virus free plants

8/2/2019 Virus Free Plants

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Pathogen/Virus Free Plants


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Most of the crop plants propagated by vegetativemeans, are systemically infected with one or more

pathogens. Strawberry plants are known to be attacked by 62

viruses and mycoplasmas

Pathogen attack does not always lead to death of the

plant.

Many viruses may not even show visible symptoms.

But, the presence of viruses in plants reduces theyield and/or quality of crops.

Yield increases up to 300% (averaging 30%) havebeen reported by replacement of virus-infected stockwith specific pathogen-free plants


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The reasons proposed for the escape of the meristemfrom virus invasion are

(a) viruses readily move in a plant body through thevascular system which is absent in the meristem.The cell-to-cell movement of the virus throughplasmodesmata is too slow to keep pace with the

actively growing tip(b) high metabolic activity in the actively dividing

meristem cells does not allow virus replication

(c) the 'virus inactivating systems' in the plant body, ifany, has higher activity in the meristem than in anyother region; thus, meristem is protected

(d) a high endogenous auxin level in shoot apices mayinhibit virus multiplication.


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Virusfree

Specific virus free or specific pathogenfree


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Virus eradication

Thermotherapy

Chemotherapy

Meristem/shoot tip culture


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Thermotherapy Principle - at temperatures higher than normal many

viruses in plant tissues are partially or completelyinactivated with little or no injury to the host tissue

Explant/whole plant exposed to higher temp, lethalonly to viruses.

38-52 C for few min to several weeks. Virus encoded movement proteins are temp

sensitive.

High temp disrupt proteins

Coat protein also disrupts as it is also tempsensitive.

Interferes in virus replication.


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The area B-C is critical in heat treatment; the greater the spreadbetween the thermal death point of the host (C) and the parasite(B), the greater are the chances for successful thermotherapy


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Thermotherapy: Limitations

Not all viruses are sensitive to the

treatment.

Prolonged heat treatment inactivates plant

resistance system

Only isometric and thread like virus are

eliminated through thermotherapy

Survival rate low.


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Chemotherapy

Virus elimination by chemicals.

Actinomycin D, Ribavirin, 2-thiouracil,

Cyclohexane, Malachite green etc Added in medium at particular conc.

Interferes in virus replication

Also cytotoxic to plants, so used in slightlyless concentrations


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Both Thermotherapy and chemotherapy,

although helps in eradication of viruses but

has certain limitation.


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Meristem Tip Culture Holmes (1948), described gradient of virus

distribution in shoot tips.

Produced virus free plants of Dahlia

through shoot tip culture.

Mainly used for virus elimination,

meristem-tip culture enables plants to be

freed from other pathogens (viroids,

mycoplasmas, bacteria and fungi)


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Meristem Tip Culture Meristem: Localized group of actively

growing and dividing cells from whichpermanent tissues are derived.

Apical meristem: Group of cells situated at

the extreme tip of root or shoot in the shapeof dome. 100 m in diameter and 250 m inlength, remains in active state and alwaysyoung.

Shoot apex: length upto 500 m, have leafprimordia

Meristem tip prefered over shoot tips.


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Meristem Tip Culture Meristem cells are highly conservative and

are genetically stable and hence plants

produced are genetically stable.

Rate of cell division is very high

Absence of plasmodesmata and vascular

bundles.


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Meristem Tip Excision Stem apices or tuber sprouts, protected with

overleaf primordia. Excision of Meristem: under aseptic

conditions, stereomicroscope or light

source required, excise active meristem,prevent it from dessication (cover in wet

paper), sterilize by 70% ethanol for few

seconds before culture.

PGRs helps in virus eradication eg.

Cytokinins interfere virus replication


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Callus cultures for virus elimination :

Shoot tip grafting or micrografting:

extremely beneficial for woody plants.

Very small tip consisting of meristem with 2-3 leaf

primordia to the in vitro grown virus free root stock.

Citrus, apple, prunes etc


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Virus Indexing or Virus Detection

Sap Transmission or Bioessay or Infectivity test:

Serological Detection (ELISA)

Nucleic acids Based: hybridization or PCR

Electron Microscopic studies


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Applications

virus free plants

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