The Development of disease resistant varieties of barley Essay

Before we take a expression at the advancement made in the development of intercrossed ( disease opposition ) assortments of barley, we have to understand the really basicss based on which these assortments were prepared. To develop disease opposition in barley harvest, Hordeum vulgar L, the research workers focused on the host and pathogen systems. In this type of system by and large the pathogen benefits from the host as it feeds and grows whereas the host suffers and withers finally due to the malfunctioning of the natural system.

The type of disease caused and the lesions observed depend on the extent of interaction that takes topographic point between the host and the pathogen. A ill or weakly developed lesion shows us that the relationship was incompatible between the two, whereas a well-developed lesion shows marks of compatibility.

Different assortments of host when infected with an single civilization of pathogen consequences in different types of lesions turn outing the fact that these interactions are conditioned by cistrons.

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The necessity for analyzing the genetic sciences of both the host and pathogen was shown by surveies conducted by Flor ( Flor, 1955 ) . These surveies were based on the relationship of cistrons conditioning the reaction of flax, and the corresponding cistrons conditioning pathogenicity of flax rust fungus.

These surveies concluded that for each cistron conditioning the reaction of the host there is a corresponding cistron conditioning the pathogenicity of the pathogen ( J.G.Moseman ) . The same relationship has been proved between barley and powdery mold fungus ( Erysiphe graminis f. ) . The familial surveies of either the host or the pathogen can supply the information sing the genetic sciences of the other being and the development of the powdery mold fungus.

Within the past decennary, two viral diseases viz. the Barley xanthous midget and Barley stripe mosaic have become more active in many countries of the universe. Barley xanthous midget has been reported in many states such as England, Norway, Australia and United provinces. Barley band mosaic has been reported from Japan, Canada and The States. Another point to observe is that these diseases are common for all the members of the grass household. In the United States major losingss are incurred from this disease in Oats than in barley.

BARLEY YELLOW DWARF DISEASE

Barley xanthous midget was foremost identified by Oswald and Houston in 1951 ; it has been a major disease in most parts of the universe particularly in The United States. Around 80 per centum decrease in output has been measured by experimentation in susceptible assortments of barley, wheat and oats ( Bruehl, 1961 ) .

Barley yellow dwarf virus ( BYDV ) is characterized by a single-stranded RNA virus ; this virus is transmitted by aphids. The isolates that have been divided into two subgroups:

Group 1

Group 2

This group contains three strains chiefly MAV, SGV and PAV.

This group contains two strains RMV and RPV.

MAV strain carried by Sitobion Avenae.

SGV and PAV carried by Rhopalosiphum padi, S.avenae and Metopolophium dirhodum.

These strains are carried by Rhopalosiphum padi and Rhopalosiphum padi

These strains have been classified as less terrible.

These strains cause terrible signifier of the disease.

The symptoms of barley xanthous midget disease differ with the affected harvest, works age at the clip of infection, present environmental conditions, and the virus strain, and can be confused with other related diseases or physiological upsets. The symptoms begin to look in about 14 yearss after the clip of infection.

Characteristic symptoms that have been found to be associated with the Barley Yellow Dwarf disease are Discoloration i.e. yellowing or reddening of the foliage and Dwarfing. Assortments that are susceptible show progressive symptom development in the signifier of bright xanthous smudges, with the coloring materials developing from the tip of the foliage and come oning downward throughout the blade. Other symptoms include stunting/dwarfing ( hence the name xanthous midget ) , decrease in root growing, delayed header or no header, and a seeable decrease in output. It is observed that the caputs of the affected workss tend to stay vertical and due to colonisation by saprophytic Fungis they become black and discolored. Due to this infection, the foliages have reduced photosynthetic ability.

BARLEY YELLOW DWARF-Virus.

Aphids are known to be vectors of the BYDV ( Barley Yellow Dwarf Virus ) . When aphids feeds on the works, BYD virions get transferred to the aphid ‘s hind intestine, the coat protein nowadays on the surface of the virus is recognized by the hindgut epithelial tissue cells of the aphid, after which the virion is allowed to go through into the hemolymph of the aphid, where it remains indefinitely, but can non multiply within the aphid. The virus is transported into the aphids salivary secretory organ so as to be released into salivary canals and canals. The virus is subsequently excreted in the aphid spit during eating.

Gildow and colleages in the twelvemonth 1999 worked on the theoretical account for transmittal of the virus through the aphids. They pointed out the three chief barriers that the virus must get the better of within the aphid for the transmittal to take topographic point.

First, after the consumption, the virus needs to undergo specific adhering with the epithelial cells of the hindgut, successful binding consequences in the virus being membrane transported to the haemocoel.

Once transported to the haemocoel, the protein coat sphere interacts with symbionin, an endo symbiotic bacterial protein ( Young and Filichkin, 1999 ) . Symbionin protects the virus from debasement within the haemocoel and let its endurance for a important clip within the aphid. Another interaction with the read through sphere of the coat protein allows the virus to travel from the haemocoel of the insect into the radical lamina of the salivary secretory organ followed by endocytosis across the plasma lemma. The spit so produced during eating contains the virus which subsequently gets transferred to phloem of the works. This theoretical account elaborates the events that occur during transmittal and can be utile in developing techniques to command the disease.

The Barley xanthous midget virus atoms are structurally isometric and are about 30 nanometers when measured utilizing shady readyings, 23-25nm when observed in thin subdivisions and 20 nanometer if negatively stained readyings of virus are used. Gill and Chong studied the ultrastructural alterations induced by MAV and RPV which are vector specific and PAV which is non-specific in oat cells.

Below here we see a thin subdivision of BYDV infected phloem parenchyma cells of bare brome ( Bromus Sterilis L. ) . This is image of the cell which has been infected with a terrible strain of BYDV which is transmitted by R.padi. This infection was achieved by exposing eight twenty-four hours old seedlings to the virus by the agencies of aphids.

Figure Electron Micrograph of Bromis sterilis L. cells infected with barley xanthous midget virus. Note the big Osmiophilic lipoid globule ( O.G ) and the rhombic crystalline array which is enveloped in an endoplasmic Reticulum cisterna ( E.R ) . ( V ) virus atoms.

Sing the genomic composing of the viruses, The Luteovirus BYDV subtypes and the Polerovirus CYDV-RPV atoms are both made up of 180 protein fractional monetary units which consists of a mixture of prevailing 22 kD population and a little figure of 50 to 55 kD read through proteins ( D’Arcy et al. 2000 ) . Among the other features which distinguishes the two, the Polerovirus genus contains a VPg genome-linked protein and a 5 ‘ ORF ( Open Reading Frame ) , designated ORF 0, both of which are absent in members of the Luteovirus subtypes. The RdRp cistrons of Luteovirus and Polerovirus besides exhibit a great trade of differences, the two genera differ in sequences at their 3 ‘ end point. The Luteovirus genome ( Fig. 4 ) consists of five major ORFs that appear to be expressed in septic workss

Figure The familial composing of the Barley Yellow Dwarf Virus and the Cereal Yellow Dwarf virus. Black lines indicate individual stranded positive sense genomic ( sRNA ) and subgenomic ( sgRNA ) . The white boxes represent the protein merchandises of the cistrons and their several weight in kilo Daltons ( K ) .

Resistance to BYDV/CYDV

The research for engendering BYDV resistive assortments of barley has been carried out for more than 50 old ages, yet the pattern of utilizing alternate control steps were given more precedence than opposition genteelness particularly in UK ( Plumb, 2002 ) . Breeding of assortments which provide strain specific opposition requires rating and choice which normally varies due to the fact that the procedure is based on the appraisal of output and the visual aspect of symptom alternatively of quantitative checks which determines the virus strain. As concluded by Burnett et.al ( 1995 ) , that tolerance instead than resistance better describes what has been traditionally measured in field surveies.

Ryd2 aka Yd2 is the cistron normally used in engendering for BYD tolerance. This semi dominant cistron was foremost found in barleys that originated in Ethiopia ( Schaller and Rasmusson, 1959 ) and was found to be located on the 3H chromosome ( Schaller, 1964 ) . Another cistron known as yd1, is recessionary by nature and was discovered in Rojo barley although relatively yd1 has a lower degree of tolerance than yd2 ( Suneson 1955 ) . Due to this, there is merely a limited sum of research work carried out on yd1. In 2004, a fresh cistron was discovered, this cistron was located in one of the Ethiopian Barley lines ( L94 ) near the kinetochore of the chromosome 6H in the Ethiopian Barley line L94 and was named Ryd3. The dominancy of this cistron is yet non known, but the opposition appeared is comparable to Ryd2.

The Ryd2 cistron nevertheless remains the most normally used cistron to develop opposition. Experiments conducted by Schaller and co-workers at Davis, California resulted in the undermentioned decision:

Out of the 6689 assortments of barley that were tested for opposition to BYDV, 117 assortments were classified as resistant. Out of this 117, 113 assortments were introduced from Abyssinia, 3 were loanblends with Abyssinian types as parent and one was from China. None of these assortments nevertheless, was found to be free of the BYDV, as indicated by the visual aspect of disease symptoms. Therefore tolerance instead than opposition was apparent and the bulk of those with tolerance may hold possessed Ryd2 or even Ryd3 cistron.

Another assortment known as the station winter barley has been released which is “ slightly immune ” to BYDV and has no relation whatsoever with Ryd2 or the Ethiopian barley. ( Grafton et al.1982 )

Subsequently, farther research showed us that the Yd2 cistron is linked with four familial markers on the chromosome 3 ; viz. streaked vs. normal foliages, slack vs. dense spike xanthan, vs. normal seedling and uzu vs. normal growing.

Besides some grounds has been gathered that makes the effectivity of the Ryd2 cistron vary with the familial background ( eg. Catherall et.al 1970 ) Some breeders have found that Ryd2 is less effectual in subsequently maturating germplasm ( Catherall and Hayes, 1966 ; B8. Resistance Mechanisms in Barley Jones and Catherall, 1970 ) . The effectivity of Ryd2 cistron can be measured by quantitative analysis of the accretion of the virus in the host works. It has been verified that the Ryd2 cistron can cut down this accretion demoing mark of opposition. ( Ranieri et al. 1993 ; Larkin et Al. 1991 ; Skaria et al.1985 ) . However, the decrease in virus accretion is effectual merely against BYDV ( eg. Luteovirus ) and non against CYD virus.

There are several tax write-offs made so as to calculate out the map of Ryd2. Larkin et.al in 1991 showed that Ryd2 does non work in leaf energids ; besides proposing in a manner that reproduction itself is non restricted. There is a possibility that Ryd2 can curtail the motion of virus from cell to cell or it can be expressed in bast cells which are non good represented in the energid cells. Thus it has been concluded that Ryd2 cistron restricts virus accretion instead than virus reproduction or spread.

Collins in the twelvemonth 1996 mapped the Ryd2 cistron, these surveies suggested that Ryd2 coseggregated with RFLP markers Xwg889 and XYIp on the long arm of chromosome 3, which is located at.5cM from the kinetochore. Sequencing revealed that Yip allelomorphs differ by merely a individual base in barley with or without Ryd2, and the tight linkage between Yip and Ryd2 gave an chance to develop specific marker fior Ryd2 choice. In 1998, Paltridge developed YLM which was a PCR based marker. The precise mechanism by which Ryd2 maps is non known. It is known that the tightly-linked Ylp encodes a vacuolar H+-translocating ATPase fractional monetary unit E ( Ford et Al. 1998 ; Dietz et Al. 1995 ) , but a function for this peptide in opposition remains bad.

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