Rice TCM 1 Encoding a Component of the TAC Complex is Required for Chloroplast Development under Cold Stress

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 wildtype 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. C are semiautonomous organelles with limited coding information for components, including plastid machinery and photosystem genes (Sato et al., 1999; Leister 2003). Chloroplast development consists of a series of complex actions and can be classified into three phases, which are coordinately regulated Plant Genome 11:160065 doi: 10.3835/plantgenome2016.07. 0065 © Crop Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA This is an open access article distributed under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). D. Lin, K. Zheng, Z. Liu, Y. Dong, Development Center of Plant Germplasm Resources, College of Life and Environmental Sciences, Shanghai Normal Univ., Shanghai 200234, China; S. Teng, Shanghai Inst. for Biological Sciences, Chinese Acad. of Sciences, Shanghai 200032, China; Z. Li, J. Xu, Inst. of Crop Sciences, Chinese Acad. of Agricultural Sciences, 12 South Zhong-Guan Cun Street, Beijing, 100081, Z. Li, J. Xu, Agricultural Genomes Inst. at Shenzhen, Chinese Acad. of Agricultural Sciences, Shenzhen 518120, China. Received 10 July 2016. Accepted 20 June 2017. *Corresponding author (dong@shnu.edu.cn; xujlcaas@126.com). Abbreviations: ΦPSII, photosynthetic efficiency; Chl, chlorophyll; Fv/ Fm, variable-to-maximum fluorescence ratio; GFP, green fluorescent protein; PCR, polymerase chain reaction; PEP, plastic-encoded polymerase; PSII, Photosystem II; pTAC, plastid TAC proteins; RT-qPCR, real-time quantitative polymerase chain reaction; TAC, transcriptionally active chromosome; WT, wild-type core Ideas • Transcriptionally active chromosome (TAC) is a component of protein–DNA complexes with RNA polymerase activity, expressed in the plastid. • Map-based cloning revealed that TCM1 encodes a novel chloroplast-targeted TAC protein in rice, in which a mutation leads to an albino phenotype and malformed chloroplasts before the three-leaf stage at low temperatures. • TAC protein TCM1 is essential for proper chloroplast development and maintaining plastid-encoded polymerase activity under cold stress conditions. Published December 14, 2017