Nitrogen (N) plays important roles as both a macronutrient and signal in plant growth and development. However, our understanding of N signaling and/or response mechanisms in plants is still limited. Here, we show that the mitogen-activated protein kinase kinase 9 (MKK9) is involved in plant N responses in Arabidopsis by regulating production of anthocyanins and the ability of N acquisition under low N conditions. Transgenic plants that express a constitutively active version of MKK9 (MKK9DD) showed decreased accumulation of anthocynanins and reduced expression of key anthocyanin biosynthetic genes under low N condition compared to the plants expressing the inactive form of MKK9 (MKK9KR). The decreased anthocyanin accumulation could be due to the increased N level in the MICK9DD plants as these plants were shown to accumulate more N and have higher expression of N acquisition-related genes under low N condition as compared with the MKK9KR plants. Taken together, our results suggest that MKK9 plays a role in plant adaptation to low N stress by modulating both anthocyanin accumulation and N status. (C) 2017 Elsevier Inc. All rights reserved.