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General
The function of the Mlo gene is generally understood as regulating a cell wall repair process, based on the growth of an intracellular papilla. which forms at the site of injury, that might be mechanical or due to penetration attempts by insects or fungi. Mutations at different sites of the gene generally lead to its malfunction, resulting in excessive papilla growth. This causes a very high level of resistance to powdery mildew, since the papilla grows usually faster than the parasite penetrates. The recessive alleles of the Mlo gene, symbolized "mlo", therefore behave as very efficient genes for resistance against powdery mildew, reducing the number of mildew conidia, that succeed to produce a colony, approx. 500 times under laboratory conditions. The absence of the regulatory function of the Mlo gene is often expressed in mlo-mutants by adverse pleiotropic effects like large necrotic flecks on leaves of adult plants and reduced yield. These side effects of mlo are considerably influenced by the genetic background and can be almost eliminated by breeding, as is demonstrated by the existence of more than 140 mlo cultivars in Europe. The actual level of mlo-based resistance under field conditions is only partly understood, depends on environmental factors and is difficult to reproduce. Exposure to elevated temperature or short term relief from water stress increase the susceptibility of mlo plants to mildew by about one order of magnitude. Presently the mlo-gene is the only source of resistance, against which highly virulent mildew races so far did not spread in agricultural environments. The mlo gene is therefore the most widely used source of resistance in spring barley varieties. The mlo-cultivars cover in central and western Europe, very roughly estimated, over 60% of the spring barley area. The Mlo gene is cloned and it's molecular structure known. Mlo-orthologs have been found also in all so far studied cereals and other higher plants.The Mlo-orthologs reverse on single cell basis the mlo based resistance of barley. Genes, similar to Mlo with almost identical structure and protein properties, have been found in very different botanical taxa, including bryophyta, indicating its very ancient origin in evolution. Mlo analogs seem to be common in higher plants. Attempts are done to utilise the resistance mechanism of mlo also in other species. Although the parasite has the capacity to overcome this kind of resistance and several genes for partial virulence to mlo have been detected in the parasite, the evolution of the mildew population towards high mlo-virulence proceeds under field conditions in small steps and slowly. The so far known most adapted mlo-virulent mildew race HL-3, produced in the laboratory about 30 years ago, and an old field isolate from Japan produce about 50 - 100 times more colonies on leaves of mlo-plants than non-adapted mildew races. A recent analysis used molecular technique to compare the mlo virulent HL-3 and its near isogenic avirulent parent line GE-3. During appresorium formation and penetration the activity of two novel Blumeria graminis genes was several times higher in HL-3 than in GE3. It was also shown, that the parasite genes responsible for higher mlo-virulence are not identical in the Japanese isolate and HL3. This makes evolution towards higher mlo virulence likely. Recently the level of mlo virulence in some field pathotypes, collected in Europe, approached 15%, relative to the fully compatible interaction, if tested on detached primary leaf leaf segments on agar. But the mlo virulence of these isolates on later leaves and under field conditions is considerably lower. The selection pressure for increased mlo virulence is growing, since the majority of new spring barley cultivars in Europe has mlo and the proportion of mlo on the spring barley acreage exceeded already 50%, see the table multiplication areas of mlo-cultivars on this page.
(Summary by the page author, updated 10.1.2006)
Alleles of the Mlo locus
Many data and links by courtesy of Dave Mathews, GrainGene
link to the GrainGene Database: click here
(still incomplete - your help to improve the table is welcome)
|
Allele |
Mutant (mother variety) |
found by |
Stock |
Reference |
Mutations at aminoacid level |
|
Mlo |
Wild type |
|
all non-mlo barleys |
|
Molecular structure in all wild-type plants identical |
|
M 66 ( Haisa) |
Freisleben & Lein 1942 |
||||
|
H3502 (Vollkorn) |
Hänsel 1971 |
? |
|||
|
MC20(Malteria Heda) |
Favret 1960 |
frame shift aft. Phe395 |
|||
|
SR1=REFOMA (FOMA) |
Wiberg 1973 |
|
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|
Risoe 5678 (Carlsberg II) |
Jorgensen1975 |
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|
Risoe 6018 (Carlsberg II) |
Jorgensen1975 |
? |
|||
|
Risoe 7085 (Carlsberg II) |
Jorgensen1975 |
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|
Risoe 7372 (Carlsberg II) |
Jorgensen1975 |
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|
SZ5139b (Diamant) |
Schwarzbach 1967 |
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|
SR7 (Foma) |
Wiberg 1973 |
|
missing Phe182 and Thr183 |
||
|
Grannenlose Zweizeilige
|
Collected in Ethiopia 1937-38, reviewed by Hoffmann & Nover 1959 |
Hor 2937, Hor4408, Hor3028, CI14017 & other |
discussed by Jorgensen 1992 |
||
|
|
F240 → L |
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|
|
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|
|
? |
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|
|
? |
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|
|
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|
|
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|
Mutant ML-3A (Azuma Golden) |
|
? |
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|
Mutant ML-4F (Fuji Nijou) |
|
? |
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|
Mutant ML-9F (Fuji Nijou) |
|
? |
|||
|
Mutant ML-13F (Fuji Nijou) |
|
? |
|||
|
Mutant B1012 (Bomi) |
|
? |
|||
|
Mutant B1101 (Bomi) |
|
? |
|||
|
Mutant B1865 (Bomi) |
|
? |
|||
|
Mutant N105 (Bomi) |
|
? |
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|
|
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|
mlo27 |
Do 2021 and 8 other mutants |
|
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|
mlo28 |
|
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|
mlo29 |
|
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|
mlo30 |
|
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|
mlo31 |
URS1 (Ursula) |
Molina-Cano et al. 2003 |
|
Molina-Cano et al. 2003 |
? |
|
mlo32 |
PRU1 (Prudentia) |
Molina-Cano et al. 2003 |
|
Molina-Cano et al. 2003 |
? |
a) The accessions are possibly of same origin, the differing names may come from drifting through collections and nurseries, see Jorgensen 1992
Molecular structure of the Mlo gene
(summary of important findings)
The wild type Mlo-allele is 1599 bp long, the coded protein contains 533 aminoacids (Büschges et al. 1997).
The deduced Mlo protein has a predicted molecular weight of 60 kDa and is predicted to be membrane-anchored by seven membrane-spanning helices. (Internat. Patent Publ. No. WO9804586)
Graphic representation of the deduced 7TM topology of barley Mlo (Devoto et al 1999)
kindly permitted by the Journal of Biological Chemistry
The lipid plasma membrane bilayer is represented by a grey horizontal bar.
Circles with letters represent amino acids identified by the single letter amino acid code.
For the exact nucleotide or aminoacid sequence see Büschges et al. 1997 or the documentation
to the Mlo patent WO9804586
Genetic maps of the Mlo locus
The Mlo gene is located in the mid of the long arm of Chromosome 4H.In the last years mapping concentrated on molecular marker maps, which are much more detailed than morphological marker maps, but not generally understandable. Below you can see thumbnails of a morphological marker map and a RFLP marker map.
To see these and some other maps in full size or to find links to more maps CLICK HERE
Recently P.M. Hayes et al., 2000, (Oregon State Univ.) assembled a page summarizing QTL and physical locations for all 7 chromosomes, see http://www.css.orst.edu/barley/nabgmp/qtlsum.htm. To see the chromosome 4 map (PDF-format) click here
The Mlo patent
The defined polynucleotide constituing the Mlo gene has been patented under WO9804586
in 1998 for the applicant
INNES JOHN CENTRE INNOV LTD (GB); PANSTRUGA RALPH (GB); BUESCHGES RAINER (GB); SCHULZE LEFERT PAUL MARIA JOSE (GB)
The patent involves 73 claims, in summary covering the isolated Mlo polynucleotide and all it's possible alterations by mutation, addition, excision etc. or parts of it, all methods and vectors for heterologous incorporation of such a polynucleotide or testing it's presence using compounds reacting with it or it's products, and transgenic plants with incorporated said polynucleotide and their descendents (for details click the link).
The patent description contains also pictures with the exact nucleotide and aminoacid sequences, of morphological, rflp, aflp and other fine structure maps.
Fortunately, the patent does not preclude the free use of mutant or spontaneous mlo-alleles in conventional breeding or testing the presence of mlo in inoculation experiments with mlo-virulent mildew.
Barley varieties with mlo-based mildew resistance
Area of mlo-varieties grown in Europe: there are no exact data available. Several national variety offices publish data on cultivar areas grown for seed certification. From these approximately the percentage of mlo-varieties actually grown can be estimated. Not all sown seed is certified, but this uncertaneity is with both mlo- and non-mlo-cultivars . The following table summarises the data available till now.