Abstract
The activation of mitogen-activated protein kinase (MAPK) cascades is an important mechanism for stress adaptation through the control of gene expression in mammals, yeast, and plants. MAPK activation has emerged as a common mechanism by which plants trigger pathogen defense responses following innate immune recognition of potential microbial pathogens. We are studying the non-host plant defense response of parsley to attempted infection by Phytophthora species using an experimental system of cultured parsley cells and the Phytophthora-derived Pep-13 peptide elicitor. Following receptor-mediated recognition of this peptide, parsley cells trigger a multifaceted innate immune response, involving the activation of three MAPKs that have been shown to function in the oxidative burst-independent activation of defense gene expression. Using this same experimental model we now report the identification of a MAPK kinase (MAPKK) that functions upstream in this pathway. This kinase, referred to as PcMKK5 based on sequence similarity to Arabidopsis thaliana AtMKK5, is activated in parsley cells following Pep-13 treatment and functions as an in vivo activator of all three MAPKs previously shown to be involved in this response. Gain- and loss-of-function mutant versions of PcMKK5, when used in protoplast co-transfection assays, demonstrated that kinase activity of PcMKK5 is required for PR gene promoter activation following Pep-13 treatment. Furthermore, using specific antibodies and immunofluorescent labeling, we demonstrate that activation of MAPKs in parsley cells correlates with an increase in their nuclear localization, which is not detectable for activated PcMKK5. These results suggest that activation of gene expression through MAPK cascades during innate immune responses in plants involves dynamic changes in the localization of the proteins involved, which may reflect the distribution of key protein substrates for the activated MAPKs.
Highlights
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AY533301 and AY533302
We have previously reported the rapid activation of 3 mitogen-activated protein kinase (MAPK) in Pep-13-treated parsley cells and have shown these activities to be necessary for triggering “early” defense gene activation, via a signaling pathway that is independent of the oxidative burst response [16]
Roles for MAPK activation in triggering defense gene expression have been demonstrated in A. thaliana following FLS2-mediated perception of bacterial flagellin [27] and in tobacco in response to Phytophthora-derived elicitins [37]
Summary
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AY533301 and AY533302. MAPKK activities that function upstream of MAPK activation have been described for osmotic stress signaling in alfalfa [22], during tobacco N-gene mediated race/cultivar-specific recognition of tobacco mosaic virus [23], following Phytophthora-derived elicitin perception in tobacco [24], as a consequence of a range of abiotic stresses in Arabidopsis [25, 26] and following perception of the flagellin-derived elicitor, Flg-22, by Arabidopsis [27] In the latter case, while identifying a complete MAPK signaling cascade triggered following perception of bacterial flagellin, a direct requirement for the activity of the MAPKK in triggering downstream defense responses was demonstrated [27]. Whether or not specific members of these sub-groups share similar functions remains to be established
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