Origin and development of adventitious shoot meristems initiated on plant chimeras.

Most studies concerning the initiation and development of the shoot apical meristem have been performed on meristems that developed during embryogenesis. We characterized the in situ formation of adventitious shoots originating from cells located near leaf axils in a series of six interspecific periclinal tobacco chimeras. Shoots were generated by decapitating the plants and removing all of the axillary buds. Eighty-four of the 413 shoots regenerated were chimeral. Many of the shoots were complex mericlinal chimeras, with several of their axillary buds possessing meristems arranged as periclinal chimeras. The adventitious shoots originated from derivatives of only the second and third cell layers in the meristem of the source plant since derivatives of the outermost meristem layer were removed with the removal of the axillary buds. With time, nearly all adventitious shoot apices that were initially chimeral became nonchimeral or stabilized as periclinal chimeras. A statistical analysis indicated that the arrangement of genetically distinct tissues in the cell layers of adventitious meristems was influenced by competition between cell types. Our method for generating these shoots can be used to create small genetically distinct sectors analogous to radiation-induced sectors as well as a complete series of periclinical chimeras, both of which have potential for use in determining tissue-to-tissue interactions. An analysis of sector length and position on chimeral shoots indicates that the first one to three leaves of adventitious shoots do not arise as derivatives of the shoot apical initials of a meristem proper.