Analysis of Mutator activity in embryogenic callus cultures and regenerated plants of Zea mays L.

Activity of the Mutator transposable element system in maize was assessed in tissue cultures and regenerated plants by Southern analysis of Mu-element modification status and genomic position. Examination of embryogenic callus cultures established from the F[subscript]1\u27s of crosses between active Mutator stock and maize inbreds A188 and H99 determined that the in planta parameters for Mutator activity were maintained in an in vitro environment. Subclonal populations from callus lines were used to demonstrate that Mu elements from active Mutator lines can remain transpositionally active in tissue culture systems. Novel Mu-homologous restriction fragments occurred in 38% of the subpopulations containing unmodified Mu elements, but not in control cultures containing modified Mu elements. The high mutagenic potential of Mutator should prove useful for in vitro mutagenesis, selection, and transposon tagging schemes, and should serve to enhance the generation of useful somaclonal variants in regenerated plants;Investigation of Mutator activity in regenerated plants demonstrated the maintenance of Mutator activity in progeny of plants regenerated from an active Mutator callus line. Activity was detected genetically by the segregation for new mutant phenotypes and molecularly by the appearance of Mu-homologous restriction fragments novel to the regenerant progeny. Segregation for new seedling mutations in third- and fourth-generation regenerant progeny of a callus culture derived from an inactive (Mu-loss) line suggests that a low-level reactivation of Mutator activity occurred in these progeny plants;Possible tissue- or development-specific influences on Mu-element excision were also explored. Embryogenic and endosperm cultures were established from maize lines in which Mu elements were known to have inserted at particular endosperm and aleurone reporter loci (wx-Mum1 and bz-Mum8). Somatic instability was demonstrated phenotypically at both mutant loci in endosperm callus tissues, indicating that excision occurs in endosperm (and aleurone) in vitro as well as in planta. Southern analysis of wx-Mum1 and bz-Mum8 embryogenic lines produced no evidence of Mu-element excision from either locus. Further studies involving tissue comparisons detected differences in Mu-element modification and genomic position in various tissues of H99/Mu[superscript]2 regenerated plants. Absence of Mu-homologous restriction fragments in a particular tissue relative to other tissues of the same plant implied that Mu-element excision occurred. Together, these findings provide preliminary evidence that Mu-element excision may be subject to tissue-specific influence or regulation