Stronger corn? Take it off steroids, make it all female

Burkhard Schulz, an assistant professor of horticulture and landscape architecture, wanted to understand the relationship between natural brassinosteroids – a natural plant steroid hormone – and plant architecture, specifically plant height. Schulz said corn could benefit by becoming shorter and sturdier, but the mechanisms that control those traits are not completely understood.

“It is essential to change the architecture of plants to minimize how much land we need to produce food and fuels,” said Schulz, whose findings are published in the early online version of the Proceedings of the National Academy of Sciences. “If you can find a natural mutation or mechanism that gives you what you need, you are much better off than using transgenic techniques that could be difficult to get approval for.”

Schulz found that when maize loses the ability to produce brassinosteroids, it becomes a dwarf, as he suspected. But another feature caught him off guard: The plants without the naturally occurring steroids could not make male organs – they had kernels where the tassels should be.

That could be a cost-saving discovery for the seed industry. Hybrid seed producers must painstakingly remove the male pollen-producing tassels from each plant so that they do not pollinate themselves. Schulz said maize plants that produce only female organs would eliminate the detasseling step.

“This would be the perfect mutation for hybrid seed production,” Schulz said. “There is no way these plants could produce pollen because they do not have male flowers.”

Schulz used a multistep process to determine brassinosteroids' role in height and, later, sex determination. He wanted to ensure that light and the addition of gibberelic acid, a hormone that promotes cell growth and elongation, would not eliminate the dwarfism.

Schulz gathered known mutants of maize with short mesocotyls, the first node on a corn stalk. He suspected that even dwarf plants that produced brassinosteroids would have elongated mesocotyls if grown in the dark as they stretched for light, a trait typical of all brassinosteroid mutants. 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 that 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.

Writer: Brian Wallheimer, 765-496-2050, bwallhei@purdue.edu

Sources: Burkhard Schulz, 765-496-3635, bschulz@purdue.edu
Gurmukh Johal, 765-494-4448, gjohal@purdue.edu