Floral Arrangements

Natural History, May, 1999 by Meredith F. Small

Geneticists are studying the ABCs of building a blossom.

In the mid-nineteenth century, the Augustinian monk Gregor Mendel began crossbreeding peas to figure out how physical traits, such as color and height, were passed from generation to generation. Mendel watched as different pairings of parent plants resulted in offspring with short or tall stems, flowers close to the stem or farther out, and seeds of various colors and textures. These experiments were the first in plant genetics. Today plant geneticists still test their hypotheses by breeding and crossbreeding plants. But there are now available a variety of molecular techniques that illuminate the evolutionary relationships between species and explain the fine details of how genes influence the development of plants.

Barring mutations, each and every body cell in an organism contains an identical set of genes. Different genes have different functions: some, for example, code for proteins; others regulate the activities of other genes. In any given cell or group of cells, only some genes are expressed (that is, activated or "turned on"). Much recent research on flowers has focused on discovering which genes influence the development of a blossom and where in that blossom they are normally expressed.

The first step toward making a flower is the production of a floral meristem, an area of actively growing cells near the tip of a stem. The cells are at first undifferentiated, but as they divide, they form the flower's sepals, petals, pollen-producing stamens, and egg-bearing carpels. These parts arise in whorls around the flower's center, with leaflike sepals outermost and carpels innermost. In the early 1990s, working with Arabidopsis thaliana, a little mustard relative with the endearing common name mouse-ear cress, Elliot Meyerowitz and his colleagues at the California Institute of Technology achieved a significant breakthrough, identifying three classes of genes--A, B, and C--that regulate development of the four whorls.

In a typical flower, sepals appear in the outer whorl only when the A genes are expressed. A and B genes expressed in the cells of the second whorl result in petals. B and C genes produce stamens in the third whorl, and C, acting alone in the fourth and innermost whorl, produces carpels. Mutant forms arise when this standard ABC pattern is tampered with--by nature or by humans. In his laboratory, Meyerowitz has produced thousands of Arabidopsis mutants--some with sepals where petals should be, for example, and others with carpels in the wrong place.

Lab-generated mutants serve to elucidate the genetics of flower development; in nature, mutations may lead to the evolution of new flower forms. One such oddity is a one-inch-high plant growing in the Selva Lacandona, a rainforest in Chiapas, Mexico. Lacking chlorophyll and leaves, the plant--Lacandonia schismatica--is as pale and naked as a distant star. You could easily walk by and not notice it. But ever since its discovery in the late 1980s, its tiny white flowers have been taken quite seriously by scientists. The sepals and petals are where one would expect them to be, but the carpels, which include ovary, style, and stigma (and which, in all other known plants, grow at the very heart of the flower) are situated outside the pollen-bearing stamens. The inner flower is inside-out: a challenge to the ABC model of floral development.

Mexican biologists Elena Alvarez-Bullya and Francisco Vergara-Silva are attempting to determine, how such an odd floral arrangement came about. Meyerowitz, a collaborator on this project, suggests that Lacandonia's ancestors may have included species in which plants produced either male or female flowers (as is true of some of the tiny plant's closest living relatives) and that a B-gene mutation in one of the female ancestors might have caused stamens to pop out of the innermost (carpel) whorl. But other explanations are equally plausible at this stage, and as Meyerowitz admits, "The jury is still out."

B genes have, however, been implicated in the evolution of another, splashier group of plants: the lobelias of the Hawaiian archipelago. Over millions of years, birds, especially the native honeycreepers, seeded the islands with lobelias, which have diversified into 110 species of wildly differing forms and colors-ranging from the short, stocky "cabbage on a stick" Brighamia genus to the tall, palmlike Cyanea.

One genus--Clermontia--is of particular interest to Victor Albert and his colleagues in the Lewis B. and Dorothy Cullman Program for Molecular Systematics Studies at the New York Botanical Garden. Like other lobelias, the twenty-two Clermontia species have striking tubular flowers that hang from the stem like curved fingers, but in two-thirds of these species, the whorl of sepals--the green underbelly of most flowers--has become a second set of colorful petals.

To get a closer look at this anatomical metamorphosis, Albert prepared thin slices of these "sepals-turned-petals" and looked at them with an electron microscope. The tissue slices confirmed that the sepals had not simply taken on the color and shape of their petal neighbors: sepal tissue had been transformed into petal tissue. The explanation? Clermontia's B genes are activated not only inside the flower bud--where they do their part to produce both petals and stamens--but also in the sepal whorl.