Cell surface expansion in polarly growing root hairs of Medicago truncatula

Fluorescent microspheres were used as material markers to investigate the r elative rates of cell surface expansion at the growing tips of Medicago tru ncatula root hairs. From the analysis of tip shape and microsphere movement s, we propose three characteristic zones of expansion in growing root hairs . The center of the apical dome is an area of 1- to 2-mum diameter with rel atively constant curvature and high growth rate. Distal to the apex is a mo re rapidly expanding region 1- to 2 mum in width exhibiting constant surges of off-axis growth. This middle region forms an annulus of maximum growth rate and is visible as an area of accentuated curvature in the tip profile. The remainder of the apical dome is characterized by strong radial expansi on anisotropy where the meridional rate of expansion falls below the radial expansion rate. Data also suggest possible meridional contraction at the j uncture between the apical dome and the cell body. The cell cylinder distal to the tip expands slightly over time, but only around the circumference. These data for surface expansion in the legume root hair provide new insigh t into the mechanism of tip growth and the morphogenesis of the root hair.