Leaves Protect From Pathogens

small white flowers with yellow stamen and green stems poking out in between, shallow focus with green blurred background

Biological Strategy

Thale cress

AskNature Team

Image: Will George / Flickr / Some rights reserved

Protect From Microbes

In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Sense Disease in a Living System

Living systems that can detect diseases within themselves, or within other living systems, can direct resources toward curing those diseases or preventing their spread. This type of sensing environmental cues is not well known, probably because of the difficulty studying disease detection, including the difficulty in providing scientific proof. However, there are a few known examples, such aschimpanzees that detect illness in themselves and self-medicate through eating plants with medicinal benefits.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Leaves of thale cress protect the plant from pathogenic microorganisms by interrupting the cellular signals that otherwise keep the stoma open and vulnerable to bacterial invasion.

Plants rely on complex innate immune systems to make up for their lack of adaptive systems. They use complex signaling cascades that can detect and respond to bacterial and fungal pathogens both inside and outside their cells. One type of response is for the plant to close their stoma–the small openings on its leaves–which denies the pathogen entry points into the plant. The RIN4 has recently been shown to be instrumental in this process. Under normal circumstances, proteins embedded in the cell membrane send signals to RIN4 to keep stoma opened. When chemical signals from pathogenic microbes are detected, these signals stop and the stoma close.