CELLULAR BASIS OF HYPOCOTYL GROWTH IN ARABIDOPSIS-THALIANA

The Arabidopsis thaliana hypocotyl is widely used to study the effects of light and plant growth factors on cell elongation. To provide a fr amework for the molecular-genetic analysis of cell elongation in this organ, here we describe, at the cellular level, its morphology and gro wth and identify a number of characteristic, developmental differences between light-grown and dark-grown hypocotyls. First, in the light ep idermal cells show a characteristic differentiation that is not observ ed in the dark. Second, elongation growth of this organ does not invol ve significant cortical or epidermal cell divisions. However, endoredu plication occurs, as revealed by the presence of 4C and 8C nuclei. In addition, 16C nuclei were found specifically in dark-grown seedlings. Third, in the dark epidermal cells elongate along a steep, acropetal s patial and temporal gradient along the hypocotyl. In contrast, in the light all epidermal cells elongated continuously during the entire gro wth period. These morphological and physiological differences, in comb ination with previously reported genetic data (T. Desnos, V. Orbovic, C. Bellini, J. Kronenberger, M. Caboche, J. Traas, H. Hofte [1996] Dev elopment 122: 683-693), illustrate that light does not simply inhibit hypocotyl growth in a cell-autonomous fashion, but that the observed g rowth response to light is a part of an integrated developmental chang e throughout the elongating organ.