SINAT5 promotes ubiquitin-related degradation of NAC1 to attenuate auxin signals

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The plant hormone indole-3 acetic acid (IAA or auxin) controls many aspects of plant development, including the production of lateral roots1,2,3. Ubiquitin-mediated proteolysis has a central role in this process. The genes AXR1 and TIR1 aid the assembly of an active SCF (Skp1/Cullin/F-box) complex that probably promotes degradation of the AUX/IAA transcriptional repressors in response to auxin4,5,6,7,8. The transcription activator NAC1, a member of the NAM/CUC family of transcription factors, functions downstream of TIR1 to transduce the auxin signal for lateral root development9. Here we show that SINAT5, an Arabidopsis homologue of the RING-finger Drosophila protein SINA, has ubiquitin protein ligase activity and can ubiquitinate NAC1. This activity is abolished by mutations in the RING motif of SINAT5. Overexpressing SINAT5 produces fewer lateral roots, whereas overexpression of a dominant-negative Cys49 → Ser mutant of SINAT5 develops more lateral roots. These lateral root phenotypes correlate with the expression of NAC1 observed in vivo. Low expression of NAC1 in roots can be increased by treatment with a proteasome inhibitor, which indicates that SINAT5 targets NAC1 for ubiquitin-mediated proteolysis to downregulate auxin signals in plant cells.

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Figure 1: Molecular and cellular properties of SINAT5.
Figure 2: E3 activity of wild-type and mutant SINAT5.
Figure 3: Phenotypes of transgenic plants.
Figure 4: Relationship between amounts of SINAT5 and NAC1 proteins in transgenic plants.


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We thank E. Ng and W.-P. Tang for technical assistance; Y.-S. Chan for taking the GUS pictures; and P. Hare for reading the manuscript. This work was supported in part by a grant from the NIH to N.-H.C. This work was initiated in the Institute of Molecular Agrobiology, Singapore, supported by Singapore A-star funding.

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Correspondence to Nam-Hai Chua.

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Xie, Q., Guo, H., Dallman, G. et al. SINAT5 promotes ubiquitin-related degradation of NAC1 to attenuate auxin signals. Nature 419, 167–170 (2002) doi:10.1038/nature00998

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