Dr. Hei Leung (right), IRRI scientist, explains the screening of rice germplasm and mutants to understand why rice is resistant to rust species. (Photo: Chris Quintana)
Wheat gets rust. Maize (corn) gets rust. Also barley, millet, triticale, and oats. In fact, all cereals, except rice, are susceptible to rust.
Rust is a disease that suffocates a plant’s photosynthetic process, rendering the crop weak and unable to provide good yields.
Rice, the “stainless steel” among cereal grasses, has long intrigued plant breeders and plant pathologists. Norman Borlaug, the father of the Green Revolution and patriarch of modern wheat varieties (see A tribute to Norman Borlaug), believed that, by discovering the genes that make rice immune to rusts, scientists might be able to introduce these genes into other cereal grains such as wheat and maize.
Quest for discovery
Finding out what makes rice resistant to rust is Hei Leung’s quest. This plant pathologist at the International Rice Research Institute in Los Baños, Philippines, focuses his research on rice genetic diversity and discovery, and meeting the needs of future generations for rice genetic resources.
“It’s not that rice does not get rust,” says Dr. Leung. “Rather, rice does not succumb to the disease even when it encounters the rust pathogen. For most rice accessions, there is no macroscopic evidence of rust colonization. Rice is a true nonhost of most rust species.”
IRRI researcher Vance Lacorte (left) and Jian Liu, a visiting scholar from China National Rice Research Institute, examine rare mutations that caused rust resistance.(Photo: Chris Quintana)
Millions of yellow- or rust-colored uredospores (the asexual spores) of more than 4,000 species of rust fungi are borne around the world and carried by the wind and jet stream. They land on a host plant like wheat or maize, germinate, and then grow toward a stomatal pore on the leaf surface to initiate infection. Rust infections produce red or yellow pustulating uredospores that give infected plants a “rusty” look.
In susceptible plants, rust cuts off the plants’ ability to photosynthesize nutrients in their leaves and transport nutrients in their stems. Infection by rusts causes stems to weaken and plants to “lodge,” or fall over, making what little yield there is nearly impossible to harvest.
A dreaded plant disease
Wheat rust is one of the world’s most dreaded plant diseases. A source of plagues since Biblical times, rust devastated the wheat crop in North America as recently as 1953. Since 1998, Ug99, a form of stem rust, has been devastating wheat farmers’ fields in East Africa, and is even now marching toward the world’s breadbasket in the Middle East and South Asia, particularly India. It threatens to disrupt food security for the millions of people who depend upon wheat as a staple.
The increasing failure of the world’s wheat crop in the face of increasingly virulent forms of wheat rusts such as stem rust and yellow rust has created a renewed interest in the wheat community in examining the one crop—-rice—-resistant to the terrible disease.
Team of experts
Dr. Leung’s colleagues on the project are spread out across the world: Shiping Wang and Zhensheng Kang at Huazhong Agricultural University and Northwest Agriculture and Forestry University in China, Yinong Yang at Penn State University, Mick Ayliffe at Australia’s Commonwealth Scientific and Industrial Research Organisation, Brian Steffenson at the University of Minnesota, and Yue Jin at the United States Department of Agriculture.
A scientific puzzle
The research team’s top priority in 2011 is to determine whether susceptibility to rust is genetically controlled. If so, these rust-sensitive mutants could reveal what genetic mechanisms are responsible for the immunity of rice to rust.
Dr. Leung emphasizes that a team approach is important to resolving this scientific puzzle and that it has important practical applications.“Understanding what constitutes long-term immunity is good for all cereals,” says Leung. “We want to discover what genes render rice nonsusceptible to rust. Such a genetic mechanism could also be useful for fighting rice diseases.”
Sarah Nell Davidson, the associate director of Durable Rust Resistance in Wheat, agrees: “Imparting nonhost resistance to wheat and other cereals could be the ultimate durable solution to the devastating family of rusts.
For farmers, discovering what leads to rust resistance in rice and transferring that knowledge to other crops could lead to stable resistance in wheat and other cereals.
“If we can identify the source of resistance in rice and transfer that to wheat, we will be able to avoid the boom-and-bust rust cycles that are so devastating to food security,” says Dr. Leung.“We will be able to take care of Ug99 and, perhaps, future Ug99s.”
