The Transcription Factor NIN-LIKE PROTEIN7 Controls Border-Like Cell Release

The root cap covers the tip of the root and functions to protect the root from environmental stress. Cells in the last layer of the root cap are known as border cells, or border-like cells (BLCs) in Arabidopsis (Arabidopsis thaliana). These cells separate from the rest of the root cap and are released from its edge as a layer of living cells. BLC release is developmentally regulated, but the mechanism is largely unknown. Here, we show that the transcription factor NIN-LIKE PROTEIN7 (NLP7) is required for the proper release of BLCs in Arabidopsis. Mutations in NLP7 lead to BLCs that are released as single cells instead of an entire layer. NLP7 is highly expressed in BLCs and is activated by exposure to low pH, a condition that causes BLCs to be released as single cells. Mutations in NLP7 lead to decreased levels of cellulose and pectin. Cell wall-loosening enzymes such as CELLULASE5 (CEL5) and a pectin lyase-like gene, as well as the root cap regulators SOMBRERO and BEARSKIN1/2, are activated in nlp7-1 seedlings. Double mutant analysis revealed that the nlp7-1 phenotype depends on the expression level of CEL5. Mutations in NLP7 lead to an increase in susceptibility to a root-infecting fungal pathogen. Together, these data suggest that NLP7 controls the release of BLCs by acting through the cell wall-loosening enzyme CEL5.