Vital Functions Of YL1in Salt Stress-Triggered Plastid-to-Nucleus Retgograde Signaling In Arabidopsis

High salinity elicits destruction of photosynthetic components and thus plant growth and development by inducing ion toxicity or osmotic stress. Previous studies have concluded that salt stress could severely inhibit the repair of PS II through ROS-induced suppression of the synthesis of PS II proteins and create imbalance between the photodamage and repair rates of PS II. CO2fixation has been demonstrated to be sensitive to salt stress, which in turn leads to deregulation of PS II repair. However, chloroplasts can regulate nuclear photosynthetic genes to adapt to a changing environment through retrograde signaling. In the present paper, we reveal that salt stress triggers plant plastid-to-nucleus retrograde response through plastid gene expression (PGE)-dependent signaling in Arabidopsis.(1) A yellow leaf mutant, yl1-1, was isolated from approximately30,000ethane methylsulfonate (EMS)-mutagenized Col-0M2seedlings; this mutant confers an evidently pale-green cotyledon phenotype and extremely low plastid protein (Dl and RbcL) levels under normal growth conditions. The yl1-1mutant showed significantly lower transcription levels of Lhcb and RbcS compared with the wild type plant under normal growth conditions. We identified a G to A base substitution, which causes a D to N change in position574of the YL1amino acid sequence through map-based cloning approach.(2) Expression pattern analysis used in β-glucuronidase (GUS) staining showed that the YL1gene is expressed in most green tissues throughout the plant growth cycle, even in germinating seeds. However, this process is hardly detected in roots, green or mature seeds, or petals. YL1gene could be significantly down-regulated by NaCl and this feature is independent of plastid-to-nucleus retrograde signaling mediated by PGE-dependent signals. YL1is mainly localized in chloroplast through YL1-GFP fusion protein detection in protoplast. Immunoblot analysis of the stroma and membrane fractions from Percoll-purified chloroplasts demonstrated that YL1protein is localized in the stroma, and not in the membrane.(3) Our Results showed that when wild type seedlings are treated with salt stress, levels of chloroplast proteins (D1and RbcL) and NEPG transcripts significantly decrease compared with those in control plants. However, no significant reduction in plastid protein and NEPG transcript levels was observed in OE plants (YL1overexpression line) after salt stress treatment, which suggests that slight PGE defect signals or plastid-to-nucleus retrograde signaling is generated. Additionally, no change in D1, RbcL, or NEPG expression in the yll-1mutant was detected under salt stress treatment compared with the control. Taken together, the results suggest that YL1participates in salt stress-triggered plastid-to-nucleus retrograde.(4) Under salt stress, yll mutant seedlings showed extremely stunted cotyledon phenotype and low chlorophyll contents compared with wild type plants. However, when seeds are germinated in continuous dark conditions, no difference in hypocotyl and root lengths between yll mutant and wild type seedlings may be observed regardless of treatment with or without salt. These results indicate that the yll phenotype is light-dependent and that salt stress mainly affects the leaf development of yll mutants. Our further observations show that Na+is overaccumulated in yll cotyledons under salt stress which is probably caused by depression of HKT1elicited by ABI4overexpression.(5) We created two double mutants, namely, yl1-2gunl and yl1-2abi4. D1and RbcL protein levels were significantly lower in double mutants, which were similar to yll single mutant and suggested defects in plastid gene expression. However, NEPG expression levels in the two double mutants were remarkably higher than that in the yl1-2mutant, which were almost similar to the wild type, suggesting ABI4or GUN1function loss could rescue the yll single mutant retrograde phenotype. Taken together, ABI4and GUN1may participate in YL1function loss-triggered plastid-to-nucleus retrograde through PGE-dependent signaling. Furthermore, the double mutants yl1-2gunl and yl1-2abi4exhibited significantly higher percentages of fully expanded cotyledons and less accumulation of cotyledons Na+after salt treatment than the yll single mutant. Higher HKT1mRNA levels were also observed in the double mutants compared with the yll single mutant. These results indicate that yll mutant sensitivity to salt stress could be rescued by disturbed PGE-dependent signaling through GUN1or ABI4function loss, which reduces HKT1depression.In a word, we conclude that YL1exerts an essential function in salt stress triggered-retrograde signaling, which is mediated by PGE-dependent signaling.