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Suppression of innate immunity mediated by the CDPK‐Rboh complex is required for rhizobial colonization in Medicago truncatula nodules
Author(s) -
Yu Haixiang,
Xiao Aifang,
Dong Ru,
Fan Yuqian,
Zhang Xianpeng,
Liu Chao,
Wang Chao,
Zhu Hui,
Duanmu Deqiang,
Cao Yangrong,
Zhang Zhongming
Publication year - 2018
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/nph.15410
Subject(s) - medicago truncatula , innate immune system , biology , immunity , microbiology and biotechnology , mutant , medicago , rhizobia , symbiosis , immune system , immunology , genetics , gene , bacteria
Summary Suppression of innate immunity is essential for rhizobial infection and colonization in compatible interactions with leguminous plants. In Medicago nad1 mutant plants, innate immunity is excessively activated, resulting in necrotic cell death after rhizobia are released from infection threads into symbiotic cells, suggesting that innate immunity plays a critical role in regulating bacteroid persistence. In this study, we identified three respiratory burst oxidase homologs (Rboh) and one calcium‐dependent protein kinase ( CDPK ) as key factors for the activation of immunity in Medicago nodules using genetic and biochemical methods. Knock‐out of either MtRbohB or MtRbohD in nad1‐1 mutant plants produced effective nodules with intact symbiotic cells, while knock‐out of MtRbohC decreased brown pigment deposition, leading to less necrosis in nad1‐1 mutant nodules. Mt CDPK 5 directly phosphorylated MtRbohB, MtRbohC and MtRbohD, which triggered immune responses in plants. Knock‐out of Mt CDPK 5 in nad1‐1 mutant plants partially restored nitrogen‐fixing nodules. Overexpression of the constitutively activated variant Mt CDPK 5 VK under the control of the NAD 1 promoter elicited strong immune responses, resulting in ineffective nodules in wild‐type plants. Our data provide direct evidence that host plants utilize innate immunity to regulate rhizobial colonization in symbiotic cells in Medicago truncatula .