How is tissue culture method useful for crop improvement?

(A) As traditional breeding techniques failed to keep pace with demand and to provide sufficiently fast and efficient systems for crop improvement,a technology called tissue culture was developed.
Scientists learned during the $1950s$ that a whole plant could be regenerated from explants,which is any part of a plant taken out and grown in a test tube under sterile conditions in special nutrient media.
This capacity to generate a whole plant from any cell/explant is called totipotency.
The nutrient medium must provide a carbon source such as sucrose,inorganic salts,vitamins,amino acids,and growth regulators like auxins and cytokinins.
By applying these methods,it is possible to achieve the propagation of a large number of plants in very short durations. This method of producing thousands of plants is called micropropagation.
Each of these plants will be genetically identical to the original plant,known as somaclones. Many important food plants like tomato,banana,and apple have been produced on a commercial scale using this method.
Another important application is the recovery of healthy plants from diseased ones. Even if a plant is infected with a virus,the meristem remains free of the virus. Hence,one can remove the meristem and grow it in vitro to obtain virus-free plants.
Scientists have also isolated single cells and,after digesting their cell walls,obtained naked protoplasts. Protoplasts from two different varieties can be fused to form hybrid protoplasts,which can be grown into new plants called somatic hybrids. This process is known as somatic hybridization.