Rice TCM1 Encoding a Component of the TAC Complex is Required for Chloroplast Development under Cold Stress

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Abstract
Transcriptionally active chromosome (TAC) is a component of protein–DNA complexes with RNA polymerase activity, expressed in the plastid. However, the function of rice TAC proteins is still poorly understood. In this paper, we first report the identification of a new rice ( Oryza sativa L.) mutant ( tcm1 ) in the gene encoding TAC. The tcm1 mutant displayed an albino phenotype and malformed chloroplasts before the three‐leaf stage when grown at low temperatures (20°C) and a normal phenotype at higher temperatures (>28°C). Map‐based cloning revealed that TCM1 encodes a novel chloroplast‐targeted TAC protein in rice. In addition, the transcript levels of all examined plastid‐encoded polymerase (PEP)‐dependent genes were clearly downregulated in tcm1 mutants at low temperatures, although partially recovering levels were obtained at high temperatures, comparable to wild‐type plants. Furthermore, the TCM1 transcripts were ubiquitously expressed in all examined tissues, with high expression levels in green tissues. The data suggest that the rice nuclear‐encoded TAC protein TCM1 is essential for proper chloroplast development and maintaining PEP activity under cold stress.

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