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Genes prevent inbreeding, aid development.

Just as butterflies have harnessed genetic material essential in development to also create beautiful wing patterns (SN: 7/9/94, p.23), plants, too, have evolved new uses for existing genes. Scientists studying the genes that help some kinds of mustard plants avoid self-fertilization have new evidence that at least one of those genes helps form the shape of the much-studied Arabidopsis, also of the mustard family.

An understanding of these genes and how they work is aiding the study not only of fertilization, but also of plant development and cell recognition, says June B. Nasrallah of Cornell University.

Like many plants, mustard flowers contain both male (pollen-producing anthers) and female (ovule-containing pistils) parts, making possible self-fertilization. Plants such as Arabidopsis survive just fine whether fertilized by their own or another Arabidopsis' pollen. But for others, including mustards in the Brassica genus, inbreeding leads to ever more sickly descendants.

So it's no surprise that protective self-incompatibility mechanisms have evolved many times in the plant kingdom (SN: 2/19/94, p.117). In Brassica species, it's likely that a pollen protein mobilizes messenger molecules in the pistil that tell the plant to reject this grain, Nasrallah reported last week in San Francisco at the annual meeting of the Society for Cell Biology.

Her team previously had identified two proteins involved in conveying this rejection message. Their genes, and possibly a third gene, exist quite close together on one chromosome and are passed along as a unit during cell division, she now reports. Furthermore, each Brassica plant inherits 1 of 60 versions of this genetic unit. As a result, each of those genes' proteins differ slightly from one individual in a species to the next -- just enough that a given plant can tell if incoming pollen is its own.

In essence, these units form the basis of a cellular identification system. "It's a self-nonself recognition system, very much like immune systems in animals," comments Teh-hui Kao of Pennsylvania State University in University Park. Like immune systems, plant recognition is rather complicated, he adds.

One of the three genes specifies a glycoprotein -- a protein with a sugar attached. The other gene codes for an enzyme called S-locus receptor kinase. The kinase and glycoprotein look a lot alike and may bind to similar molecules, Nasrallah says. She thinks the putative third gene codes for such a molecule, a pollen protein.

Normally, the glycoprotein resides in the walls of cells at the pistil's tip. Because the glycoprotein dissolves in water, Nasrallah predicts it can relay the pollen's protein from outside the cell wall to the S-locus receptor kinase in the cell membrane. That kinase then stimulates a cascade of chemical reactions that signals the cell to prevent fertilization. Other kinases relay messages similarly in animal cells, likewise helping animal cells communicate, she adds.

Her team now has evidence that S-locus receptor kinase is active at the base of lateral stems in sprouting Arabidopsis. At these sites, the kinase may help shape the plant. When the Cornell scientists cause Arabidopsis to make too much of the kinase, much smaller cells and dwarf plants result. Perhaps the kinase tells cells to stop expanding, Nasrallah suggests.

"Even though self-compatibility is a very specific phenomenon and restricted to a very specific subset of plants, the genes have been recruited from genes that have a very different function," Nasrallah concludes.
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Title Annotation:plant genes that have evolved new uses
Author:Pennisi, Elizabeth
Publication:Science News
Date:Dec 24, 1994
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