Mutations in the Arabidopsis Gene IMMUTANS Cause a Variegated Phenotype by Inactivating a Chloroplast Terminal Oxidase Associated with Phytoene Desaturation

Abstract

The immutans ( im ) mutant of Arabidopsis shows a variegated phenotype comprising albino and green somatic sectors. We have cloned the IM gene by transposon tagging and show that even stable null alleles give rise to a variegated phenotype. The gene product has amino acid similarity to the mitochondrial alternative oxidase. We show that the IM protein is synthesized as a precursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane. The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the carotenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for carotenoid desaturation. The suggested terminal oxidase function of IM appears to be essential to prevent photooxidative damage during early steps of chloroplast formation. We propose a model in which IM function is linked to phytoene desaturation and, possibly, to the respiratory activity of the chloroplast. INTRODUCTIONIn plant cells, plastid differentiation is intimately linked to organogenesis and is affected by both developmental regulatory mechanisms and environmental conditions. The complex mechanisms involved in the phototransformation of the plastids of dark-grown seedlings (etioplasts) to photosynthetically active plastids (chloroplasts) have been described extensively (reviewed in Mullet, 1988;Taylor, 1989;Chory and Susek, 1994). Alternatively, developing grass tissues provide a unique system in which a gradient of developmental stages of cells and plastids (proplastids to chloroplasts) is present (Mullet, 1988;Bilang and Bogorad, 1996; Inada et al., 1996).Mutants visibly impaired in chloroplast differentiation can be selected by their pale green, yellow, or albino color. The chlorophyll-deficient mutant olive of Antirrhinum has this phenotype (Hudson et al., 1993). Not all of these mutants are altered directly in pigment synthesis. Putative functions can be proposed for mutated genes based on the similarity of their coding sequences to known proteins. The Arabidopsis mutant albino3 (Sundberg et al., 1997) is impaired in chloroplast membrane biogenesis, as suggested by amino acid sequence similarity between ALBINO3 and the yeast mitochondrial OXA1 protein involved in mitochondrial biogenesis. The Arabidopsis cla1 mutation (Mandel et al., 1996) identifies an enzyme also present in cyanobacteria. Mutations also have been described that affect the proplastid-tochloroplast transition. The dag mutant from Antirrhinum (Chatterjee et al., 1996) and dcl1 from tomato (Keddie et al., 1996) do not contain chloroplasts but rather plastids resembling nondifferentiated proplastids. These genes together with the PALE CRESS gene from Arabidopsis (Reiter et al., 1994) affect both chloroplast development and leaf architecture.One class of mutations gives rise to variegated plants that have a mutant phenotype in some sectors and a wild-type phenotype in others. The iojap (Han et al., 1992) mutant of maize or the chloroplast mutator in ma...

Biochemistry and Molecular Biology of Carotenoid Biosynthesis in Chili Peppers (Capsicum spp.)

“…Mutant plants affected in PTOX gene ( immutants and ghost ) are defective in carotenoid production and accumulate phytoene; as a result, these mutants exhibit photobleaching in high light conditions because of a lack of photoprotective carotenoids [69,70,87–90]. PTOX is a plastid-located plastoquinol:oxygen oxidoreductase linked to carotenoid biosynthesis and chlororespiration to prevent photooxidative damage [91].…”

“…PTOX is a plastid-located plastoquinol:oxygen oxidoreductase linked to carotenoid biosynthesis and chlororespiration to prevent photooxidative damage [91]. PTOX is synthesized as a precursor polypeptide, which is imported into chloroplasts and inserted into the thylakoid membrane [69]. In Arabidopsis a PTOX cDNA showing a 350-amino acid ORF encoding a putative 40.5 kD protein was isolated [69].…”

Characterization of the influence of chlororespiration on the regulation of photosynthesis in the glaucophyte Cyanophora paradoxa

“…Nevertheless, the interaction between photosynthesis and respiration still exists in the form of chlororespiration21. In chloroplasts of land plants, plastidial NDH-1 complexes can transfer electrons from stromal NADPH to PQ pool222324, while plastid terminal oxidases (PTOX) pull out electrons from PQ pool and transfer them to molecular oxygen2526. Eukaryotic algae also possess PTOX27, but plastidial ndh genes have not been widely reported in plastid genome of eukaryotic algae28.…”

Ultrastructural and Physiological Characterization of YELP, a Novel, Yellow, Chlorophyll-Deficient Cell Mutant Line That Develops Etioplast-Like Plastids in the Light

“…Reduced carotenoid synthesis causes photo-oxidative damage, particularly under high-light conditions or extreme temperatures. However, in carotenoid-deficient plants, even standard light conditions trigger photo-oxidation [52].…”