Killer Plants

Carnivorous plants around the world all developed their killer habit in surprisingly similar fashion, according to a genetic study of distantly related pitcher plants from Australia, Asia and America.

The plants became flesh-eaters as a means of making their own “fertiliser”.

Findings published in Nature Ecology and Evolution show while the three species evolved independently in different parts of the world over time, the biological processes they use to digest insects are incredibly alike.

Co-author Victor Albert, from Buffalo University, said the finding suggested there were limited evolutionary pathways by which plants could develop flesh-eating traits.

Carnivorous plants grow in very nutrient-poor habitats and produce carbon through the usual photosynthetic processes, just like other plants.

However, Dr Albert said, because their habitats failed to provide enough phosphorus and nitrogen, nature’s equivalent of “fertiliser”, they had to develop unusual ways to collect these nutrients.

“The animals [they prey on] have this in abundance in the form of the proteins and nucleic acids that are released when the plant digests them,” he said.

 

Pitcher plants capture insects by luring them into a trap, a cupped leaf with a waxy, slippery interior that makes it difficult to climb out. A “soup” of digestive fluids sits at the bottom of this chamber and breaks down the flesh and exoskeletons of prey.

To better understand how these flesh-eating plants evolved, Dr Albert and colleagues mapped the genome of Cephalotus follicularis, a pitcher plant that is native to south-west Australia and closely related to star fruit.

“Cephalotus was a great plant to study because the plants bear two different kinds of leaves, one being the flat leaves, specialised for light gathering as in other plants, and the other being the elaborate pitchers used by the plants to capture prey,” Dr Albert said.

By mapping its genome the team was able to uncover which genes were expressed only in pitcher leaves, and those related to photosynthetic leaves, or both.

“In this way, we were able to discern that several genes encoding digestive enzymes were preferentially expressed in [the pitcher plant] traps,” he said.

The team then sequenced the digestive enzymes and digestive fluid proteins of this species and two other species: Asian pitcher plant Nepenthes alata, which is related to buckwheat, and the American pitcher plant Sarracenia purpurea, which is related to kiwi fruit.

This showed that over time, plant proteins previously used to fight disease and stress had evolved into the digestive enzymes that break down the insects’ bodies.

“While we knew that carnivorous pitcher plants had evolved three different times, independently, from within different lineages of flowering plants, we didn’t realise until now that the protein families used by these plants for their digestive functions had been repurposed independently as well,” he said.

“Even more exciting was that within some of these enzyme families, changes in the amino acid sequences of the proteins also occurred convergently.”

In other words, some of the critical enzymes evolved entirely separately, in different places and lineages.

These included chitinase, which breaks down chitin, the major component of insects’ exoskeletons and purple acid phosphatase, which enables plants to obtain phosphorus from its prey’s body parts.

Dr Albert said the finding was an example of “nested convergence”, convergent evolution at several hierarchical levels.

These were morphological (pitchers are very similar yet develop totally differently in the three pitcher plant groups); genetic (convergent use of the same or similar enzyme groups); and molecular (amino acids show some convergent changes only in the carnivores).


 

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