The PAMP-Flg22-MPK6-ERF104-ERF104 Signaling Pathway in Plants

The conserved peptide flagellin is recognized in plants by plasma membrane receptors and induces local as well as systemic immune responses. However, the fate of pathogen-derived ligands that enter plant cells via endocytosis has remained unclear, including their transport through the vasculature and to distant organs. We used biologically active fluorescence and radiolabeled flg22 to show that flg22 is trafficked to the closest vascular region after entering the cell and that this requires FLS2. Moreover, long-distance transport of flg22 is facilitated by a signaling pathway that involves ERF104 and MPK6.

Our data support the model that flg22 binds directly to MPK6 to promote its phosphorylation and activation, and that the MPK6-ERF104 complex is required for ET production in response to a bacterial pathogen. However, the requirement for ACC for ERF104 release also suggests that a second signaling cascade is involved in addition to the direct binding of MPK6 to ERF104. This signaling cascade is likely mediated by a reversible phosphorylation of the ERF104 protein, as phospho-ERF104 mutants fail to promote the ET response to flg22 or the basal resistance to P. syringae pv. phaseolicola (Psp).

We conclude that the PAMP-flg22-MPK6-ERF104 signaling pathway is required for basal disease resistance to Psp and other bacterial pathogens. Interestingly, the erf104 mutant and an ERF104 RNAi line exhibit enhanced root growth inhibition in response to Psp but not B. cinerea, a non-adapted B. cinerea pathogen. We propose that the difference between these two responses is a result of changes in ERF104 function and/or in the balance between ERF104-dependent and -independent pathways of PAMP signaling to vascular tissues.


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