Mitogen-Activated Protein Kinase signalling in plant immunity
Alpha 1-4-linked oligogalacturonides (OGs) derived from plant cell wall pectin function as damage-associated molecular patterns (DAMPs) and activate the plant immune response. OGs also regulate developmental responses, likely due to their ability to antagonize auxin. So far, little is known about the intracellular elements involved in the early events triggered by OGs. In both animal and plants, Mitogen-Activated Protein Kinase (MAPK) phosphorylation cascades represent a highly conserved signal transduction mechanism. A MAPK cascade consists of a core module of three kinases that act in sequence: a MAP kinase kinase kinase (MAPKKK) that activates, via phosphorylation, a MAP kinase kinase (MAPKK), which in turn activates a MAP kinase (MAPK). The objectives of our work is to elucidate the role of the Arabidopsis NPK1-RELATED PROTEIN KINASEs (ANPs), a MAPKKK gene family composed by ANP1, ANP2 and ANP3, in the signal transduction cascade activated by OGs and in immunity. Here we show that ANPs are involved in signalling cascade activated by OGs and by the microbe associated molecular pattern (MAMP) elf18, a peptide derived from the bacterial EF-Tu. Single and double anp KO mutants as well as a triple mutant, obtained by conditionally expressing, in the anp2 anp3 double mutant background, an artificial microRNA having as target the third member of the gene family, exhibit defective responses to both elicitors. Moreover, confocal microscopy analyses performed in elicited plants overexpressing GFP-tagged ANPs show that these kinases are targeted to plant organelles mainly associated with ROS generation, and to the nucleus. These results reveal that ANPs are elements of the DAMP and MAMP signaling pathways.