An MAP kinase interacts with LHK1 and regulates nodule organogenesis in Lotus japonicus
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Symbiosis receptor-like kinase (SymRK) is a key protein mediating the legume-Rhizobium symbiosis. Our previous work has identified an MAP kinase kinase, SIP2, as a SymRK-interacting protein to positively regulate nodule organogenesis in Lotus japonicus, suggesting that an MAPK cascade might be involved in Rhizobium-legume symbiosis. In this study, LjMPK6 was identified as a phosphorylation target of SIP2. Stable transgenic L. japonicus with RNAi silencing of LjMPK6 decreased the numbers of nodule primordia (NP) and nodule, while plants overexpressing LjMPK6 increased the numbers of nodule, infection threads (ITs), and NP, indicating that LjMPK6 plays a positive role in nodulation. LjMPK6 could interact with a cytokinin receptor, LHK1 both in vivo and in vitro. LjMPK6 was shown to compete with LHP1 to bind to the receiver domain (RD) of LHK1and to downregulate the expression of two LjACS (1-aminocyclopropane-1-carboxylic acid synthase) genes and ethylene levels during nodulation. This study demonstrated an important role of LjMPK6 in regulation of nodule organogenesis and ethylene production in L. japonicus.
Keywordscytokinin ethylene biosynthesis LHK1 Lotus japonicus MAPK cascade root nodule symbiosis
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We thank Dr. K. Szczyglowski (Agriculture and Agri- Food Canada, University of Western Ontario, Canada) for kindly providing LHK1 mutants, Dr. A. Downie (John Innes Centre) for providing M. loti strain R7A carrying pMP2112, Dr. G. Wu (Shanghai Jiao Tong University, China) for providing M. loti MAFF303099, Dr. S. Wang (Huazhong Agricultural University, China) for providing pCAMBIA1301U, L. japonicus LORE1 mutant collection (Centre for Carbohydrate Recongnition and Signaling, Aarhus University, Denmark) for providing LjMPK6 mutants. This work was supported by the National Key R&D Program of China (2016YF0100700), the National Natural Science Foundation of China (31670240 and 31870219), the State Key Laboratory of Agricultural Microbiology (AMLKF201503 and AMLKF201608), the Graduate Education Innovation Fund of Huazhong Agricultural University (to Z.Z.), and Graduate Student Research Innovation Project of Huazhong Agricultural University (to J.Y.).
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