Flowering plants face a problem that we mammals are thankfully spared: how to avoid self-fertilization and, hence, inbreeding. To look at a flower, autopollination might seem inevitable--the long, slender pistil, which houses the egg cells, sits right next to the pollen-bearing stamens. But these plants have evolved a unique mechanism called self-incompatibility to prevent just that. Biologists have understood the pistils contribution to this defense for some time. The male half of the equation has long eluded them, however. Until now. Two reports published today in the journal Nature shed light on what pollen does to steer clear of self-fertilization.

A single gene known as the S-RNase gene regulates the pistil's role in self-incompatibility. This gene has many variants, or alleles, and in order for reproduction to occur, the egg and pollen must carry different alleles. When pollen lands on a flowers pistil, proteins on the pistils surface allow it to compare the pollens S-allele with its own. If it is different, the pollen is allowed to travel down a tunnel called the style to fertilize the egg inside. But if it is the same, the pistil rejects the pollen. Ten years after the discovery of that gene, Teh-hui Kao of Pennsylvania State University and his colleagues, working with the petunia plant, have finally identified the pollen component of self-incompatibility--a gene dubbed PiSLF.

The second study, conducted by Stephen G. Thomas and Veronica E. Franklin-Tong of the University of Birmingham in England, reveals that rejection by the pistil in effect causes the pollen to commit suicide. Experimenting with the common poppy plant, Papaver rheas, the team uncovered the biochemical events induced in the pollen when it encounters an incompatible pistil. The specific chemicals produced in the pollen suggest that its demise occurs by way of a mechanism called programmed cell death, which has never before been observed in the context of self-fertilization avoidance.

In an accompanying article, Bruce McClure, a plant biochemist at the University of Missouri, discusses the importance of the two studies for understanding how self-incompatibility controls plant reproduction. "Given the new findings," he writes, "research can now shift to decoding the biochemical details of the exchange between pollen and pistil." --Alla Katsnelson