The Arabidopsis NAC transcription factor SOMBRERO (SMB/ANAC033) is a key regulator of root cap differentiation. Expressed directly after the formative division of root cap stem cells, it regulates exit from the stem cell fate and promotes root cap maturation. SMB also controls the timely execution of programmed cell death (PCD) and post-mortem corpse clearance of root cap cells. I hypothesise that the precise timing of SMB activities is achieved via post-translational regulation. Because SMB guides differentiation at several steps, I assume that its regulation is multi-layered, and it entails phosphorylation and modulation by interacting partners. The SMB phosphorylation is implied by our preliminary data and previous reports on the phosphorylation-dependent activity of other NAC proteins. I will determine the physiological relevance of SMB phospho-sites by overexpression, and smb phenotype complementation assays. To investigate the possibility of interaction partners fine-tuning the activity of SMB, I will identify SMB interactors by root cap-specific proximity labelling and I will investigate SMB interactors via gain- and loss-of-function approaches. Understanding the regulation of SMB will unravel the mechanisms driving differentiation and PCD. In this respect, the Arabidopsis root cap is a powerful model system, suitable to inform on the general mechanisms governing NAC transcription factors in other PCD contexts, e.g. in the xylem or during senescence.