How Plants Avoid Incest

There's no such thing as a cultural taboo if you are a plant. But then, how do plants avoid the negative effects of inbreeding? Scientists have known since the early 20th century that many plants reject their own pollen as well as that from close relatives. But how do they do it?

This question is not only of academic interest. Due to the fact that commercial plants have been cultivated by humans for millennia, most of them lost this natural ability. Thus, scientists now hope to be able to put it back into the plants in order to create more robust specimens.

In 1980s the researchers finally managed to get a glimpse of how plants' natural system of avoiding inbreeding works. Scientists discovered a class of enzymes (called S-RNases) in the pistil (the female part of the plant) that destroys the unwanted pollen. How is it that these enzymes don't also destroy all pollen was not known. That is - until now.

Most scientists speculated that the S-RNases are probably somehow cleared away when good pollen comes along. This is not so, Bruce McClure, a biochemist from the University of Missouri at Columbia and colleagues have now discovered.

They have discovered that the enzyme gets inside the pollen irrespective of whether the pollen is compatible or incompatible with the pistil. The trick goes on inside the pollen.

Both compatible and incompatible pollen trap the toxic enzyme inside a compartment. But the incompatible pollen manages to keep the enzyme trapped only for 36 hours. Another protein, called HT-B, which helps the disintegration of the compartment's wall, was also found to exist in larger quantities in case on incompatible pollen. Thus it seems that the incompatible pollen is less able to protect itself from the HT-B protein and is also less capable to keep the toxin captured.

Other biochemists and plant geneticists are thrilled by the new findings and say they are "quite revolutionary" and that they will have a "huge impact" on the field.

Photo Credit: PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY