Turning defence into a carnivorous offence in the Cape sundew

Insects landing on the carnivorous Cape sundew (Drosera capensis) don’t stand a chance. The sticky mucilage secreted from their leaves holds the victim in place while the leaf curls over to get a better grip. The plant then releases digestive enzymes that break down the insect so its nutrients can be absorbed into the leaf, supplementing the limited nutrients gained by the plant from the poor soil in which they grow.

Rapid reflexes

Like the animal nervous system, plants use electrical signals to carry messages over long distances, helping them to mount a rapid response to wounding, infection, or herbivore attack. Instead of travelling along nerves, plant electrical signals are believed to travel through the phloem or xylem vascular tissues, telling plants to accumulate the wound-response phytohormones; abscisic acid and jasmonates.

For centuries, carnivorous plants have been known to use electrical signals to react to prey too. Researchers hypothesised that these carnivory signals evolved from the plant defence response, and it was this theory that Miroslav Krausko and colleagues investigated in their recent paper in New Phytologist.

Testing tentacles

Krausko and the team investigated the sundew responses to touch, wounding and prey, using polystyrene balls or fruit flies (Drosophila melanogaster) to stimulate the tentacles, or mimicking herbivory by piercing the leaf 20 times with a needle.

Image: Responses of sundew traps to different stimuli. Image credit: Krausko et al. (2016). New Phytologist.
Responses of sundew traps to different stimuli. Image credit: Krausko et al. (2016). New Phytologist.

While mechanical stimulation using the polystyrene balls did lead to some minor leaf bending in the sundews, it was the prey stimulus that really made them move! Wounding the leaf did not cause the traps to bend, but the application of jasmonic acid, which plays a role in both the wound and carnivory responses, resulted in a tightly curled trap. This suggested that the plants might be able to differentiate between the patterns of jasmonate release in the wounding and prey responses.

Krausko et al. used electrodes on the surface of the tentacles and leaves to measure the electrical signals involved in the different responses. Wounding led to a systemic electrical signal travelling throughout the plant, while prey and mechanical stimulation of the tentacles caused a very localised response. These differing electrical signals resulted in precise patterns of jasmonate accumulation.

Drooling Drosera

Image: A close-up look at Cape sundew tentacles, and their sticky dew drops of mucilage. Image credit: Incidencematrix. Used under licence: CC BY 2.0.
A close-up look at Cape sundew tentacles, and their sticky dew drops of mucilage. Image credit: Incidencematrix. Used under licence: CC BY 2.0.

The localised electrical signal from the prey stimulus lead to an accumulation of jasmonates at that location and caused the traps to bend, as demonstrated previously by Nakamura and colleagues. Krausko et al. found that the jasmonates in turn induced the production of digestive enzymes to break down the prey, while other plant defence phytohormones (abscisic acid and salicylic acid) were not able to stimulate enzyme production. Using gas chromatography-mass spectrometry, they also identified seven new proteins in the digestive fluid, which now require further investigation.

To test whether the increased jasmonates in the prey response were simply a result of more mechanical stimulation by struggling fruit flies, Krausko and colleagues applied dead, crushed flies to the traps and demonstrated that the fly molecules themselves were enough to stimulate the enzyme activity.

Carnivory evolved from plant defence

In contrast to localised carnivorous response, the wounded plants accumulated higher levels of jasmonates in all leaves, leading to an increase in digestive enzyme activity and providing direct evidence that carnivory and plant defence are evolutionarily related mechanisms.

“We do not know yet what the purpose of this behaviour is. Why does the plant produce digestive enzymes when there is nothing to eat? We suggest that it is a result of evolutionary history, when ordinary plants used, as they do today, electrical signals and jasmonates for initiation of a defence response. The digestive enzymes are very similar to defence proteins. Carnivory has probably evolved from plant defence mechanisms,” concluded Andrej Pavlovič, corresponding author of the study.

But with such similar characteristics, how do carnivorous plants distinguish between the wounding and carnivory responses? Krausko et al. suggest that the systemic electrical signals and jasmonates spreading throughout wounded plants can trigger a false alarm, but the sundew quickly recognises the localised pattern of electrical signalling and jasmonate accumulation in combination with chemicals released by the prey, causing it to ramp up its secretion of digestive enzymes in the carnivorous response at the appropriate place.

Read the paper:

Krausko, M., Perutka, Z., Šebela, M., Šamajová, O., Šamaj, J., Novák, O. and Pavlovič, A. (2016). The role of electrical and jasmonate signalling in the recognition of captured prey in the carnivorous sundew plant Drosera capensis. New Phytologist. doi: 10.1111/nph.14352


 

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Sarah Jose
@JoseSci

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