The evolution of carnivorous plants is a story of job-changing genes

Before the 18th century, people generally believed that plants could not move and could only exist as food for animals. It was even more unbelievable that plants could prey on animals.

Darwin spent 16 years conducting meticulous experiments on plants. He observed that the leaves of some plants had specialized into strange structures that could not only capture insects and other small animals, but also digest and absorb the nutrients in them. This discovery was later recorded in detail in his book "Insectivorous Plants".

If the publication of "Carnivorous Plants" changed the image of plants as "harmless to humans and animals", then the book "The Power of Plants to Move" "liberated" plants from the inherent impression that they are rigid and immobile.

Suddenly, plants became no different from animals, able to move and kill people. This new knowledge made horror stories about plants killing people a very popular story theme, but on the other hand, it also inspired generations of biologists to understand and decipher these incredible and weird plants.

A 19th-century writer described a man-eating tree. Source: JW BUEL / PUBLIC DOMAIN

So strange

Take another look

Currently, there are 12 families, 20 genera and about 860 species of known carnivorous plants recorded in the world. Most of these plants grow in nutrient-poor areas, such as humid acidic swamps or poor tropical soils. All habitats are characterized by a lack of nitrogen and phosphorus necessary for plant growth. Therefore, in order to survive, they need to find alternative sources of important nutrients.

Image credit: Royal Botanic Gardens Victoria

Driven by evolution, plants living in "barren lands" turned their targets to insects and other small invertebrates, which are rich in protein and various elements needed for plant growth. In order to deal with them, carnivorous plants have also acquired unique predation methods.

In terms of trapping methods, all carnivorous plants currently use six basic mechanisms, namely traps, adhesives, clamp-type insect traps, lobster cage principle insect traps that cannot find the exit or entrance after entering, pigeon cage principle insect traps that can only move forward but not backward after entering, and sac-type insect traps that can create a vacuum to suck out prey.

Among them, most of the trapping methods are very "dull". For example, the leaves of pitcher plants such as Nepenthes, Heliamphora, and Sarracenia, which are good at making traps for hunting, are mostly highly specialized and become complex bottle-shaped containers filled with digestive juices.

Nepenthes species specimens collected in Malaysian Borneo. Image courtesy of A. van der Ent (1) & A. Robinson

These specialized leaves can also store water, or attract animals through the smell of nectar glands or the reflection of different spectra. When the "melon-eating crowd" who don't know the truth lands on the leaves, due to the special smooth structure at the top of the bottle mouth, many insects accidentally fall into the bottle while walking on it.

Image source: See image watermark

It is basically difficult for prey that falls into the bottle to climb out, because the bottle wall is covered with a smooth wax layer. At the same time, the digestive fluid at the bottom of the bottle has a low surface tension and is very viscous, which means that once the prey falls into the bottle, it will quickly sink to the liquid surface and finally be decomposed by the digestive fluid for its own use.

Of course, there are also more flexible trapping methods, such as plants of the genus Drosera, which have relatively three-dimensional and movable sticky tentacle insect traps that secrete mucus that attracts insects. When the prey is stuck, the tentacles that were originally stretched out will actively bend in the direction of the prey, allowing more mucus to wrap around the prey, suffocating it to death, while increasing the contact range between the digestive gland and the prey, thereby killing the prey more quickly to obtain nutrition.

Image source: ISTOCK.COM / CATHY KEIFER

However, the one that can be called an "elite hunter" is the Venus flytrap (Dionaea muscipula), which has a sophisticated and complex response system. Its unique clip-shaped insect trap is highly specialized from leaves and connected by the midrib. There are sensitive tactile hairs on the inner surface of each clip. When the tactile hairs bend, they will trigger the ion channels of the base cells to open, generate an action potential and transmit it to the midrib, closing the clip. What's more amazing is that the Venus flytrap can also distinguish whether the action potential is caused by the touch of an insect or a falling raindrop or dead leaf, and make corresponding judgments.

The Venus flytrap's trap closes in an instant, trapping its prey. Credit: ANDIA / ALAMY STOCK PHOTO

Although carnivorous plants vary greatly in shape, form and trapping methods, the basis of their prey is through specialized leaves or parts of leaves, which also means that carnivorous plants mainly obtain nutrients through leaves rather than roots.

This gene is useless.

Why don't you change your position?

So how did leaves, originally used for photosynthesis, become weapons of attack? The answer may lie in the genes.

For plants to eat meat, they need to solve two key problems: the first is digestion, and the second is absorption.

From now on, it seems that the evolutionary process of carnivorous plants was cunning and flexible. They used existing "resources" to gain new abilities. After all, it is easier to utilize "waste" than to recreate it.

As early as the 1970s, researchers discovered that the digestive juices of carnivorous plants contain a variety of enzymes, including chitinase, which can break down chitin in insect exoskeletons, proteases, which break down proteins in meat, and purple acid phosphatases, which can extract elemental phosphorus from prey. Initially, researchers did not know whether these enzymes were produced by the carnivorous plants themselves or by microorganisms in the digestive juices.

Image credit: H. ZELL / WIKIMEDIA COMMONS

It wasn't until the rapid development of DNA sequencing technology that molecular scientists were able to identify many of the genes that encode these enzymes that they discovered they had remained unchanged, but had been repurposed by carnivorous plants because the enzymes acted in a manner very similar to the chemical defenses that plants originally used to fight bacteria, fungi, and herbivorous insects.

For example, chitinase may have originally been used to fight fungi, which contain chitin in their cell walls, and later, as arthropods evolved, chitinase was also used to defend against them.

As the trapped animal is digested, large molecules such as chitin and protein are broken down into smaller molecules, which the plant must transport from the digestive fluid into the body. In normal plants, absorbing nutrients is the job of the roots, and transporters continuously transport nutrients from the soil to the plant. In carnivorous plants, in order to allow the leaves to absorb nutrients, they put the transporter gene that works in the roots in the leaves. However, the gene will always remain active in the roots, but in the leaves, the transporter will only start when nutrients need to be absorbed.

For ancient plants, the reuse of a large number of repeated genes allows them to gain stronger adaptability in the environment, which may be the reason why plants can acquire the ability to eat carnivores.

References:

1.on the Origin of Carnivory: Molecular Physiology and Evolution of Plants on an Animal Diet

https://doi.org/10.1146/annurev-arplant-080620-010429

2.Venus Flytrap: How an Excitable, Carnivorous Plant Works

https://www.sciencedirect.com/science/article/abs/pii/S1360138517302807

3. Venus flytrap carnivorous lifestyle builds on herbivore defense strategies

https://genome.cshlp.org/content/early/2016/04/28/gr.202200.115.full.pdf

Author: Fish

Winner of the Silver Award for Outstanding Science Popularization Works of China Science Writers Association

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