Mechanical properties that underlie important cotton fiber traits

The economic value of cotton is based on the long, thin, strong, and twisted fiber cells that emerge from the seed epidermis. The mature dried fiber morphology reflects the outcome of tapering of the cell within days and elongation across weeks, followed by persistent synthesis of a cellulose-rich secondary cell wall. However, the biomechanical details of their morphogenetic control are poorly understood, limiting direct connections between cell wall properties and fiber morphogenesis. In this work, we developed novel quantitative phenotyping pipelines to analyze fiber growth behaviors at a daily resolution and coupled experimental observation with mechanical modeling. We uncovered unexpected variability in growth rate, cell wall properties, and cell geometry across a critical window of fiber development. Treating the cell wall as a fiber-reinforced composite, finite element analysis was used to analyze the sensitivity of cell diameter to material property variation and to predict how spatial gradients of fiber and matrix material properties can dictate the patterns of shape change. Our analyses point to the central importance of cellular-scale patterning of the microtubule and microfibril arrays in controlling fiber shape and size and provide targets for fiber quality improvement.