The Component of the TAC Complex, TCD7, Controls Rice Chloroplast Development at the Early Seedling Stage under Cold Stress

Transcriptionally active chromosome (TAC) is a component of protein-DNA complexes with RNA polymerase activity found in chloroplasts. Although TAC in Arabidopsis thaliana has been extensively investigated, how the rice (Oryza sativa L.) TAC complex functions remains largely unknown. We report the characterization of the mutant thermosensitive chlorophyll-deficient7 (tcd7) and the cloning of TCD7. tcd7 mutant seedlings displayed an albino phenotype specifically at low temperatures and before the four-leaf stage. We identified TCD7 by map-based cloning followed by transgenic rescue and genome editing tests, showing that TCD7 encodes the putative TAC component FRUCTOKINASE-LIKE 2 (OsFLN2). TCD7 transcripts were highly abundant in green tissues, and the protein localized to chloroplasts. In agreement with the albino phenotype, transcript levels of genes controlling chloroplast development and the establishment of photosynthetic capacity were severely reduced in tcd7 seedlings at low temperatures, but were expressed as in the wild type at high temperatures, implying that TCD7 regulates the PEP pathway and chloroplast development. Moreover, TCD7 interacted with the thioredoxin OsTRXz to form an OsTRXz-TCD7 regulatory module, which might regulate plastid transcription under cold stress. Our results demonstrate that the nucleus-encoded TAC protein TCD7 protects chloroplast development from cold stress via a TRXz-FLN regulatory module.