Highways and detours: routes for protein and polysaccharide secretion into the plant cell wall

Plant cell wall assembly is a highly dynamic and regulated process involving a complex network of secretory pathways that coordinate the delivery of proteins, polysaccharides and various other components to the apoplast. While the conventional endomembrane route—comprising the endoplasmic reticulum, Golgi apparatus, trans-Golgi network, and plasma membrane—has long been recognized as the primary trafficking highway, recent discoveries have evidenced the relevance of alternative and unconventional routes, including Golgi bypassing, exocyst-positive organelles, and multivesicular bodies. These findings have expanded our understanding of the molecular logistics underlying wall construction, remodeling, and defense. This review synthesizes classical and emerging models of exocytosis in plant cells, highlighting mechanisms of protein and polysaccharide trafficking, with special attention to cellulose synthase complex dynamics, glycosylphosphatidylinositol-anchored proteins, and leaderless protein secretion. Dedicated to Professor Gabriella Piro, whose insightful contributions have significantly advanced our understanding of polysaccharide and protein secretion, as well as the interplay between trafficking specificity and cell wall biosynthesis. Her scientific legacy continues to inspire current research into the secretory pathways that shape the plant cell wall, a vital interface for growth, development, and environmental adaptation.