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Taking out the blast

Rice blast fungus is the most deadly pathogen of rice. This devastating disease infects both upland and lowland varieties of rice in more than 85 countries, reducing yields by up to 75 per cent. The most serious losses occur in Asia and Latin America. However, efforts to control rice blast fungus have led scientists at Purdue University in Indiana, USA to discover the biochemical pathways that could prevent blast infestation. If this pathogen could be eradicated, the increase in rice yields would feed an additional 60 million people each year.

Rice blast fungus - the most deadly pathogen of rice (IRRI)
Rice blast fungus - the most deadly pathogen of rice
IRRI

Rice blast, Magnaporthe grisea, is so widespread because its spores are present year-round in the high humidity common in rice growing areas. Additionally, this fungus can infect the plant at any growth stage. According to the International Rice Research Institute (IRRI) in the Philippines, using resistant varieties is the most practical and economical method of control. However, rice blast is constantly evolving, and therefore resistant varieties soon become susceptible to the disease. Plants high in silica have been shown to fare better against the disease than those grown in soils deficient in the mineral. Therefore, adding calcium silicate slag to the soil, if affordable, can reduce the fungal impact, as can applying nitrogen sparingly. Fungicides are not commonly used to control rice blast due to cost and risks to farmer health as well as the environment.

Rice blast fungus spreads by releasing spores which are blown to other rice plants where they stick on the leaves. The spores can also overwinter in fallen rice grains and rice stubble, and so infect new crops the following year. Once a spore comes into contact with a leaf, it develops into a round, highly pressurized structure known as an appressorium. This fungal bubble grows until it blasts through the leaf surface with a force that scientists report is equal to 40 times the pressure in a bicycle tyre. After the pathogen enters the rice leaf, infected cells die, a strategy used by the plant to stop the attack. However, this usually results in the death of the entire plant if it is young, and probably the loss of the rice grain if the plant is older.

Rice blast solutions

Purdue molecular biologists Jin-Rong Xu, Xinhua Zhao, Yangseon Kim and Gyungsoon Park have discovered the biochemical pathways of the enzyme that coordinates the fungal attack known as pathogenicity mitogen-activated protein (MAP) kinase. The enzyme's biochemical pathway includes three key genes that control a cascade of events involved in the infection process. Xu's team found that blocking these genes prevents the fungus from creating appressoria and infecting the plant. This discovery could lead to either the production of new fungicides or the development of new resistant rice plants that are able to withstand the pathogen's attack.

Xu's research seems to indicate that a fungicide will be easier to create than resistant plants. "For developing new resistant rice plants," he says, "we have to understand which particular plant molecules trigger the MAP kinase pathway. Once we know this, we can breed plant cultivars that do not produce or produce much fewer signal molecules. Theoretically, that is possible but it takes a lot of effort to do so. Practically, it may be impossible because these signal molecules may be essential for plant growth or differentiation." However, the genes Xu has discovered could be directly used for developing assays to screen for new fungicides, although it is difficult to determine how long this may take. "If there is a company interested in developing assays, that should be quite easy," he notes, "and they could screen lots of chemical libraries."

Complicated interactions

Rice researchers in Bhutan in fields affected by rice blast (IRRI)
Rice researchers in Bhutan in fields affected by rice blast
IRRI

Unfortunately, rice blast holds many more puzzles for scientists. More than one biochemical pathway is involved in multiple possible infection processes used by rice blast fungus. This is most likely a result of the centuries-old relationship between this disease and its host. Scientists like Xu will continue to study the complex biochemical communication involved in the infection process; for example, it appears that the rice plant gives a signal to the receptor on the fungus which allows disease penetration to proceed.

Scientists have also discovered that rice blast fungus boasts a unique family of protein detectors that can recognize rice leaf surfaces from those of other plants, discriminate among good and poor host plants, and even determine how the plant in question might fight a fungal invasion. These biochemical investigators, known as G-protein-coupled receptors (GPCR), also play a role in initiating leaf penetration.

Written by: Treena Hein

Date published: September 2005

 

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