Whether plants make complex, divided leaves like those of ferns or tomatoes instead of simple, single leaves such as blades of grass is controlled by whether a set of genes, called KNOX1, is switched on in the leaves, according to researchers led by Neelima Sinha, an associate professor of plant biology at the University of California, Davis.

The group of genes, called KNOX1, is found in all plants. KNOX1 genes were switched on in the leaves of all plants with complex leaves, such as tomato, carrot, fennel, cycads and ferns. KNOX1 was not active in the leaves of simple-leaved plants, such as maize, rice, tobacco and Arabidopsis.

The findings are published in the June 7 issue of Science.

At first sight, the researchers found KNOX1 gene expression in the leaves of some plants with "simple" leaves. But when they studied those leaves under a microscope, they found that they actually started as complex leaves but grew into simple-looking leaves.

In one plant, the researchers found both patterns. Neobeckia aquatica (lake cress) is a water plant with simple leaves above the water and complex leaves below. KNOX1 is expressed in the submerged leaves, but not in the aerial ones.

KNOX1 is a group of homeobox genes. Homeobox genes control the development of plants and animals from fertilized eggs to mature adults by telling other genes what to do. They have been found in all kinds of living things, including yeasts, fruit flies, mammals and plants. For scientists studying development and evolution, homeobox genes are intriguing because small changes in their behavior can cause big changes in the appearance of an animal or plant.

Leaves in the earliest types of vascular land plants, such as ferns and cycads, are complex, Sinha said. The first flowering plants, or angiosperms, are thought to have had simple leaves. The complex leaf trait then reappeared several times as different groups of flowering plants evolved, she said. The researchers surveyed more than 400 different types of plants from collections at UC Davis, State University of New York-Stony Brook, UC Santa Cruz and elsewhere. They found that, except in one small group of plants related to peas, KNOX1 was used whenever compound leaves reappeared.

"It's interesting that evolution goes back to the same tool kit again and again," Sinha said.

Sinha teamed up with Geeta Bharathan, now an assistant professor at the State University of New York, Stony Brook, to look at how genes regulate the evolution of leaf shape. The other contributors to the paper are researcher Thomas Goliber; graduate student Sharon Kessler; undergraduate students Christopher Moore and Thinh Pham, all at UC Davis.