Download Regarding question 1:

Regarding question 1:
1. Callus and Meristem contain both undifferentiated
embryonic cells that can give rise to a whole
organism. Nevertheless there are major differences
between them. The Meristem differentiation is part of
the normal development and originates from true
embryonic cells.
The callus originates from a
differentiated cell that underwent dedifferentiation —
some genetic changes may be present already in the
differentiated cell. Moreover, the hormonal treatment
that promote rapid cell division in a callus may affect
the genetic integrity of the plant: Many DNA repair
mechanisms in the cell take place during specific cell
cycle checkpoints. If those checkpoints are not
respected repair may not take place and mutation rate
might increase.
2. Many viruses are excluded from seeds and from plants
derived from tissue culture (from embryonic calli). A plant that
underwent such propagation might thus be clean of viruses. If it is
grown in an environment where it is exposed to viruses it will
become slowly, infected. Infection might take time, or be partially
controlled by control of insects, and such plant has a better start in
life than a plant that is infected from birth after normal vegetative
propagation.
3. Protoplast fusion enables to overcome sexual barriers of
reproduction via gametes. However an hybrid obtained by
protoplast fusion will be sterile if it contains two nuclei, each with a
different genome.
4. You are right; getting mutants does not require an apriori
knowledge of the genetic basis of the character for which a
mutation is searched. However in some cases, a new trait cannot
be obtained by a simple mutation, and if this trait exists in another
species it might be transferred following protoplast fusion. Note
however that the technique of protoplast fusion is not very efficient
either to transfer traits and is not widely used.