Welcome to the section on Micronaut.ch which explores, analyzes and explains fascinating microscopical structures.
With the summer storm comes death. The freshening wind has blown a single spore of a rust fungus on a plant leaf. There it takes a rest and waits for the rain. When the humidity finally reaches 100 percent, it germinates and begins its deadly work.
Text by science writer Atlant Bieri
Photography by Martin Oeggerli
From it grows a tube that slowly gropes its way ahead. After a while it suddenly stops. It has found a bump – a mere 0.5 micrometres high ledge. The fungus has found what it has been looking for: a stoma. This is one of millions of little mouths with which the plant breathes. On this picture they are blue and framed in red. Exactly above it, the fungus forms a kind of suction cup that connects it inseparably with the leaf.
During the entire time the plant remains, of course, not idle. The chemical trail of the fungus has already been a warning. It responses by closing the stoma, in order to make life as difficult as possible for the intruder. But the fungus is well prepared. At the bottom of the suction cup it now grows an infection wedge, which is the fungal equivalent of a drill. With this superb tool, it forges a way through the closed stoma. The plant has no chance. Immediately below the stoma lies a cavity. There, the fungus forms a spherical structure. That’s its base camp. The tube on the outside of the leaf and the suction cup are no longer needed and die off.
The life cycle of a fungus – Eat beyond the own nose, have sex and spread your spores!
At its new location, it grows another tube. Since its arrival as a spore, it has taken no food and will starve to death if it does not hurry. The tube attaches itself to the nearest cell and penetrates through its outer wall. Right behind it, a thin membrane emerges. It is the last layer of protection before the cell leaks like a stuck egg. This, however, is something the fungus wants to prevent. Because as soon as it destroys a single cell, the plant will make use of its most potent defence weapon: suicide. It kills off all the cells in the vicinity of the injury, and thus deprives the intruder of its food source.
Because of these deadly consequences, the fungus does not break through the membrane, but only invaginates it, like a finger that pierces a balloon without destroying its plastic skin. At this intimate contact point, the fungus now steals the plants sugars. It’s an inexhaustible source of nutrients. Soon after, it grows more tubes to other cells and thus the infection spreads.
After a week the fungus has gorged itself and it is time to reproduce. For this, it forms a bump just below the leaf surface, similar to a plague-spot. In it, one million spores mature. Finally, the pressure becomes so great that it bursts open to the outside and tears a big hole into the leaf. The spores (orange and turquoise dots) are thrown out and carried away by the next gust of wind.
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