Abstract
Salt stress causes rapid accumulation of nonexpressor of pathogenesis-related genes 1 (NPR1) protein, known as the redox-sensitive transcription coactivator, which in turn elicits many adaptive responses. The NPR1 protein transiently accumulates in chloroplast stroma under salt stress, which attenuates stress-triggered down-regulation of photosynthetic capability. We observed that oligomeric NPR1 in chloroplasts and cytoplasm had chaperone activity, whereas monomeric NPR1 in the nucleus did not. Additionally, NPR1 overexpression resulted in reinforcement of morning-phased and evening-phased circadian clock. NPR1 overexpression also enhanced antioxidant activity and reduced stress-induced reactive oxygen species (ROS) generation at early stage, followed with transcription levels for ROS detoxification. These results suggest a functional switch from a molecular chaperone to a transcriptional coactivator, which is dependent on subcellular localization. Our findings imply that dual localization of NPR1 is related to proteostasis and redox homeostasis in chloroplasts for emergency restoration as well as transcriptional coactivator in the nucleus for adaptation to stress.
Highlights
Salt stress causes rapid accumulation of nonexpressor of pathogenesis-related genes 1 (NPR1) protein, known as the redox-sensitive transcription coactivator, which in turn elicits many adaptive responses
It is well known that NPR1 protein is a key transcriptional coactivator in broad-spectrum immunity of plants against phytopathogens[21], NPR1 has not been fully characterized in relation to abiotic stresses
We observed that the transcription levels of NPR1 rapidly increased from 1 h to 6 h, after which they gradually returned to basal levels in leaves of Nicotiana tabacum after salt stress (Fig. 1a)
Summary
Salt stress causes rapid accumulation of nonexpressor of pathogenesis-related genes 1 (NPR1) protein, known as the redox-sensitive transcription coactivator, which in turn elicits many adaptive responses. NPR1 overexpression enhanced antioxidant activity and reduced stress-induced reactive oxygen species (ROS) generation at early stage, followed with transcription levels for ROS detoxification. These results suggest a functional switch from a molecular chaperone to a transcriptional coactivator, which is dependent on subcellular localization. Because the rapid generation of ROS represents a common plant response to almost all environmental challenges[4,9], it is suggested that ROS and the redox system in chloroplasts represent primary sources within the plant signaling battery. Chaperones are considered as powerful buffers for multiple stress resistance[14]
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