Role of the Transcription Factor Nin-like Protein 7 (NLP7) in Root Development and Stress Responses.

Abiotic stresses such as nutrient stress and drought have a substantial impact on agricultural production, as they reduce yield in a wide range of crops. Because roots are the ‘first responders’ to nutrient and drought stress, identifying genes that link stress responses to root growth and development is an important part of crop improvement. NIN-LIKE PROTEIN 7 (NLP7) is a member of the NLP transcription factor family and is a key regulator of nitrate responses. NLP7 is expressed within the root cap, root elongation and root differentiation zones in Arabidopsis. We have recently shown that NLP7 regulates root cap development by modifying expression of cell wall loosening enzymes that regulate release of the last layer of cells from the root cap. Here we show that NLP7 regulates root growth by modulating reactive oxygen species in the root meristem. Root meristem size is controlled both through plant hormones and reactive oxygen species. Mutations in NLP7 lead to slower root growth under standard conditions, and a smaller root meristem compared to wild type. Both phenotypes appear to be independent of cytokinin and auxin, the major hormones involved in root meristem growth. The nlp7 mutant has increased levels of H₂O₂ in the root, and NLP7 expression is activated by H₂O₂. Microarray analysis revealed that several genes with roles in the ROS pathway are differentially expressed in the mutant. Mutations in the transcription factor UPBEAT1 (UPB1) lead to longer root growth through changes in ROS. Double mutant analysis with upbeat1 and nlp7 showed that UBP1 is epistatic to NLP7. Our work ties a key regulator of plant nitrate signaling directly to changes in root growth through the ROS signaling pathway.