Control of root cap maturation and cell detachment by BEARSKIN transcription factors in Arabidopsis

The root cap supports root growth by protecting the root meristem, sensing gravity and interacting with the rhizosphere through metabolite secretion and cell dispersal. Sustained root cap functions therefore rely on balanced proliferation of proximal stem cells and regulated detachment of distal mature cells. Although the gene regulatory network that governs stem cell activity in the root cap has been extensively studied in Arabidopsis, the mechanisms by which root cap cells mature and detach from the root tip are poorly understood. We performed a detailed expression analysis of three regulators of root cap differentiation, SOMBRERO, BEARSKIN1 and BEARSKIN2, and identified their downstream genes. Our results indicate that expression of BEARSKIN1 and BEARSKIN2 is associated with cell positioning on the root surface. We identified a glycosyl hydrolase 28 (GH28) family polygalacturonase (PG) gene as a direct target of BEARSKIN1. Overexpression and loss-of-function analyses demonstrated that the protein encoded by this PG gene facilitates cell detachment. We thus revealed a molecular link between the key regulators of root cap differentiation and the cellular events underlying root cap-specific functions. Summary: Arabidopsis NAC transcription factors BRN1 and BRN2 are specifically expressed in the outer root cap layer and activate genes associated with its cellular functions.

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