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.
Highlights
Mitogen-activated protein kinases (MAPKs) are key regulators of immune responses in animals and plants
Mpk3 and mpk4 display major and partially overlapping transcriptional changes under standard growth conditions In control conditions, we observed 1,235 genes differentially expressed in mpk4, 496 genes in mpk3 and only 61 genes in mpk6 in comparison to Col-0 (Table 1, Additional file 1: Table S1 and Additional file 2: Table S2)
As MAPKs are known to regulate MAMPinduced transcriptional responses, we wondered if the absence of one kinase would trigger basal changes in the transcriptome that resemble those triggered by flg22 treatment
Summary
Mitogen-activated protein kinases (MAPKs) are key regulators of immune responses in animals and plants. The first layer of induced defences relies on pattern recognition receptors (PRRs) that detect conserved microbe-associated molecular patterns (MAMPs) and initiate a defence program called pattern-triggered immunity (PTI). ETI is an amplified PTI response that results in disease resistance and is often associated with the accumulation of the hormone salicylic acid (SA) and a localised programmed cell death referred to as hypersensitive response (HR). While this response is efficient against biotrophic pathogens, necrotrophic pathogens that kill host cells are fought through activation of defences mediated by the hormones jasmonic acid (JA) and ethylene (ET) [6]
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