Bearskin2 mediates the coordinated secretion of xylogalacturonan and root cap polygalacturonase in Arabidopsis border-like cells

Abstract

AbstractBorder-like cells (BLCs) are sheets of cells that are continuously sloughed off and replenished at the Arabidopsis root cap surface.ROOT CAP POLYGALACTURONASE (RCPG)encodes a putative pectinase involved in BLC shedding. Xylogalacturonan (XGA) is a pectic polysaccharide whose synthesis is associated with cell detachment and secreted separately from other cell wall polysaccharides.BEARSKIN1 (BRN1)andBRN2areArabidopsisNAC family transcription factors, andRCPGexpression is inhibited inbrn1/2. To explore the link between XGA and RCPG, we examined XGA synthesis inArabidopsislines with altered RCPG levels. We found that RCPG was contained in XGA-carrying vesicles budding from thetrans-Golgi, but XGA synthesis was not affected in thercpgmutant. XGA was absent in BLCs ofbrn2, but not ofbrn1, indicating thatBRN2is necessary for XGA synthesis. Overexpression of functional RCPG-GFP (oeRCPG-GFP) caused upregulation ofBRN2, ectopic XGA synthesis, overaccumulation of endogenous RCPG, and accelerated BLC turnover, suggesting a positive regulatory loop between RCPG and BRN2. Inactivation ofBRN2inoeRCPG-GFPsuppressed RCPG-GFP expression, excess RCPG, and XGA synthesis. Our data provide evidence that XGA and RCPG are secreted together and that BRN2 controls XGA synthesis, which facilitates RCPG export and BLC separation.