Perfect timing: fungi use plant cues (some fruits increase production of ethylene as they ripen, causing fungi spores to germinate) (Brief Article)

Scientists have long wondered how a green, unblemished tomato can arrive at market a black, moldy mess. Now they know that the tomato surface may harbor invisible fungal spores that sense the production of a gas called ethylene as the fruit ripens. The ethylene causes the spores to germinate just as the fruit becomes most vulnerable, says Pappachan E. Kolattukudy, a biochemist at Ohio State University in Columbus. Certain plants--bananas, avocados, and tomatoes, for example--sharply increase their production of ethylene as they ripen.

Thus, for the fungus to use this cue "is a simple, logical, failsafe mechanism," Kolattukudy says.

He and visiting Israeli plant pathologist Moshe A. Flaishman observed that spores placed on a microscope slide reacted to ethylene but not to other, similar hydrocarbon gases such as methane. The spores also became active when placed on unripe fruit and exposed to ethylene or to a compound that produced this gas. However, they did not germinate when placed on a transgenic tomato incapable of making ethylene, the researchers report in the July 5 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

After exposure, the dormant spores send out thin tubes. A spoonlike plate forms at the end of each tube, attaches to the fruit surface, and penetrates the surface with a pointed peg. "The infection then takes off," Kolattukudy says.

The same chemicals that inhibit a plant's ability to react to ethylene also make the spores insensitive to this gas, an observation suggesting that the fungi may have co-opted the plant's ethylene receptor for their own use, he adds.