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Toxic blocker.
Enzymes and other proteins (blue and green dots) that prevent inbreeding are taken up by pollen tubes (outlined in light blue) and released when inbreeding is imminent.

Ariel Goldraij, Mayandi Sivaguru, and Bruce McClure, University of Missouri at Columbia

How Plants Avoid Incest

Cultural taboos keep most humans from inbreeding, but plants must rely on a complex internal warning system. In a new study, scientists have teased apart how this botanic chastity belt operates. As many commercial plant species have lost the ability to guard against incest, the findings may help breeders circumvent inbreeding to create more robust plants.

Since the early 20th century, scientists have known that many plants reject their own pollen as well as that from close relatives. Only in the 1980s did researchers find a class of enzymes in the pistil, or female part of plants, that could destroy "incompatible" pollen. If compatible pollen entered the flower, the reasoning went, these enzymes, called S-RNases, were somehow cleared away before they could do any damage.

Now Bruce McClure, a biochemist from the University of Missouri at Columbia, and colleagues have upset this notion. When the researchers examined compatible and incompatible pistil-pollen combinations in tobacco plants, they found that S-RNase got inside the pollen in both cases. So how does compatible pollen escape the enzyme's wrath?

The answer lies in the pollen's ability to keep the toxin bottled up. By tracking S-RNase with glowing molecules, the scientists found that--in both compatible and incompatible breeding--the pollen trapped the enzyme within a compartment, where it couldn't do any harm. But after 36 hours, the compartments containing the incompatible pollen disintegrated, unleashing S-RNase. Further experiments indicated that a protein called HT-B is present in much higher levels during incompatible pollination and may help break down the compartment walls, the team reports 16 February in Nature.

The new findings "are quite revolutionary," says Veronica Franklin-Tong, a biochemist at the University of Birmingham in the United Kingdom. Plant geneticist Amy Iezzoni of Michigan State University in Lansing agrees. Many details in the mechanism still need to be worked out, she says, but this paper will have "a huge impact" on the field.

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