Dual-targeted transcription factors are required for optimal photosynthesis and stress responses in Arabidopsis thaliana

Chloroplast to nucleus retrograde signaling is essential for cell function, acclimation to fluctuating environmental conditions, plant growth and development. The vast majority of chloroplast proteins are nuclear-encoded and must be imported into the organelle after synthesis in the cytoplasm. This import is essential for the development of fully functional chloroplasts. On the other hand, functional chloroplasts act as sensors of environmental changes and can trigger acclimatory responses that influence nuclear gene expression. Signaling via mobile transcription factors (TFs) has been recently recognized as a way of communication between organelles and the nucleus. In this study, we performed a targeted reverse genetic screen to identify novel dual-localized TFs involved in chloroplast retrograde signaling during stress responses. We found that CHLOROPLAST IMPORT APPARATUS 2 (CIA2), a TF with putative plastid transit peptide can be detected in chloroplasts and the nucleus. Further, we found that CIA2, along with its homolog CIA2-like (CIL) act in an unequally redundant manner and are involved in the regulation of Arabidopsis responses to UV-AB, high light, and heat shock. Finally, our results suggest that both CIA2 and CIL are crucial for chloroplast translation. Our results contribute to a deeper understanding of signaling events in the chloroplast-nucleus cross-talk.\n\nSignificanceWe found that a transcription factor CIA2 can be located in chloroplasts and nucleus. CIA2 and is close homolog CIL are involved in protein translation and abiotic stress responses, and we suggest that they play an essential role in retrograde signaling between these organelles.