Rescue of a maize mitochondrial cytochrome oxidase mutant by tissue culture

The maize NCS6 mitochondrial mutation is a partial deletion of the cytochrome oxidase subunit 2 gene (cox2) that survives heteroplasmically in the plant. Mutant mitochondria segregate from normal mitochondria during somatic development giving rise to defective sectors on the plants, including areas of kernel abortion on the ears. Embryos from NCS6 kernels can be rescued by tissue culture. Slowly growing Type II callus derived from one of these embryos has been shown by PCR analysis to be homoplasmic for the mutation, carrying only the defective mitochondrial cox2 gene. Most of the rescued embryos were heteroplasmic for normal and mutant genes and heteroplasmy was maintained in the callus cultures. However, when suspension cultures were initiated from heteroplasmic calli, normal cells were shown to have a selective advantage. When the homoplasmic cox2 mutant callus cultures were placed on regeneration medium, plantlets did not regenerate. Heteroplasmic calli were capable of regeneration under the same conditions. These studies suggest that the functioning of mitochondrial cytochrome oxidase is not essential for growth as callus, but is required for the differentiation and development of plants.