In the meristem there is no cells differentiation. Virus can't move from cell to cell through plasmodesmesta using their Movement protein.

according to me, I  think that the phloem is not enough  developped in the meristem, so that  the virus can't spread very quickly at the meristem. Another hypothesis, there is a very intense metabolic activity in the meristem

In the meristem there is no cells differentiation. Virus can't move from cell to cell through plasmodesmesta using their Movement protein.

no vascular system in meristem is available for virus movement, viral replication (viral inactivating system) cannot cope with faster meristem cell division and auxin level might also limit viruses.

Apical meristem are actively diving cells they dont have well established vascular system like old tissues do. I suppose that these actively dividing cell maybe secreating some sort of chemical( plant hormones) that keep away viruses and bacteria from entering or surviving. I also think maybe its due to high metabolic activity and high level of chemicals( high concentration) which is too much for microorganism to coop with.

Dear Hervé!

I think even when you do tissue culture, using meristems there is no guarantee that all plants will be free of viruses. You'll get some free plants viruses and another infected. You should test samples from plants produced from each meristem to see if they are actually virus free. Thus, also in the early stages of laboratory is important to keep plants produced from each meristem identified and separated until have the results of the testing of virus. The efficiency of the elimination of virus will depend on the size of the meristem used, the ability of the technician to remove the meristem and which viruses were present in the plant.

Three main reasons-

* High metabolic processes(Enzymes activity)

*High concentration of auxin and Cytokinin hormone (Low pH)

*Cell multiplication very high and Plamodesmata size very small which prevent the movement of the virus from one cell to another cell.

Dear Hervè!

A recent paper about this topic: 

Citrus leaf blotch virus invades meristematic regions in Nicotiana benthamiana and citrus

JESÚS AGÜERO, MARÍA CARMEN VIVES, KARELIA VELÁZQUEZ, SUSANA RUIZ-RUIZ†,

JOSE JUÁREZ, LUIS NAVARRO, PEDRO MORENO AND JOSÉ GUERRI*

MOLECULAR PLANT PATHOLOGY (2013) 14(6), 610–616

Sincerely yours,

Paulo

Dear all,

probably something to see with silencing (again!).

Some papers going in this direction:

http://www.ncbi.nlm.nih.gov/pubmed/16040651

http://www.ncbi.nlm.nih.gov/pubmed/20015979

Best wishes

Franck

on newly growing meristem, there is high rate of cell divition, so it is a result of virus free cell duplication

Perhaps it's the evolutionary gain of plant. If the virus overcome this barrier, one time infection in the meristem can make whole system infective. And as some studies says female (in case of humans) are more resistant to infections than male counter parts to avoid protection to progeny.

Meristematic cells are considered as virus free because:

1). Meristematic cells have high multiplication rate than bacterial cell multiplication rates.

2). Viruses generally move in plant body through vascular bundles. There is the absence of vascular bundles in meristematic regions.

3). High auxin concentration and cytokinin concentration and low pH value at which viruses could not survive.

4). High metabolic activity.

5). RNA silencing mechanism. According to some findings, it has been hypothesized that the defense systems in meristematic regions treat viruses as foreign particles thus prevent the viruses into the meristematic regions.

beacause Cell differentiaion in meristem is fast .

The viruses are associated with the vascular system of the plants. In the newly developed meristem as the the growth is very fast, the vascular tissues are not yet developed. As the virus moves via vascular tissues, so it can not reach the apical meristem.