Glycosylation-dependent sorting of an Arabinogalactan protein SLEEPING BEAUTY mediates apical tip growth and osmosensing in Physcomitrium patens

Plant terrestrialization is a key event that profoundly transforms the Earths surface, during which early land plants acquired cell walls with sophisticated modifications to overcome osmotic stress. Here, we describe the cell wall rigidifying function of SLEEPING BEAUTY (SB), a wall-associated Arabinogalactan protein (AGP) from the bryophyte Physcomitrium patens. The highly-glycosylated SB is intrinsically disordered and rigidifies the cell wall to facilitate orderly depositions of cellulose microfibril, which when affected, compromised the cell wall integrity and rendered a soften cell wall. We further pinpointed critical glycosylation sites on the SB that contribute to its disordered propensity and osmosensing function. Pharmacological inhibition of endocytic trafficking pathways showed that SB is actively endocytosed and trafficked to the tonoplast, which was enhanced under mild osmotic stress. Importantly, sorting of SB to tonoplast requires functional glycosylations at Prolines 55, 92 and 94. Finally, we showed that the wall-rigidifying function of SB is conserved in the flowering plant Arabidopsis, thus highlighting a common cell wall organization across land plants. We propose that the highly disordered AGP may act as a cell wall scaffold protein to mediate wall materials biosynthesis/deposition and play a role in apical tip growth as well as extracellular osmosensing.