Differential cellulose distribution drives polarized growth of cotton fibers.

Plant cell morphogenesis relies on the mechanical properties of the primary cell wall, yet it remains unclear which components predominantly regulate wall extensibility. Cotton fibers, highly elongated single cells, offer a unique system to investigate polarized cell expansion. Here, by combining atomic force microscopy and cellulose labeling, we find a basipetal gradient of cellulose microfibril density from the apex to the shank that underlies cell wall heterogeneity and directed cotton fiber elongation. Live-cell imaging shows that cellulose synthase complexes accumulate more densely toward the shank, which is guided by specific microtubule organization and is supported by genetic disruption and microtubule perturbation. A mechanical model further demonstrates that a cellulose gradient is sufficient to reshape axial strain for directional growth. Collectively, our findings provide single-cell evidence for a cellulose-dependent mechanism of directional growth, expanding our understanding of primary cell wall extensibility in plant morphogenesis and offering potential strategies to improve cotton fiber quality.