closing in the canals of the aphid stylet, or whether the aphid has punctured a living plant cell. These events will show up as different aspects of the EPG waveforms.
Once an aphid has pierced the phloem, it injects a watery type of saliva for up to 60 seconds, which shows up as one type of EPG waveform. The aphid will then start feeding up its food canal, but will still be injecting saliva down its saliva canal. This saliva is then taken back up again into the food canal, mixed with sap. The aphid does this to help overcome the plant's protein defence responses, which are activated to block the damage in phloem cells. For example, in extremely damaged cells the plastids can burst and release their contents which coagulate to plug sieve elements. Another response involves proteins associated with the endoplasmic reticulum of the sieve element. Released from their anchored position on the cell wall, they coagulate in response to wounding. It is vital for the aphid that these proteins do not coagulate in its stylet and prevent it from feeding either by the saliva injected into the sieve element before feeding or by adding the saliva to the ingested sap during feeding.
Prof. Aart van Bel, with Torsten Will at the University of Giessen14, work on broad bean plants (Fabacae) and are interested in one specific protein defence response. Fabaceen plants have a unique type of protein in their sieve elements called forisomes that seem to be an evolutionary novelty. “Forisomes probably act as 'stop-cocks' - as soon as a sieve element is damaged, calcium flows into the cell and the forisomes respond by expanding and blocking it off” says Prof. van Bel. If the damage is not too great, calcium is pumped out of the tube and the forisome contracts again, leaving the phloem free for transport. These mechanisms have recently been discovered and described by Dr. Michael Knoblauch et al. at the University of Giessen15. Forisomes are one of a whole catalogue of closure mechanisms available to plants when they are damaged16, and work in a matter of seconds. An important question was raised by the observation that a microcapillary the same diameter as an aphid stylet, was inducing the wound response, whereas when the aphid pierces the phloem, these responses were not occurring. The aphids clearly have the means to deal with the plant's defences, probably by affecting the flow of calcium into damaged cells. “The aphid probably restricts the flow of calcium in the plant in two ways. Firstly, by closing off the stylet wound with gelling saliva so calcium can't flow in from the outside.” The second mechanism probably involves the aphid preventing the plant from detecting changes in turgor pressure. “When a microcapillary is inserted into the cell you get a massive loss of turgor, which is probably detected by mechanoreceptive calcium channels” says Prof. van Bel. Calcium will then flow in and the closure mechanisms will set in motion, however this doesn't occur when the aphid stylet, which has a few million times lower inner volume than the microcapillary, is inserted.
There are still a few more hurdles the aphid has to overcome before it can enjoy its sugary meal. Whilst an aphid's diet sounds like heaven to anyone with a sweet tooth, it does pose problems, “Phloem sap is an extreme diet”, explains Prof. Angela Douglas, of the University of York17. “There are two main nutritional barriers [to using plant sap as a main food source]. The first is that sap has a very high sugar content, and therefore a very high osmotic pressure” she says. Aphids transform much of the sugar into long chain oligosaccharides to overcome this, otherwise they could lose body water to the gut as they feed. Indeed, treating aphids with an inhibitor of the sucrase enzyme, essential for oligosaccharide production has a dramatic effect: the aphids actually shrivel as they feed, losing body water to the gut. This discovery has applications for pest management - the aphid sucrase enzyme could be a suitable target for specific inhibitors that would abolish its dual role in osmoregulation and carbon nutrition of the aphid.
The second problem with feeding on sap is that it is low in essential amino acids, nutrients which animals cannot make themselves and therefore must obtain from their diet. “Plant sap contains nitrogen, but very few essential amino acids, so one wouldn't expect the aphids to be able to make protein” says Prof. Douglas. This problem is solved by vertically transmitted symbiotic microorganisms18 (i.e. transmitted from mother to offspring). The symbiotic bacteria in aphids are called Buchnera and provide the aphid with essential amino acids, in a relationship that has evolved to the point that they cannot survive without each other. Prof. Douglas is interested in how effective Buchnera are in providing the aphids with their amino acid requirement on different host plants. “In some host plants, they are pretty ineffective, producing fewer amino acids and having a direct impact on aphid performance” she says. Current research is focussing on determining the mechanism - it's possible that an anti-microbial with the symbiotic bacteria as the primary target may have evolved in certain plants.
Much current research on aphids is focussing on aspects of the interactions with plants that could be exploited as control measures, with genetic and molecular studies giving insights into how aphids and plants interact with each other. All in all, the relationship between an aphid and its host is extremely complex, worth considering next time you discover an aphid infestation on your roses or favourite pot plant!
University of Cambridge
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15. Knoblauch et al. (2001) Plant Cell 13:1221-1230; Knoblauch et al., (2003) Nature Materials 2:600-603