REARRANGEMENTS IN SUGAR-BEET MITOCHONDRIAL-DNA INDUCED BY CELL-SUSPENSION, CALLUS-CULTURES AND REGENERATION

Structural alterations in mitochondrial DNAs (mtDNAs) from a plant of a sterile sugar beet line, callus derived from it, suspension-cultured cells and plants regenerated from the callus were studied. BamHI rest riction analysis revealed that structural alterations between the mtDN As of the callus and the control plant had occurred. Multiple rearrang ements were also demonstrated in the mtDNA from the suspension culture , of which some were similar to those appearing in the callus, and oth ers had arisen de novo. Rearrangements were also identified by means o f blot hybridization of BamHI-digested mtDNA from suspension cultured cells with the genes encoding subunit II of cytochrome oxidase (cox II ) and subunit 1 of NADH-dehydrogenase (Nd1). No alterations were obser ved in the mitochondrial genome of the callus and regenerants. The loc ation of the genes for the alpha-subunit of F1-ATPase (atpA) and apocy tochrome b (cob) in the mtDNA remained unchanged. Our salient finding was of a plant with an altered mitochondrial genome as judged by EcoRI and BamHI restriction analysis. This exceptional plant had retained t he sterile phenotype like all of the other regenerants and the parent. The set of plasmid-like molecules of mtDNA remained the same as that in the control plant and in all of the regenerants, callus and suspens ion-cultured cells. The only type of plasmid-like molecule found in al l of the DNAs was the 1.6-kbp minicircle, which is a feature of steril e cytoplasms. These structural changes in mtDNA were obviously a conse quence of somaclonal variation during the in vitro cultivation of the sugar beet cells.