Translocation of chloroplast NPR1 to the nucleus in retrograde signaling for adaptive response to salt stress in tobacco

Chloroplasts play a pivotal role in biotic and abiotic stress responses, accompanying changes in the cell reduction/oxidation (redox) state. Chloroplasts are an endosymbiotic organelle that sends retrograde signals to the nucleus to integrate with environmental changes. This study showed that salt stress causes the rapid accumulation of the nonexpressor of pathogenesis-related genes 1 (NPR1) protein, a redox-sensitive transcription coactivator that elicits many tolerance responses in chloroplasts and the nucleus. The transiently accumulated chloroplast NPR1 protein was translocated to the nucleus in a redox-dependent manner under salinity stress. In addition, immunoblotting and fluorescence image analysis showed that chloroplast-targeted NPR1-GFP fused with cTP (chloroplast transit peptide from RbcS) was localized in the nucleus during the responses to salt stress. Chloroplast functionality was essential for retrograde translocation, in which the stomules and cytoplasmic vesicles participated. Treatments with H2O2 and an ethylene precursor enhanced this retrograde translocation. Compared to each wild-type plant, retrograde signaling-related gene expression was severely impaired in the npr1-1 mutant in Arabidopsis, but enhanced transiently in the NPR1-Ox transgenic tobacco line. Therefore, NPR1 might be a retrograde signaling hub that improves a plants adaptability to changing environments.