Intracellular trafficking of an Arabinogalactan protein SLEEPING BEAUTY influences cell wall integrity and apical tip growth in Physcomitrium patens

Polarized tip growth is a key morphological adaption that facilitated land plant terrestrialization, and arabinogalactan proteins (AGPs) are central regulators of this process. However, how glycosylation modulates AGP function during tip growth remains poorly understood. Here, we show that the AGP SLEEPING BEAUTY (SB) is required for cell wall integrity during protonemal tip growth in the moss Physcomitrium patens, a function that is conserved in the distantly related vascular plant Arabidopsis thaliana. Loss of SB function disrupts cellulose microfibril organization and compromises cell wall integrity, leading to altered tip growth dynamics and geometry. Using a hypoglycosylated SB variant, we demonstrate that glycosylation of Pro55, Pro92, and Pro94 within a highly intrinsically disordered region is required for proper SB secretion and vacuolar trafficking; loss of these modifications results in pronounced inhibition of tip growth. We propose that glycosylation of the intrinsically disordered domain in AGP fine-tunes SB abundance at the cell surface, thereby facilitating cellulose microfibril biosynthesis and/or assembly to regulate robust polarized tip growth.