The trihelix transcription factor MdSIP1-2 interacts with MdNIR1 promoter to regulate nitrate utilization in apple

Nitrate is not only the main nitrogen (N) source for plant growth and development, but also an important signaling molecule for plants to cope with the changing environment. To adapt to N fluctuations in the environment, plants have established sophisticated regulatory networks. Although some nitrate signaling regulatory genes have been determined, whether the trihelix transcription factors (TFs) regulate N metabolism remains unclear. Here, an apple trihelix TF-encoding gene 6B -INTERACTING PROTEIN1-2 (SIP1-2) was identified, which is inducible upon nitrate treatment. Under N deficiency condition, the MdSIP1-2-OE transgenic apple calli and Arabidopsis repressed the expression of genes involved in N transport and metabolism, and repressed the activity of both nitrate reductase and nitrite reductase, leading to decreased N content and biomass. These results indicated that MdSIP1-2 inhibited the utilization of nitrate, and MdSIP1-2 may play a negative regulatory role in nitrate signal transduction in plant. In vivo and in vitro experiments showed that MdSIP1-2 directly bound to the GT1 motif of the MdNIR1 promoter to repress its transcription. Taken together, these results revealed that the trihelix TF family gene MdSIP1-2 plays a crucial role in regulating nitrate transport and metabolism, and providing new insights for plants to adapt to N fluctuation in environment.