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

Abstract

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.

Further insights into the role of NIN-LIKE PROTEIN 7 (NLP7) in root cap cell release

“…3A), which is similar to the expression level of CEL5 in nlp7-1. 15 Although a defect in CEL5 leads to additional root cap layers, 15 examination of the cel3-1 mutant (Salk_057689) revealed no difference in the number of root cap layers between WT and cel3-1 (Fig. 3B), possibly due to redundancy in the CELLULASE gene family.…”

“…1 The release of BLC depends on the action of enzymes that modify components of the cell wall including cellulases and pectin methylesterases. [12][13][14][15] Mutants deficient in cell-wall-homogalacturonan (HG) shows defects in the release of BLCs, with single cells released from the root cap instead of an intact layer. 12,14,15 BLC release is also influenced by degradation of cellulose by cellulases, as a mutant defective in CELLULASE5 (CEL5) shows a sticky root cap.…”

“…12,14,15 BLC release is also influenced by degradation of cellulose by cellulases, as a mutant defective in CELLULASE5 (CEL5) shows a sticky root cap. 13,15 Figure 1. The columella root cap marker PET111 has similar expression in both Col-0 and nlp7-1.…”

“…We have recently shown that the transcription factor NIN-LIKE PROTEIN 7 (NLP7), previously characterized as a regulator of nitrate signaling, [16][17][18] modulates BLC release in Arabidopsis. 15 Mutations in NLP7 lead to the release of BLCs as single cells and to the upregulation of several cell wall modifying enzymes including CELLULASE5. 15 The nlp7 BLC release phenotype is dependent upon a functional CEL5, because the nlp7-1cel5 double mutant releases BLCs as an intact layer.…”

“…15 Mutations in NLP7 lead to the release of BLCs as single cells and to the upregulation of several cell wall modifying enzymes including CELLULASE5. 15 The nlp7 BLC release phenotype is dependent upon a functional CEL5, because the nlp7-1cel5 double mutant releases BLCs as an intact layer.…”