He also added gibberellic acid to the plants to ensure that a deficiency of that hormone was not causing the dwarfism.

The dwarf plants that did not grow in the dark or with the addition of the gibberellic acid were compared to regular maize plants that had been dwarfed by subjecting them to a chemical that disrupts the creation of brassinosteroids. Both exhibited short stalks with twisted leaves and showed the feminization of the male tassel flower.

Schulz then used information that was already known from the research plant Arabidopsis about genes that control brassinosteroid production. He found the same genes in the maize genome.

In the dwarf maize plants, those genes were mutated, disrupting the biosynthesis of the steroids. A chemical analysis showed the compounds produced along the pathway of gene to steroid were greatly diminished in the maize dwarfs.

Cloning of the gene revealed that an enzyme of the brassinosteroid pathway was defective in the mutant plants. A related enzyme in humans has been reported as essential for the production of the sex steroid hormone testosterone.

Mutations in this enzyme in humans also resulted in feminization.

 While Schulz expected brassinosteroids to affect plant height, he said he did not expect those steroids to affect sex determination.

 "We don't know if this is a special case for corn or if this is generally the same in other plants," he said. "If it is the same in other plants, it should be useful for creating plants or trees in which you want only males or females."

Gurmukh Johal, a professor of botany and plant pathology and collaborator on the research, identified the mutant used in the research, nana plant1, years ago. He said better understanding the steroid-production pathways could be important to strengthening maize plants and increasing yields.

"Maize produces too much pollen and it actually wastes a lot of energy on that," Johal said. "This implies that by using this gene or the pathway it controls, we could manipulate the plants to improve their quality."

Schulz said he would look at other plants, such as sorghum, to determine if the same genes and pathways control sex determination and height.

The project was an international collaboration with George Chuck from the Plant Gene Expression Center at the University of California Berkeley, Shozo Fujioka of RIKEN Advanced Science Institute in Japan, Sunghwa Choe of Seoul National University in South Korea, and Devi Prasad Potluri of Chicago State University.

The National Science Foundation and the U.S. Department of Agriculture funded the research.