Establishment of Tissue Cultures

By now you are familiar with the term "Explants". After a few days in culture the explant becomes slightly rough in texture and its surface glistens in reflected light. This is a sign of the beginning of callus formation. A callus is defined as an irregular tissue mass varying considerably in texture, appearance and growth rate. In nature plants produce callus as a result of mechanical injury (wound tissue), invasion by microorganisms or by insect feeding. Callus formation has been observed in angiosperms, gymnosperms, ferns, mosses and liverworts. Infection by Agrobacterium tumefaciens causes the production of tumors or 'Crown gall' in dicotyledonous plants. The stimulus for cell proliferation in these cases is provided by endogenous hormones, auxins and cytokinins. Plant material typically cultured included vascular cambia, storage parenchyma, pericycle of roots, cotyledons, leaf mesophyll and provascular tissue. You should know that all multicellular plants are potential sources of explants for callus initiation. In the laboratory you can easily establish callus cultures from the stem pith tissues of tobacco or slices of carrot root on MS medium supplemented with 2, 4-D.