''Magnaporthe grisea'', also known as rice blast fungus, rice rotten neck, rice seedling blight, blast of rice, oval leaf spot of graminea, pitting disease, ryegrass blast, Johnson spot, neck blast, wheat blast, and Imochi (

Japanese Japanese may refer to: * Something from or related to Japan, an island country in East Asia * Japanese language, spoken mainly in Japan * Japanese people, the ethnic group that identifies with Japan through ancestry or culture ** Japanese diaspor ...

:稲熱) is a plant-pathogenic

fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...

and model organism that causes a serious disease affecting

rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...

. It is now known that ''M. grisea'' consists of a

cryptic species complex In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

containing at least two biological species that have clear genetic differences and do not interbreed. Complex members isolated from ''

Digitaria ''Digitaria'' is a genus of plants in the grass family native to tropical and warm temperate regions but can occur in tropical, subtropical, and cooler temperate regions as well. Common names include crabgrass, finger-grass, and fonio. They are ...

'' have been more narrowly defined as ''M. grisea''. The remaining members of the complex isolated from rice and a variety of other hosts have been renamed ''Magnaporthe oryzae'', within the same ''M. grisea'' complex. Confusion on which of these two names to use for the rice blast pathogen remains, as both are now used by different authors. Members of the ''Magnaporthe grisea'' complex can also infect other agriculturally important cereals including

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...

, rye,

barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley p ...

, and pearl millet causing diseases called blast disease or blight disease. Rice blast causes economically significant crop losses annually. Each year it is estimated to destroy enough rice to feed more than 60 million people. The fungus is known to occur in 85 countries worldwide and was the most devastating fungal plant pathogen in the world.

Hosts and symptoms

''M. grisea'' is an ascomycete fungus. It is an extremely effective plant pathogen as it can reproduce both sexually and asexually to produce specialized infectious structures known as appressoria that infect aerial tissues and hyphae that can infect

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...

tissues. Rice blast has been observed on rice strains M-201, M-202, M-204, M-205, M-103, M-104, S-102, L-204, Calmochi-101, with M-201 being the most vulnerable. Initial symptoms are white to gray-green lesions or spots with darker borders produced on all parts of the shoot, while older lesions are elliptical or spindle-shaped and whitish to gray with necrotic borders. Lesions may enlarge and coalesce to kill the entire leaf. Symptoms are observed on all above-ground parts of the plant. Lesions can be seen on the

leaf collar A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...

, culm, culm nodes, and panicle neck node. Internodal infection of the culm occurs in a banded pattern. Nodal infection causes the culm to break at the infected node (rotten neck). It also affects reproduction by causing the host to produce fewer seeds. This is caused by the disease preventing maturation of the actual grain.

Disease cycle

The pathogen infects as a spore that produces lesions or spots on parts of the rice plant such as the leaf, leaf collar, panicle, culm and culm nodes. Using a structure called an appressorium, the pathogen penetrates the plant. The appressorium cell wall is chitinous and its inner side contains

melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...

. which is necessary to damage host structures. The

turgor pressure Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibriu ...

generated during this process is sufficient to penetrate the plants' cuticles routinely, and experimentally can penetrate

Kevlar Kevlar (para-aramid) is a strong, heat-resistant synthetic fiber, related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, the high-strength material was first used commercially in the early 1970s a ...

. This impressive turgor is produced by synthesis of

glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

and maintained by the aforementioned appressorial melanin. The pathogen is able to move between the plant cells using its invasive hyphae to enter through

plasmodesmata Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and spec ...

. ''M. grisea'' then sporulates from the diseased rice tissue to be dispersed as conidiospores. After overwintering in sources such as rice straw and stubble, the cycle repeats. A single cycle can be completed in about a week under favorable conditions where one lesion can generate up to thousands of spores in a single night. Disease lesions, however, can appear in three to four days after infection. With the ability to continue to produce the spores for over 20 days, rice blast lesions can be devastating to susceptible rice crops. Infection of

induces phosphorylation of the light-harvesting complex II protein . LHCB5 is required for a

reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...

burst produced by the host which provides resistance against this pathogen.

Environment

Rice blast is a significant problem in temperate regions and can be found in areas such as irrigated lowland and upland. Conditions conducive for rice blast include long periods of free moisture and/or high humidity, because leaf wetness is required for infection.

Sporulation In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

increases with high relative humidity and at , spore germination, lesion formation, and sporulation are at optimum levels. In terms of control, excessive use of nitrogen fertilization as well as drought stress increase rice susceptibility to the pathogen as the plant is placed in a weakened state and its defenses are low. Flooding and draining fields is normal in rice growing, however leaving a field drained for extended periods also favors infection as that will aerate the soil, converting ammonium to nitrate and thus causing stress to rice crops, as well.

Geographical distribution

Wheat blast was found in the 2017-2018 rainy season in

Zambia Zambia (), officially the Republic of Zambia, is a landlocked country at the crossroads of Central, Southern and East Africa, although it is typically referred to as being in Southern Africa at its most central point. Its neighbours are t ...

, in the

Mpika Mpika is a town in the Muchinga Province of Zambia, lying at the junction of the M1 Road to Kasama and Mbala and the Tanzam Highway ( Great North Road) to Dar es Salaam, Tanzania in the north-east and Lusaka in the south-west. It also has a r ...

district of the

Muchinga Province Muchinga Province is one of the ten provinces of Zambia. It is located in the northeast of the country and borders with Tanzania in the north, Malawi in the east, Eastern Province in the south, Central Province in the southwest, Luapula Province ...

. In February 2016 a devastating wheat epidemic struck

Bangladesh Bangladesh (}, ), officially the People's Republic of Bangladesh, is a country in South Asia. It is the eighth-most populous country in the world, with a population exceeding 165 million people in an area of . Bangladesh is among the mos ...

. Transcriptome analysis showed this to be an ''M. grisea'' lineage most likely from

Minas Gerais Minas Gerais () is a state in Southeastern Brazil. It ranks as the second most populous, the third by gross domestic product (GDP), and the fourth largest by area in the country. The state's capital and largest city, Belo Horizonte (literally ...

São Paulo São Paulo (, ; Portuguese for ' Saint Paul') is the most populous city in Brazil, and is the capital of the state of São Paulo, the most populous and wealthiest Brazilian state, located in the country's Southeast Region. Listed by the Ga ...

Brasília Brasília (; ) is the federal capital of Brazil and seat of government of the Federal District. The city is located at the top of the Brazilian highlands in the country's Central-West region. It was founded by President Juscelino Kubitsche ...

, and

Goiás Goiás () is a Brazilian state located in the Center-West region. Goiás borders the Federal District and the states of (from north clockwise) Tocantins, Bahia, Minas Gerais, Mato Grosso do Sul and Mato Grosso. The state capital is Goi ...

states of

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

and not from any geographically proximate strains. This successful diagnosis shows the ability of genetic surveillance to untangle the novel biosecurity implications of transcontinental transportation and allows the Brazilian experience to be rapidly applied to the Bangladeshi situation. To that end the

government A government is the system or group of people governing an organized community, generally a state. In the case of its broad associative definition, government normally consists of legislature, executive, and judiciary. Government is ...

has set up an early warning system to track its spread through the country.

Management

This fungus faces both fungicides and genetic resistance in some types of rice developed by

plant breeders Plant breeding is the science of changing the traits of plants in order to produce desired characteristics. It has been used to improve the quality of nutrition in products for humans and animals. The goals of plant breeding are to produce cro ...

. It is able to establish both resistance to those chemical treatments and

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ...

to crop resistance by genetic change through mutation. In order to most effectively control infection by ''M. grisea'', an integrated management program should be implemented to avoid overuse of a single control method and fight against genetic resistance. For example, eliminating crop residue could reduce the occurrence of overwintering and discourage inoculation in subsequent seasons. Another strategy would be to plant resistant rice varieties that are not as susceptible to infection by ''M. grisea''. Knowledge of the pathogenicity of ''M. grisea'' and its need for free moisture suggest other control strategies such as regulated irrigation and a combination of chemical treatments with different modes of action. Managing the amount of water supplied to the crops limits spore mobility thus dampening the opportunity for infection. Chemical controls such as Carpropamid have been shown to prevent penetration of the appressoria into rice epidermal cells, leaving the grain unaffected. Papajani et al. 2015 finds the essential oils of both ''

Origanum vulgare Oregano (, ; ''Origanum vulgare'') is a species of flowering plant in the mint family Lamiaceae. It was native to the Mediterranean region, but widely naturalised elsewhere in the temperate Northern Hemisphere. Oregano is a woody perennial pl ...

'' and ''

Rosmarinus officinalis ''Salvia rosmarinus'' (), commonly known as rosemary, is a shrub with fragrant, evergreen, needle-like leaves and white, pink, purple, or blue flowers, native to the Mediterranean region. Until 2017, it was known by the scientific name ''Rosmar ...

'' to be effective

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...

, and provides treatment thresholds. The wheat blast strain can be diagnosed by sequencing. Thierry ''et al.'', 2020 presents a set of

genetic marker A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species. It can be described as a variation (which may arise due to mutation or alteration in the genomic loci) that can be ...

s which can be found by

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) ...

(PCR),

real-time PCR A real-time polymerase chain reaction (real-time PCR, or qPCR) is a laboratory technique of molecular biology based on the polymerase chain reaction (PCR). It monitors the amplification of a targeted DNA molecule during the PCR (i.e., in real ...

(RT-PCR), and

loop-mediated isothermal amplification Loop-mediated isothermal amplification (LAMP) is a single-tube technique for the amplification of DNA and a low-cost alternative to detect certain diseases. Reverse transcription loop-mediated isothermal amplification (RT-LAMP) combines LAMP with ...

(LAMP). The big advantages of the Thierry markers are that they do not miss

isolates {{About, , the linguistics term dealing with languages unrelated to any other language in the world, Language isolate, other uses, Isolate (disambiguation){{!Isolate Isolates is a term used in developmental psychology and family studies, to describ ...

lacking the Mot3 sequence, for example , and its great sensitivity.

Importance

Rice blast is the most important disease concerning rice crops in the world. Since rice is an important food source for much of the world, its effects have a broad range. It has been found in over 85 countries across the world and reached the United States in 1996. Every year the amount of crops lost to rice blast could feed 60 million people. Although there are some resistant strains of rice, the disease persists wherever rice is grown. The disease has never been eradicated from a region.

Strains

Three strains, ''albino'' (defined by a mutation at the ''ALB1'' locus), ''buff'' (''BUF1''), and ''rosy'' (''RSY1'') have been extensively studied because they are nonpathogenic. This has been found to be due to nonuse of

, which is a virulence factor in ''M. grisea''., "Three mutants of ''M. grisea'', ''albino'', ''buff'', and ''rosy'' (corresponding to the ''ALB1'', ''BUF1'', and ''RSY1'' loci, respectively), have been studied extensively and are nonpathogenic. This is due to an inability to cross the plant cuticle because of the lack of melanin deposition in the appressorium." The (''M. o.'' pv. ''triticum'') causes the wheat blast disease.

Genetics

Whole-genome sequences were just becoming possible, and being made available, in 2003. A mitogen-activated protein kinase (MAPK) called '' pmk1'' is genetically close to one necessary for

mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...

and cell morphology in

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...

s, '' FUS3''/'' KSS1''. Defective mutant yeast are somewhat or entirely restored in mating function if they are given a copy of ''pmk1''. It was therefore assumed that this must only be a mating and development gene in ''M. grisea'', however it turns out to be both vital to the female mating process and in appressorium function and pathogenicity as a whole. Because signal links between MAPKs and

cyclic adenosine monophosphate Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and used for intracellular signal transd ...

s were shown to be required for mating in several other models, including ''

Ustilago maydis Corn smut is a plant disease caused by the pathogenic fungus ''Ustilago maydis'' that causes smut on maize and teosinte. The fungus forms galls on all above-ground parts of corn species. It is edible, and is known in Mexico as the delicacy ' ...

'' and several others, this was assumed to be true for ''M. grisea'', and yet that was then shown to be unnecessary in this model. This demonstrates significant variety in cellular function within fungi. The

transaminase Transaminases or aminotransferases are enzymes that catalyze a transamination reaction between an amino acid and an α- keto acid. They are important in the synthesis of amino acids, which form proteins. Function and mechanism An amino acid ...

alanine: glyoxylate aminotransferase 1 (AGT1) has been shown to be crucial to the pathogenicity of ''M. grisea'' through its maintenance of redox homeostasis in peroxisomes. Lipids transported to the appressoria during host penetration are degraded within a large central vacuole, a process that produces

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

s. β-Oxidation of fatty acids is an energy producing process that generates Acetyl-CoA and the reduced molecules FADH₂ and

NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...

, which must be oxidized in order to maintain redox homeostasis in appressoria. AGT1 promotes lactate fermentation, oxidizing NADH/FADH₂ in the process. ''M. grisea'' mutants lacking the AGT1 gene were observed to be nonpathogenic through their inability to penetrate host surface membranes. This indicates the possibility of impaired lipid utilization in ''M. grisea'' appressoria in the absence of the AGT1 gene.

References

External links

GROMO - Genomic Resources of ''Magnaporthe oryzae''

''Magnaporthe grisea'' Genome

The official Website of the International Rice Blast Genome Consortium

Index Fungorum

''Magnaporthe grisea'' at MetaPathogen: stages, tissues, mating types, strains, references
{{DEFAULTSORT:Magnaporthe Grisea Magnaporthales Biological weapons Fungal plant pathogens and diseases Rice diseases Fungi described in 1880 Biological anti-agriculture weapons Wheat diseases Model organisms