RESCUE OF A MAIZE MITOCHONDRIAL CYTOCHROME-OXIDASE MUTANT BY TISSUE-CULTURE

The maize NCS6 mitochondrial mutation is a partial deletion of the cyt ochrome oxidase subunit 2 gene (cox2) that survives heteroplasmically in the plant. Mutant mitochondria segregate from normal mitochondria d uring somatic development giving rise to defective sectors on the plan ts, including areas of kernel abortion on the ears. Embryos from NCS6 kernels can be rescued by tissue culture. Slowly growing Type II callu s derived from one of these embryos has been shown by PCR analysis to be homoplasmic for the mutation, carrying only the defective mitochond rial cox2 gene. Most of the rescued embryos were heteroplasmic for nor mal and mutant genes and heteroplasmy was maintained in the callus cul tures. However, when suspension cultures were initiated from heteropla smic calli, normal cells were shown to have a selective advantage. Whe n the homoplasmic cox2 mutant callus cultures were placed on regenerat ion medium, plantlets did not regenerate. Heteroplasmic calli were cap able of regeneration under the same conditions. These studies suggest that the functioning of mitochondrial cytochrome oxidase is not essent ial for growth as callus, but is required for the differentiation and development of plants.