SMB controls decompartmentalization in Arabidopsis root cap cells to execute programmed cell death

Programmed cell death (PCD) is a fundamental cellular process crucial to development, homeostasis, and immunity in multicellular eukaryotes. In contrast to our knowledge on the regulation of diverse animal cell death subroutines, information on execution of PCD in plants remains fragmentary. Here we make use of the accessibility of the Arabidopsis thaliana root cap to visualize the execution process of developmentally controlled PCD. We identify a succession of selective decompartmentalization events and ion fluxes that are controlled by a gene regulatory network downstream of the NAC transcription factor SOMBRERO (SMB). Surprisingly, breakdown of the large central vacuole is a relatively late and variable event, preceded by an increase of intracellular calcium levels and acidification, release of mitochondrial matrix proteins, leakage of nuclear and endoplasmic reticulum lumina, and release of fluorescent membrane reporters into the cytosol. Elevated intracellular calcium levels and acidification are sufficient to trigger cell death execution specifically in cells that are rendered competent to undergo PCD by SMB activity, suggesting that these ion fluxes act as PCD-triggering signals.