Abstract
Hemicellulosic xyloglucan is abundant in the primary cell walls of eudicots and non-gramineous monocots and contributes to their mechanical integrity. Its deficiency results in morphologically abnormal root hairs in Arabidopsis mutants lacking either two XYLOGLUCAN XYLOSYLTRANSFERASE (xxt1 xxt2) or five CELLULOSE SYNTHASE-LIKE C (cslc-∆5) genes, but the cellular and biomechanical mechanisms underlying these abnormalities have remained unexplored. We combined multiple imaging modalities and quantitative modeling to unmask these mechanisms, first observing that xyloglucan-deficient mutants produced fewer and shorter root hairs that grew slower than wild type. Xyloglucan-deficient trichoblasts showed root hair initiation defects, helping to explain the sparseness of root hairs in these mutants. The mutant root hairs also had lower turgor pressure, thicker walls, and wider cross-sections, that explain lack of anisotropic growth, but cellulose organization appeared normal in xyloglucan-deficient root hairs. Brillouin microscopy suggested that xyloglucan-deficient root hairs were softer than wild-type root hairs. Finite element modeling of mutant root hairs predicted lower surface stress and growth rates, which were directly proportional to turgor pressure and inversely proportional to wall thickness. Stress patterns on models hint at potential mechanisms of bulging in xyloglucan-deficient root hairs. We propose that though cellulose orientation specifies the axis of cell expansion, it cannot enforce it without xyloglucan that interlinks microfibrils into a load-bearing network, identifying matrix-cellulose interaction as a distinct and necessary determinant of anisotropic plant cell growth.