Healthy rice panicles (top) and leaves of plants infected with the devastating blast fungus (bottom) (Pictures from USDA-ARS)
Rice is life for more than half of the world's population. It is the
second most widely grown cereal in the world, next to wheat. The crop
was planted in more than 155 million hectares in 2008. Around 2.5 billion
people in Asia, mostly from developing countries such as India, China,
Indonesia and Bangladesh, obtain more than 70 percent of their caloric
intake from rice and its derived products. In addition, more than a billion
households in Asia, Africa and America, depend on rice for their main
source of livelihood.
The global rice production, however, is faced with numerous challenges. According to the United Nations Food and Agriculture Organization (FAO), diseases, insects and weeds are responsible for yield losses of up to 30 percent. The blast disease is one of the most serious and widespread diseases of rice. Caused by the fungus Magnaporthe oryzae, the disease is capable of wiping out entire rice fields. Most rice cultivars are susceptible to the rice blast.
The devastating nature of the disease stems from the fungus' ability to develop structures that block the transport of water and minerals in the plants' vascular system. This results to lesions in above-ground plant parts. The fungus is also capable of infecting rice plants in any growth stage, from the seedling to the tillering stage. Once inside the plant, the fungus produces thousands of spores that can be blown by the wind to neighboring plants or fields.
Rice blast was first reported in Asia and is now present in more than 80 countries. It is estimated to cause production losses of up to USD 60 million annually in South and Southeast Asia alone. According to the International Rice Research Institute (IRRI), more than 266,000 tons of rice are lost to the disease in India every year. This is equivalent to 0.8 percent of the country's total yield.
In Japan, as much as 200,000 tons are lost annually to the disease, according to Dr. Shuichi Fukuoka, a scientist at the country's National Institute of Agrobiological Sciences.
A Moving Target
Magnaporthe oryzae is no easy target. Numerous researchers have identified genes that provide resistance against blast. But the pathogen can easily evolve resistance against these genes. What's more, rice cultivars equipped with these genes are usually inferior in terms of agronomic performance.
Recently, a team of Japanese researchers led by Shuichi Fukuoka identified a novel gene that promises longer lasting resistance against the blast-causing fungus. The gene, pi21, encodes a proline-rich protein with putative heavy metal-binding domain and putative protein-protein interaction motifs. It was traced to a Quantitative Trait Locus (QTL) in a blast resistant rice cultivar that has long been grown in Japan. The cultivar, however, is unpopular because it produces lower quality rice.
Using map-based cloning, the team found that the resistant variant of the gene has two deletions that cause a partial loss of function. This loss of function enables the plant to defend itself against the blast fungus. Pi21 is different from previously identified blast resistance genes (race-specific or R genes), which work by interacting with a specific gene in a particular fungal strain.