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Nuclear gene-induced variegation mutants provide a powerful system to dissect interactions between the genetic systems of the nucleus-cytoplasm, the chloroplast, and the mitochondrion. The immutans (im) variegation mutation of Arabidopsis is nuclear and recessive and results in the production of green- and white-sectored leaves. The green sectors contain cells with normal chloroplasts, whereas the white sectors are heteroplastidic and contain cells with abnormal, pigment-deficient plastids as well as some normal chloroplasts. White sector formation can be promoted by enhanced light intensities, but sectoring becomes irreversible early in leaf development. The white sectors accumulate the carotenoid precursor phytoene. We have positionally cloned IM and found that the gene encodes a 40.5-kD protein with sequence motifs characteristic of alternative oxidase, a mitochondrial protein that functions as a terminal oxidase in the respiratory chains of all plants. However, phylogenetic analyses revealed that the IM protein is only distantly related to these other alternative oxidases, suggesting that IM is a novel member of this protein class. We sequenced three alleles of im, and all are predicted to be null. Our data suggest a model of variegation in which the IM protein functions early in chloroplast biogenesis as a component of a redox chain responsible for phytoene desaturation but that a redundant electron transfer function is capable of compensating for IM activity in some plastids and cells.
INTRODUCTION
Variegation mutants have played a prominent role in the history of genetics (reviewed in Granick, 1955). As a notable example, they were used by E. Bauer in his seminal studies in the early 1900s to describe the phenomenon of non-Mendelian inheritance. The cells in the green sectors of these plants have morphologically normal chloroplasts, whereas cells in the white sectors have abnormal plastids deficient in pigments and organized lamellar structures. One common mechanism of variegation involves the induction of defective mitochondria or chloroplasts in some but not all cells by mutations in nuclear genes (Tilney-Bassett, 1975). Because the products defined by these genes are required for normal chloroplast biogenesis, they provide an excellent starting point to dissect the poorly understood pathways of communication between the nuclear-cytoplasmic, chloroplast, and mitochondrial genetic systems (Taylor, 1989).
Several well-known examples of variegated plants induced by nuclear gene mutations include the maize iojap and nonchromosomal stripe...