Functional analysis of Arabidopsis immune-related MAPKs uncovers a role for MPK3 as negative regulator of inducible defences

Abstract

BackgroundMitogen-activated protein kinases (MAPKs) are key regulators of immune responses in animals and plants. In Arabidopsis, perception of microbe-associated molecular patterns (MAMPs) activates the MAPKs MPK3, MPK4 and MPK6. Increasing information depicts the molecular events activated by MAMPs in plants, but the specific and cooperative contributions of the MAPKs in these signalling events are largely unclear.ResultsIn this work, we analyse the behaviour of MPK3, MPK4 and MPK6 mutants in early and late immune responses triggered by the MAMP flg22 from bacterial flagellin. A genome-wide transcriptome analysis reveals that 36% of the flg22-upregulated genes and 68% of the flg22-downregulated genes are affected in at least one MAPK mutant. So far MPK4 was considered as a negative regulator of immunity, whereas MPK3 and MPK6 were believed to play partially redundant positive functions in defence. Our work reveals that MPK4 is required for the regulation of approximately 50% of flg22-induced genes and we identify a negative role for MPK3 in regulating defence gene expression, flg22-induced salicylic acid accumulation and disease resistance to Pseudomonas syringae. Among the MAPK-dependent genes, 27% of flg22-upregulated genes and 76% of flg22-downregulated genes require two or three MAPKs for their regulation. The flg22-induced MAPK activities are differentially regulated in MPK3 and MPK6 mutants, both in amplitude and duration, revealing a highly interdependent network.ConclusionsThese data reveal a new set of distinct functions for MPK3, MPK4 and MPK6 and indicate that the plant immune signalling network is choreographed through the interplay of these three interwoven MAPK pathways.

Changes in PUB22 Ubiquitination Modes Triggered by MITOGEN-ACTIVATED PROTEIN KINASE3 Dampen the Immune Response

“…At the same time, MPK3 displays an overlap in transcriptome, substrates, and functions with MPK4 (Popescu et al, 2009;Frei dit Frey et al, 2014). Unlike mpk4 mutants, which display a constitutively active immunity phenotype, the negative regulatory function of mpk3 is restricted to pathways activated during the immune response, in analogy to the function of PUB22 and its homologs (Trujillo et al, 2008;Frei dit Frey et al, 2014). In line with this, mpk3 phenocopies the pub22 pub23 pub24 triple mutant, both of which display increased responses (Trujillo et al, 2008;Frei dit Frey et al, 2014).…”

“…Unlike mpk4 mutants, which display a constitutively active immunity phenotype, the negative regulatory function of mpk3 is restricted to pathways activated during the immune response, in analogy to the function of PUB22 and its homologs (Trujillo et al, 2008;Frei dit Frey et al, 2014). In line with this, mpk3 phenocopies the pub22 pub23 pub24 triple mutant, both of which display increased responses (Trujillo et al, 2008;Frei dit Frey et al, 2014). However, MPK4 function during immunity needs to be reassessed, as phenotypes in mpk4 may originate from the activation of the NODlike receptor SUMM2 (Zhang et al, 2012).…”

“…Both MPK3 and MPK6 are commonly described as partially redundant positive regulators, while MPK4 is appreciated as a negative regulator. However, recent studies have helped to draw a more precise picture, indicating that MPK3 and MPK6 are far more functionally distinct than initially assumed (Frei dit Frey et al, 2014;Lassowskat et al, 2014). At the same time, MPK3 displays an overlap in transcriptome, substrates, and functions with MPK4 (Popescu et al, 2009;Frei dit Frey et al, 2014).…”

“…However, recent studies have helped to draw a more precise picture, indicating that MPK3 and MPK6 are far more functionally distinct than initially assumed (Frei dit Frey et al, 2014;Lassowskat et al, 2014). At the same time, MPK3 displays an overlap in transcriptome, substrates, and functions with MPK4 (Popescu et al, 2009;Frei dit Frey et al, 2014). Unlike mpk4 mutants, which display a constitutively active immunity phenotype, the negative regulatory function of mpk3 is restricted to pathways activated during the immune response, in analogy to the function of PUB22 and its homologs (Trujillo et al, 2008;Frei dit Frey et al, 2014).…”

“…While MPK3 and MPK6 are believed to function as partially redundant positive regulators of immunity, MPK4 has been attributed a negative regulatory role (Rodriguez et al, 2010). However, recent studies have helped to draw a more precise picture, revealing more complex interactions (Popescu et al, 2009;Zhang et al, 2012;Frei dit Frey et al, 2014;Lassowskat et al, 2014).…”