Isolation and Characterization of a Plastid Terminal Oxidase (PTOX) Gene from Rice (Oryza sativa L.)

We identified an EMS-induced japonica rice mutant that is characterized by excessive tillering and semi-dwarfism as well as variegated leaves.  A mapping population generated by crossing the mutant to indica rice was used to initially map the candidate mutation to a region of about 650 kb.  EcoTILLING (Targeting Induced Local Lesions In Genomes) polymorphisms identified between the two parents were employed as markers to further delineate the candidate locus to 170 kb.  TILLING of the candidate region identified a single base substitution that introduced a premature stop codon in a gene that encodes a plastid terminal oxidase, which shares homology with the Arabidopsis IMMUTANS and tomato GHOST genes.  Genetic complementation and RNAi analysis confirmed that the mutation identified is responsible for the developmental phenotypes of the mutant.  Plastid terminal oxidase is required for carotenoid synthesis and mutant seedlings germinated and grown in darkness showed enhanced mesocotyl elongation consistent with a deficiency in the carotenoid-derived hormone strigolactone (SL). Application of the synthetic strigolactone GR24 was able to rescue both the elongated mesocotyl and the high tillering phenotypes of the rice ptox mutant confirming that strigolactone-deficiency is the primary cause of these developmental and growth phenotypes.  Additional experiments are underway to further characterize the mutant.