ORANGE Represses Chloroplast Biogenesis in Etiolated Arabidopsis Cotyledons via Interaction with TCP14.

The conversion of etioplasts into chloroplasts in germinating cotyledons is a crucial transition for higher plants, enabling photoautotrophic growth upon illumination. Tight coordination of chlorophyll biosynthesis and photosynthetic complex assembly is critical for this process. ORANGE (OR), a DnaJ-like zinc finger domain-containing protein, was reported to trigger the biogenesis of carotenoid-accumulating plastids by promoting carotenoid biosynthesis and sequestration. Both nuclear and plastidic localizations of OR have been observed. Here, we show that Arabidopsis (Arabidopsis thaliana) OR physically interacts with the transcription factor TCP14 in the nucleus and represses its transactivation activity. Through this interaction, the nucleus-localized OR negatively regulates expression of EARLY LIGHT-INDUCIBLE PROTEINS (ELIPs), reduces chlorophyll biosynthesis, and delays development of thylakoid membranes in the plastids of germinating cotyledons. Nuclear abundance of OR decreased upon illumination. Together with an accumulation of TCP14 in the nucleus, this derepresses chloroplast biogenesis during de-etiolation. TCP14 is epistatic to OR and expression of ELIPs is directly regulated by the binding of TCP14 to Up1 elements in the ELIP promoter regions. Our results demonstrate that the interaction between OR and TCP14 in the nucleus leads to repression of chloroplast biogenesis in etiolated seedlings and provide new insights into the regulation of early chloroplast development.plantcell;31/12/2996/FX1F1fx1.

The DnaJ-like Zinc Finger Protein ORANGE Promotes Proline Biosynthesis in Drought-Stressed Arabidopsis Seedlings.

Orange (OR) is a DnaJ-like zinc finger protein with both nuclear and plastidial localizations. OR, and its orthologs, are highly conserved in flowering plants, sharing a characteristic C-terminal tandem 4× repeats of the CxxCxxxG signature. It was reported to trigger chromoplast biogenesis, promote carotenoid accumulation in plastids of non-pigmented tissues, and repress chlorophyll biosynthesis and chloroplast biogenesis in the nucleus of de-etiolating cotyledons cells. Its ectopic overexpression was found to enhance plant resistance to abiotic stresses. Here, we report that the expression of OR in Arabidopsis thaliana was upregulated by drought treatment, and seedlings of the OR-overexpressing (OE) lines showed improved growth performance and survival rate under drought stress. Compared with the wild-type (WT) and OR-silencing (or) lines, drought-stressed OE seedlings possessed lower contents of reactive oxygen species (such as H2O2 and O2-), higher activities of both superoxide dismutase and catalase, and a higher level of proline content. Our enzymatic assay revealed a relatively higher activity of Δ1-pyrroline-5-carboxylate synthase (P5CS), a rate-limiting enzyme for proline biosynthesis, in drought-stressed OE seedlings, compared with the WT and or lines. We further demonstrated that the P5CS activity could be enhanced by supplementing exogenous OR in our in vitro assays. Taken together, our results indicated a novel contribution of OR to drought tolerance, through its impact on proline biosynthesis.

The Nuclear Localization of the DnaJ-Like Zinc Finger Domain-Containing Protein EDA3 Affects Seed Development in Arabidopsis thaliana.

The DnaJ-like zinc finger domain-containing proteins are involved in different aspects of plastid function and development. Some of these proteins were recently reported to have dual subcellular localization in the nucleus and plastids. One member of this family, PSA2 (AT2G34860), was found to localize to the thylakoid lumen and regulate the assembly of photosystem I (PSI). However, PSA2 was also annotated as Embryo sac Development Arrest 3 (EDA3) from the observation that its embryo sac development was arrested at the two-nuclear stage. In this study, we characterized the eda3 mutant, and demonstrated that, as compared with the wild-type (WT) plants, the mutant has shorter siliques, fewer siliques per plant, and fewer seeds per silique. Both aborted and undeveloped ovules were observed in siliques of the mutant. By immunoblot analysis, we found that, different from the chloroplast localization in mature leaves, EDA3 localizes in the nucleus in seeds. A nuclear localization signal was identified from the deduced amino acid sequence of EDA3, and also proved to be sufficient for directing its fusion peptide into the nucleus.

At3g53630 encodes a GUN1-interacting protein under norflurazon treatment.

Chloroplasts are semi-autonomous organelles, with more than 95% of their proteins encoded by the nuclear genome. The chloroplast-to-nucleus retrograde signals are critical for the nucleus to coordinate its gene expression for optimizing or repairing chloroplast functions in response to changing environments. In chloroplasts, the pentatricopeptide-repeat protein GENOMES UNCOUPLED 1 (GUN1) is a master switch that senses aberrant physiological states, such as the photooxidative stress induced by norflurazon (NF) treatment, and represses the expression of photosynthesis-associated nuclear genes (PhANGs). However, it is largely unknown how the retrograde signal is transmitted beyond GUN1. In this study, a protein GUN1-INTERACTING PROTEIN 1 (GIP1), encoded by At3g53630, was identified to interact with GUN1 by different approaches. We demonstrated that GIP1 has both cytosol and chloroplast localizations, and its abundance in chloroplasts is enhanced by NF treatment with the presence of GUN1. Our results suggest that GIP1 and GUN1 may function antagonistically in the retrograde signaling pathway.