Abstract
Root cap has several important physiological functions, such as protection of the root meristem and gravity sensing. In Arabidopsis thaliana, root cap cells are generated by coordinated cell division and differentiation of two types of stem cells with distinct cell lineage. After initial differentiation, root cap cells rapidly go through a maturation process and slough off within several days. We isolated a dominant Arabidopsis mutant, urp7-D, in which stem cells that give rise to the root cap were lost and root cap-like character was given in the epidermis. In urp7-D, a NAC domain transcription factor, SOMBRERO (SMB) was ectopically expressed throughout the root. Consistently, loss-of-function smb mutants showed enhanced cell division of the root cap stem cells, as well as delayed root cap differentiation. Expression of SMB initiates in the daughter cells produced by the division of the root cap stem cells, and then gradually increases as they mature. These results indicate that SMB is a key regulator of root cap differentiation and maturation.
