Quantitative Assessment of Ultrastructural Changes in Primary Roots of Corn (Zea mays L.) after Geotropic Stimulation

Abstract

In outer cortical cells of corn (Zen mays L.) roots we made ultrastructural comparisons between the geotropicafly responding (661 m irradiated) and nonresponding (dark control) roots in both the curving and noncurving regions of the root. In the control treatment, Golgi apparatus (dictyosomes) and mitochondria exhibited centrifugal distribution (taking the stele as the center) in both regions of the roots (the organeUes localized in the top of the ceUs in the upper tissues, and in the bottom of the ceUs in the lower tissues). In the geotropicaUy responding roots, the distribution patterns were the same as those of the controls. However, in the zone of curvature the dictyosomes (but not the mitochondria) were randomly distributed in the cels of the upper tissues. This change in pattern of dictyosome distribution could be related to the change In cell elongation of upper cels.In a previous paper (6), we compared ultrastructural differences in root cap cells of maize between the geotropically responding, red light-exposed roots, and the nonresponding dark control roots. The root cap is the site of perception in the geotropic response. In a light microscopy examination (8), we established that the curving zone of geotropically responding corn roots is located in the 1.5-to 2.5-mm region from the root meristem, or 2 to 3 mm from the tip of the root. The outer cortical cells of corn roots, the row of cells next to the epidermis, exhibit the greatest elongation among all root cells (8). In the present study, we have compared cell ultrastructure between the geotropically responding and nonresponding corn roots in the outer cortex of the curving regions and in regions behind the curving zone of corn roots. MATERIALS AND METHODSPlanting and Irradiation. The detailed procedure can be found in reference 6. Corn seeds (Wisconsin hybrid 64A x 22R) were soaked in warm water, drained, and refrigerated overnight, and planted on both edges in Lucite bars wrapped with moist filter paper. The bars were kept moist and away from light. The primary roots grew to a 1.5-to 3-cm length within 48 hr of planting, and extended horizontally from the embryo. These roots were used for experimentation. The horizontal pattern of growth subjected the roots to a constant gravitational stimulation during the entire period of growth and experimentation. As long as the roots were not exposed to light, they remained horizontal and showed no By acceptance of this article, the publisher or recipient acknowledges the U.S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering the article.' Work supported by the U.S. National Aeronautics and Space Administration and the U.S. Energy Research and Development Administration.geotropic curvature. A red irradiation (661 nm at I J m-2) would induce them to curve toward gravity (Fig. 1), i.e. becoming positively geotropic (7). Thus, all manipulations were carried out in darkness before the irradiation, except for seed planting which was done in a dim green light...

The r�le of abscisic acid in root growth and gravireaction: A critical review

“…Dictyosomes in cells of the upper and lower halves of the root showed no difference in morphology or numbers that could be related to the gravitropic reaction. Similar results were found for roots of Zea mays [168] . Recently, it has been shown [42] that in the lower half of the proximal part of the meristem of a horizontal, gravireacting Lens root there was less intense DNA synthesis and an increased mitotic index, which may indicate a slower rate of mitosis .…”