Mutants are a valuable tool for solving many problems in physiology, genetics, and molecular biology. Soon after cell suspension and protoplast culture emerged as techniques in plant biology, they were applied to the isolation of selectable markers that were unavailable through classical methods using whole plant systems. This field has been pioneered by Widholm in 1972 1,2, and see Chapter 38 and 40 this vol.) and reviewed several times (3-8). As far as selectable markers are concerned, mostly nuclear-coded mutations have been characterized, with theexceptionof chloroplast-encodedstreptomycinresistance (9,10) and lincomycin resistance (21). No selectable mutations are yet known for the mitochondrial genome in higher plants.