Plants signal herbivore attacks using positive and negative regulators

After recognizing a biotic stress, plants activate signaling pathways to fight against the attack. Typically, these signaling pathways involve the activation of phosphorylation cascades mediated by Mitogen-Activated Protein Kinases (MAPKs). In the plant-herbivore model Arabidopsis thaliana-Tetranychus urticae, several Arabidopsis MAP kinases are induced by the mite attack. In this study, we demonstrate the participation of the MEKK-like kinases MAPKKK17 and MAPKKK21. Leaf damage caused by the mite was assessed using T-DNA insertion lines. Differential levels of damage were found when the expression of MAPKKK17 was increased or reduced, respectively. In contrast, reduced expression of MAPKKK21 resulted in less damage caused by the mite. In addition, MAPKKK17 and MAPKKK21 are coexpressed with different sets of genes and encode proteins with low similarity in the C-terminal region. Overall, our results demonstrate that MAPKKK17 and MAPKKK21 have opposite roles. MAPKKK17 and MAPKKK21 act as positive and negative regulators, respectively, on the plant response. The induction of MAPKKK17 and MAPKKK21 after mite infestation would be integrated in the bulk of signaling pathways activated to balance the response of the plant to a biotic stress.