Cell division activity during apical hook development

Growth during plant development is predominantly governed by the combined a ctivities of cell division and cell elongation. The relative contribution o f both activities controls the growth of a tissue. A fast change in growth is exhibited at the apical hypocotyl of etiolated seedlings where cells gro w at different rates to form a hook-like structure, which is traditionally assumed to result from differential cell elongation. Using new tools we sho w asymmetric distribution of cell division during early stages of hook deve lopment. Cell divisions in the apical hook were predominantly found in sube pidermal layers during an early step of hook development, but were absent i n mutants exhibiting a hookless phenotype. In addition, during exaggeration of hook curvature, which is mediated by ethylene, a rapid change in the co mbined activities of cell division and cell elongation was detected. Our re sults indicate a fast change in cell division activity during apical hook d evelopment. We suggest that cell division together with cell elongation con tributes to apical hook growth. Our results emphasize the change in the rel ative contribution of cell division and cell elongation in a fast growing s tructure like the apical hook.