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 caro-tenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for caro-tenoid 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 de-saturation and, possibly, to the respiratory activity of the chloroplast.

The Arabidopsis Spontaneous Cell Death1 gene, encoding a ζ-carotene desaturase essential for carotenoid biosynthesis, is involved in chloroplast development, photoprotection and retrograde signalling

“…These developmental defects are partly attributed to the loss of or reduced carotenoid-mediated antioxidant activity, impaired chlorophyll biosynthesis and reduced PS II capacity [4,[26][27][28]. Moreover, the ABAdeficient phenotype is often associated with precocious seed germination (viviparous), reduced seed dormancy and compromised stress response [10].…”

Activation of the Stt7/STN7 Kinase through Dynamic Interactions with the Cytochrome b6f Complex

“…In the case of overreduction of the acceptor side of PSI, excess electrons can reduce molecular oxygen through the Mehler reaction to superoxide, which is then converted to hydrogen peroxide by a plastid superoxide dismutase and ultimately to water by a peroxidase (Asada, 2000). Over-reduction of the PQ pool can be alleviated by PTOX, the plastid terminal oxidase responsible for oxidizing PQH 2 to form hydrogen peroxide, which is subsequently converted to water (Carol et al, 1999;Cournac et al, 2000;Wu et al, 1999).…”

Disruption of phytoene desaturase gene results in albino and dwarf phenotypes in Arabidopsis by impairing chlorophyll, carotenoid, and gibberellin biosynthesis

“…Another Arabidopsis mutant, immutans, accumulates phytoene in variegated white tissues. The IMMUTANS gene encodes plastid terminal oxidase, the disruption of which prevents oxidation during chlorophyll biosynthesis and results in the accumulation of phytoene [16,17]. However, mutations in the gene encoding the phytoene desaturase itself (designated PDS3) [14] have not been reported.…”

“…First-strand cDNA synthesis was performed in a 20-µL-reaction volume consisting of 3 µg of total RNA, oligodT [12][13][14][15][16][17][18] and SuperScript TM reverse transcriptase (GIBCO BRL). The reaction was allowed to proceed at 42 ºC for 50 min before being terminated by treatment at 70 ºC for 15 min.…”