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PLANTS INTERACTING WITH OTHER ORGANISMS

A Novel ARID DNA-Binding Protein Interacts with SymRK and Is Expressed during Early Nodule Development in Lotus japonicus^{1,[C],[W],[OA]}

State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, China (H.Z., T.C., M.Z., Q.F., H.K., Z.Z.); and Department of Microbiology, Molecular Biology and Biochemistry, University of Idaho, Moscow, Idaho 83844–3052 (Z.H.)

During the establishment of symbiosis in legume roots, the rhizobial Nod factor signal is perceived by the host cells via receptor-like kinases, including SymRK. The NODULE INCEPTION (NIN) gene in Lotus japonicus is required for rhizobial entry into root cells and for nodule organogenesis. We describe here a novel DNA-binding protein from L. japonicus, referred to as SIP1, because it was identified as a SymRK-interacting protein. SIP1 contains a conserved AT-rich interaction domain (ARID) and represents a unique member of the ARID-containing proteins in plants. The C terminus of SIP1 was found to be responsible for its interaction with the kinase domain of SymRK and for homodimerization in the absence of DNA. SIP1 specifically binds to the promoter of LjNIN but not to that of LjCBP1 (a calcium-binding protein gene), both of which are known to be inducible by Nod factors. SIP1 recognizes two of the three AT-rich domains present in the NIN gene promoter. Deletion of one of the AT-rich domains at the NIN promoter diminishes the binding of SIP1 to the NIN promoter. The protein is localized to the nuclei when expressed as a red fluorescence fusion protein in the onion (Allium cepa) epidermal cells. The SIP1 gene is expressed constitutively in the uninfected roots, and its expression levels are elevated after infection by Mesorhizobium loti. It is proposed that SIP1 may be required for the expression of NIN and involved in the initial communications between the rhizobia and the host root cells.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Zhongming Zhang (zmzhang{at}mail.hzau.edu.cn).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.119164

^* Corresponding author; e-mail zmzhang{at}mail.hzau.edu.cn.

Received March 17, 2008; accepted June 13, 2008; published July 16, 2008.

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