The mitogen-activated protein (MAP) kinase cascades are important signaling components that mediate various biological pathwaysin all eukaryotic cells. In our recent publication,1 we identified AtMPK4 as one of the downstream targets of AtMKK6 that is required for executing male-specific meiotic cytokinesis. Here we provide evidence that another target, AtMPK13, is developmentally co-expressed with AtMKK6 in Arabidopsis, and both AtMPK13 and AtMKK6 display high Promoter::GUS activity in the primary root tips and at the lateral root primordia. Partial suppression of either AtMKK6 or AtMPK13 expression significantly reduces the number of lateral roots in the transgenic lines, suggesting that the AtMKK6-AtMPK13 module positively regulates lateral root formation.