Carnivorous plants turned defense into highly successful offense

Carnivorous plants have evolved in many different taxa of plants, and in locations all around the world. They use a range of modifications to entrap insect prey and then digest them, absorbing the nutrients from the insect bodies to supplement the minerals normally available in the soil. Although carnivorous plants have fascinated people for hundreds of years, the evolution of their ability to catch and digest live prey has remained obscure.

In new research, scientists from the Salk Institute, along with collaborators from Washington University in St. Louis, have investigated the molecular basis of plant carnivory to establish how carnivorous plants developed prey-capture-associated behaviors, such as leaf movements and digestive enzyme secretion.  

Previous research has indicated that many proteins involved in the digestion of live prey by carnivorous plants are related to those used by other flowering plants to defend themselves against pathogens and pests. In the current study, the experts investigated whether perhaps carnivorous sundew plants evolved the ability to capture and consume prey by repurposing molecular mechanisms used originally for similar defense purposes. 

“If we can learn more about how plants such as these and others have adapted to respond to their unique environments, then perhaps we can alter these molecular pathways in the future, to develop plants that can survive in harsher conditions,” said study co-author Professor Joanne Chory.

Biologists believe that plants such as the spoon-leaved sundew (Drosera spatulata) likely adopted carnivory to survive in nutrient-poor conditions. However, sundews are difficult to cultivate and their DNA wasn’t sequenced until recently, so scientists could not examine how carnivory works in these plants at a cellular level. 

“Carnivorous sundew plants are not model organisms,” said study co-author Carl Procko. “Less than a handful of labs in the world have previously been able to genetically modify them, so we’ve had to learn new techniques to closely examine them.”

In Venus flytraps (Dionaea muscipula), the closure response of the leaves has been related to the movement of calcium ions from sensory hairs into the leaf blade, in response to mechanical stimulation of the hair. For the current study, the researchers investigated how calcium molecules move dynamically within cells in the leaves of carnivorous sundew plants, in response to touch from live prey. They applied genetic tools to image the dynamic changes of calcium molecules in the leaves as insect prey landed on sundew leaves and were captured there by sticky secretions.  

In non-carnivorous plants, calcium signaling plays many important roles, such as triggering the jasmonic acid defense pathway to repel unwanted insect pests. Jasmonic acid also responds to electrical activity, which is a critical element of prey capture in some carnivorous plants, including sundews. 

The scientists wanted to know if this same defense pathway in non-carnivorous plants might also be required for the sundew’s carnivorous behavior. They found that changes in calcium levels within plant cells did lead to leaf movements that enabled the capture of prey. Fluctuations in calcium within the plant cell resulted in the activation of genes typically targeted by jasmonic acid, as the leaf bent inwards and trapped the insect in sticky digestive secretions. 

The researchers further observed that sundew leaves bent less when they were given non-living prey and when their calcium channels were blocked. These findings demonstrate that calcium aids in insect prey-capture responses in sundews and, together with the work of others, supports the idea that jasmonic acid is involved in insect digestion.

“It was fascinating to see how these plants respond to prey-associated mechanical stimulation, like touch,” said study co-author Ivan Radin. “The ability to sense and respond to mechanical forces is something most people don’t associate with plants, especially on this rapid timescale. Our work provides a beautiful visual of this fact.”

“The findings show that calcium is also involved and likely ramps up jasmonic acid responses, similar to how non-carnivorous plants respond defensively to mechanical stimulation from pests,” said Chory. “This provides further credence to the notion that carnivory in sundews may be an evolved defense pathway.”

In future, the researchers wish to apply similar genetic techniques to study other carnivorous plants that have been too difficult to examine until now. They hope to investigate the molecular basis of prey-capture mechanisms to understand better how carnivory evolved in distantly related species and to see if crops could benefit from adapting their existing environmental and insect response pathways to survive in challenging environments.   

The study is published today in the Proceedings of the National Academy of Sciences.

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By Alison Bosman, Earth.com Staff Writer