West Lafayette, Indiana
October 2, 2003

A team of Purdue University researchers has recently uncovered the genetic mechanism that prevents certain crop plants from growing tall - a finding that has future crop production applications since some grains produce greater yields if plants are kept short.

A dwarf form of corn called brachytic2 (br2) was recognized in 1951, but until now, scientists have not understood the genetic mechanism underlying the plant's mutation. These dwarf mutants are somewhat unusual, as their lower stalks are highly compressed but the upper portions of the plant, including the ears and tassels, are normal. A related mutant in sorghum called dwarf 3 (dw3) has been put into widespread cultivation because it displays ideal crop characteristics, such as increased grain yield and improved stalk strength and quality, Johal said.

Johal and his colleagues found that loss of a gene product called a p-glycoprotein generates these dwarf corn and sorghum plants by interfering with the movement of auxin, an essential hormone in plant growth and development. They also have identified the genetic mechanism that causes dwarf sorghum plants to spontaneously revert to a taller form.

In corn, the normal gene Br2 produces a p-glycoprotein, and the researchers found that a mutation in this gene is responsible for the altered growth of the dwarf plant. They also found that the dwarf mutants, while shorter than their taller counterparts, have more cells per unit area in the stalk, which makes the stalks stronger and perhaps more effective at retaining water.

Although p-glycoproteins are involved in transporting molecules across cell membranes, their exact function still has not been conclusively shown.

"This finding in br2 dwarf mutants demonstrates the 'real-world' impact of research involving model plants," said Angus Murphy, assistant professor of horticulture and a collaborator on the study. Murphy recently demonstrated that in Arabidopsis, a plant commonly used as a model system in plant genetics and molecular biology, mutations in a p-glycoprotein gene similar to Br2 disrupt auxin flow, leading to alteration of the plant's form.

"After discovering that p-glycoproteins control hormonal movement in Arabidopsis, we were able to apply that information to demonstrate that the same mechanism underlies a well-described phenomenon in corn," Murphy said. "The kind of collaboration that produced this discovery is one of the unique characteristics of the Purdue research environment."

Johal and Murphy work in different academic departments located in different buildings - but they both agree that the combination of their diverse areas of expertise was key to their success.

"This study has been a perfect match between genetics and physiology," Johal said. "Geneticists have known about this mutation for years, but without this collaboration, we would not have been able to reveal the physiological changes that cause it. Our combined areas of research complement one another very well."

Johal and his colleagues also report in the current study that a genetic phenomenon involving a direct duplication of a part of a normal gene causes instability in the sorghum dwarf mutant dw3. A direct

duplication occurs in a gene when a portion of its DNA sequence is repeated elsewhere in the gene. In the case of the dw3 mutant, Johal and his colleagues show that a direct duplication in the normal gene

not only generates the dwarf mutation, but also is responsible for the mutant occasionally reverting to its tall form.

"Direct duplications, like the one we see in dw3, are unstable because they can self-correct," Johal said.

In another phenomenon of genetics, called recombination, a duplicated portion of a gene can be removed by a process called unequal crossing over, during which pairs of chromosomes slightly misalign to exchange corresponding segments of DNA. The end result of this unequal crossing over is that the dw3 dwarf reverts back to its normal form.

Curiously, one of the sorghum plants in the study had the dwarfed appearance typical of dw3, but Johal found that it lacked the duplication responsible for dwarfing in other dw3 plants they studied. According to Johal, the dw3 gene in this plant experienced unequal crossing over, which, by removing the direct duplication, should have restored normal height. However, this crossing over event introduced a few minor changes in the gene that were significant enough to disrupt its function and still cause the plant's dwarfed growth.

Because this gene lacks the duplication, Johal said it is a stable mutant that will not revert back to a tall form.

"This single discovery of a stable mutant will have an immediate impact on sorghum breeding," Johal said. "Now that we have identified this stable mutant, the dw3 mutant can be corrected for commercial breeding."

Unlike dwarf sorghum, dwarf corn has not been put into commercial use partly because corn hybrids grown in the United States are not excessively tall. In addition, br2 tends to produce barren plants when grown at high densities. Furthermore, the equipment in use in the United States today would not be able to effectively harvest significantly shorter plants, he said.

However, he said the discovery of the dwarfing mechanism may renew interest in developing a dwarf corn with improved yield, which could be of particular interest in developing countries.

Dwarf varieties of rice and wheat, introduced during the 1960s throughout the Indian subcontinent and Southeast Asia, were largely responsible for thwarting famine, Johal said.

"The population explosion in those regions placed many people at risk of starvation," he said. "The introduction of dwarfing lines tripled or even quadrupled the yield of wheat and helped prevent massive food shortages."

This increase in crop yield, brought on by the introduction of dwarf crops and other technologies, is often referred to as the "green revolution" in agriculture.

According to Johal, sorghum may be crucial to the future impact of the green revolution.

"The next round of the green revolution must impact Africa," he said. "Sorghum, which is a staple in many parts of Africa, especially sub-Saharan Africa, could play a key role there."

Other cereal crops, including teff, a grain grown primarily in Ethiopia, and basmati rice, grown in India, which both grow unusually tall, also may benefit from the discovery reported in this study, Johal said.

This research was supported by start-up funds made available to Johal by Purdue University and a National Science Foundation grant awarded to Murphy. Other collaborators on the study included Dilbag S. Multani and Mark Chamberlin of Hi-Bred International Inc., Steven P. Briggs of Diversa Corp., and Joshua J. Blakeslee of Purdue University.

Writer:
Jennifer Cutraro, (765) 496-2050, jcutraro@purdue.edu

Sources: 
Guri Johal, (765) 494-4448, gjohal@purdue.edu
Angus Murphy, (765) 496--7956, murphy@purdue.edu

Related Web sites:

Guri Johal: http://www.btny.purdue.edu/Faculty/Johal/

Angus Murphy: http://www.hort.purdue.edu/hort/people/faculty/murphy.shtml

ABSTRACT

Loss of an MDR transporter in compact stalks of maize br2 and sorghum dw3 mutants.

Dilbag S. Multani, Steven P. Briggs, Mark A. Chamberlin, Joshua A. Blakeslee, Angus S. Murphy, Gurmukh S. Johal

Agriculturally advantageous reduction in plant height is usually achieved by blocking the action or production of gibberellins. Here we describe a different dwarfing mechanism found in maize brachytic2 (br2) mutants characterized by compact lower stalk internodes. The height reduction in these plants results from the loss of a P-glycoprotein that modulates polar auxin transport in the maize stalk. The sorghum ortholog of br2 is dwarf3 (dw3), an unstable mutant of longstanding commercial interest and concern. A direct duplication within the dw3 gene is responsible for its mutant nature and also for its instability, because it facilitates unequal crossing-over at the locus.

The article is accessible to subscribers at http://www.sciencemag.org