The root cap is a small tissue located at the tip of the root with critical functions for root growth. Present in nearly all vascular plants, the root cap protects the root meristem, influences soil penetration, and perceives and transmits environmental signals that are critical for root branching patterns. To perform these functions, the root cap must remain a relatively stable size and must integrate endogenous developmental pathways with environmental signals, yet how it does so is not clear. We previously showed that low pH conditions alter root cap development, and these changes are mediated by the NIN LIKE PROTEIN 7 (NLP7) transcription factor, a master regulator of nitrate signaling. Here we show that NLP7 integrates nitrate signaling with auxin pathways to regulate root cap development. We find that low nitrate conditions promote aberrant release of root cap cells in Arabidopsis. Nitrate deficiency impacts auxin pathways in the last layer of the root cap, and this is mediated in part by NLP7. Mutations in NLP7 abolish the auxin minimum in the last layer of the root cap and alter root cap expression of the auxin carriers PIN-LIKES 3 (PILS3) and PIN-FORMED 7 (PIN7) as well as transcription factors that regulate PIN expression. Together, our data reveal NLP7 as a link between endogenous auxin pathways and nitrate signaling in the root cap.
The root cap surrounds the root tip and promotes root growth by protecting the root apical meristem, influencing root branching, and sensing environmental signals like nitrate. The root cap maintains a constant size through the coordination of cell production in the root meristem with cell release at the tip of the root, a process that requires an auxin minima in the last layer of the root cap. To perform its functions, the root cap must maintain a constant size and synchronize external cues with development, but mechanisms underlying such coordination are not well understood. Mutations in the NIN LIKE PROTEIN 7 (NLP7) transcription factor, a master regulator of nitrate signaling, lead to defects in root cap cell release and cell production. Nitrate impacts root development through crosstalk with auxin. Therefore, we hypothesized that NLP7 regulates root cap cell release and cell production by modulating auxin pathways. Here we show that mutations in NLP7 abolish the auxin minima required for root cap cell release and alter root cap expression levels of the auxin carriers PIN-LIKES 3 (PILS3) and PIN-FORMED 7 (PIN7). We find that NLP7 is required for proper root cap cell production and differentiation and for expression of transcription factors that regulate these processes. Nitrate deficiency impacts auxin pathways in the last layer of the root cap, and this is mediated in part by NLP7. Together, our data suggest that NLP7 integrates nitrate signaling with auxin pathways to optimize root cap development in response to external nitrate cues.
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