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Chih-Cheng Lin, Ya-Ting Chao, Wan-Chieh Chen, Hsiu-Yin Ho, Mei-Yi Chou, Ya-Ru Li, Yu-Lin Wu, Hung-An Yang, Hsiang Hsieh, Choun-Sea Lin, Fu-Hui Wu, Shu-Jen Chou, Hao-Chung Jen, Yung-Hsiang Huang, Deli Irene, Wen-Jin Wu, Jian-Li Wu, Daniel J. Gibbs, Meng-Chiao Ho*, and Ming-Che Shih* (2019) Regulatory cascade involving transcriptional and N-end rule pathways in rice under submergence. Proc. Natl. Acad. Sci. (2019) 116(8): 3300-3309. (https://doi.org/10.1073/pnas.1818507116)

The rice SUB1A-1 gene, which encodes a group VII ethylene response factor (ERFVII) plays a pivotal role in rice survival under flooding stress, as well as other abiotic stresses. In Arabidopsis, five ERFVII factors play roles in regulating hypoxic responses. A characteristic feature of Arabidopsis ERFVIIs is a destabilizing N-terminus, which functions as an N-degron that targets them for degradation via the oxygen-dependent N-end rule pathway of proteolysis, but permits their stabilization during hypoxia for hypoxia-responsive signaling. Despite having the canonical N-degron sequence, SUB1A-1 is not under N-end rule regulation, suggesting a distinct hypoxia signaling pathway in rice during submergence. Herein we show that two other rice ERFVIIs, ERF66 and ERF67, are directly transcriptionally up-regulated by SUB1A-1 under submergence. In contrast to SUB1A-1, ERF66 and ERF67 are substrates of the N-end rule pathway, which are stabilized under hypoxia and may be responsible for triggering a stronger transcriptional response to promote submergence survival. In support of this, overexpression ERF66 or ERF67 leads to activation of anaerobic survival genes and enhanced submergence tolerance. Furthermore, using structural and protein-interaction analyses, we show that the C-terminus of SUB1A-1 prevents its degradation via the N-end rule and directly interacts with the SUB1A-1 N-terminus, which may explain enhanced stability of SUB1A-1 despite bearing an N-degron sequence. In summary, our results suggest that SUB1A-1, ERF66, and ERF67 form a regulatory cascade involving transcriptional and N-end rule control, which allows rice to distinguish flooding from other SUB1A-1-regulated stresses.The proposed model of the regulatory cascade of SUB1A-1, ERF66, and ERF67 that involves transcriptional and N-end rule pathways in response to submergence stress in submergence-tolerant rice cultivars.

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