Editors' ChoicePlant Immunity

Activating the MAPK pathway with a kinase instead of a GTPase

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Science Signaling  22 Nov 2016:
Vol. 9, Issue 455, pp. ec277
DOI: 10.1126/scisignal.aal4278

Upon binding to microbe-derived molecules, pattern recognition receptors (PRRs) initiate immune responses, which in plants include the mitogen-activated protein kinase (MAPK) pathway. LYK5 and CERK1 are Arabidopsis thaliana PRRs that form a complex and induce MAPK signaling in response to binding to the fungal cell wall component chitin. Yamada et al. found that the CERK1-associated cytoplasmic kinase PBL27 enabled the LYK5-CERK1 complex to directly activate MAPK signaling. PBL27 interacted with the C-terminal domain of the MAP kinase kinase kinase MAPKKK5 in a yeast two-hybrid assay. Compared with wild-type Arabidopsis and similar to pbl27 mutant plants, mapkkk5 mutants exhibited reduced phosphorylation (activation) of MAPK in response to chitin treatment and increased susceptibility to infection by a fungal pathogen. The expression of immune response markers following chitin treatment was also altered in mapkkk5 mutants compared with wild-type control plants. Tagged forms of PBL27 and MAPKKK5 coimmunoprecipitated from membrane fractions when expressed in tobacco leaves or in cultured Arabidopsis protoplasts (dissociated cells devoid of cell walls). PBL27 phosphorylated MAPKKK5 in vitro, and the addition of CERK1 to the in vitro kinase assay increased PBL27-mediated phosphorylation of MAPKKK5. PBL27 phosphorylated several residues in the C-terminal domain, not the kinase domain, of MAPKKK5, and a form of MAPKKK5 in which these residues were mutated did not restore chitin-induced MAPK activation in mapkkk5 mutants. These results shed light on how a plant pathogen recognition system elicits MAPK signaling and suggest that MAPK signaling in plants can be initiated independently of receptor-mediated recruitment and activation of guanosine triphosphatases.

K. Yamada, K. Yamaguchi, T. Shirakawa, H. Nakagami, A. Mine, K. Ishikawa, M. Fujiwara, M. Narusaka, Y. Narusaka, K. Ichimura, Y. Kobayashi, H. Matsui, Y. Nomura, M. Nomoto, Y. Tada, Y. Fukao, T. Fukamizo, K. Tsuda, K. Shirasu, N. Shibuya, T. Kawasaki, The Arabidopsis CERK1‐associated kinase PBL27 connects chitin perception to MAPK activation. EMBO J. 35, 2468–2483 (2016). [PubMed]

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