Effects of Indoleacetic Acid on Dictyosomes of Apical and Expanding Cells of Oat Coleoptiles

Abstract

We found that the auxin-induced growth is mediated through the activation of the dictyosomes (collectively, the Golgi apparatus). Incubation of oat (Avena sativa) coleoptile segments in indoleacetic acid-sucrose-phosphate buffer changes significandy the number of dictyosomes in the expanding cells. A further indication of auxin enhancement of dictyosome activity is a decrease in dictyosomal cisternae (flattened membranous sacs) number. This decrease occurred after 6 minutes of incubation in auxin, and then was followed by a reduction in the organelle number per se. These times are in keeping with the rapid action of auxin-induced cell elongaton, and the latent period of geotropism. In the apical cells, the effect of indoleacetic acid is more subtle and complex. The 3To whom reprint requests should be addressed.Individual stacks of cistemae (flattened sacs containing fluid). The association of dictyosomes within a cell forms the Golgi apparatus (14). regulated by the hormone auxin. In this study we ask whether auxin, IAA, changes the activity of the dictyosomes. And if so, are the kinetics of the changes in keeping with those of the tropistic and growth responses. We examined the quantity and activity of this organelle in the extreme apex as well as in the elongating cells of oat coleoptiles after different times of incubation in IAA solutions. MATERIALS AND METHODSPlanting and Incubation. Oat seeds (Avena sativa, Victory I) were soaked in warm tap water (initially 40 C) for 2 hr, drained, and kept in the dark for 20 hr at 4 C. The soaked seeds were then aligned, embryo up, on lucite bars covered with moist filter paper. The seeds were exposed to red light (G.E. Ruby Red tungsten filament lamp, 10 w) for 24 hr, and grown for an additional 48 hr in the dark. At the end of this period, the coleoptiles reached a length of 2 to 3 cm. The apical 1 mm, and the subapical 3 mm (4th through 6th mm) of the coleoptile were sectioned with spaced razor blades. The segments were placed in Petri dishes containing buffer, 0.1 M KH2PO4 and 2% sucrose (pH 4.6). The dishes were set on a slow reciprocating horizontal shaker for a minimum of 2 hr, in order to deplete the tissues of their endogenous auxin. The segments were then transferred to small culture dishes containing the same buffer solution with or without 1 ,M IAA (sodium salt). The dishes were then placed on the shaker, and the segments were harvested at 6-min intervals for a period of 60 min for examinations by EMS and for the determination of cell lengths. All manipulations were carried out in a dim green light (21) at 25 C and 70% relative humidity.Electron and Light Microscopy. At 6-min intervals, the segments (n = 6) were transferred to a cold fixative (approximately 4 C) containing 4% glutaraldehyde, 1 % acrolein buffered with 0.1 M Na-cacodylate to pH 7.4, and 0.1% CaCI2. Fixation was carried out in a refrigerator (4-6 C) overnight. The aldehyde-fixed segments were then washed with the same buffer in which they were fixed, and postfixed for 3 hr in cold 1 %...

Effects of Indoleacetic Acid on the Quantity of Mitochondria, Microbodies, and Plastids in the Apical and Expanding Cells of Dark-grown Oat Coleoptiles

“…IAA incubation exerted no effect on the number of any of the organlles examined in these cells (P > 0.02). As previously shown (8), the number of dictyosomes in these same cells also showed no change. However, in the cells of the outer epidermis of this region (Table II), IAA did show a (10).…”

“…The tropistic response is a growth response involving the plant hormone IAA (32). We found that incubation of oat coleoptiles with IAA increases the activity of dictyosomes in the expanding cells (8). The response is rapid, beginning within 6 min (cisternal changes), and continues throughout a 1-hr incubation period.…”

“…The procedure used in this study was identical to that in (8). Oat seeds (A vena sativa, Victory I) were soaked in initially warm (45 C) tap water, decanted, and kept at 4 to 6 C for approximately 20 hr.…”

“…When the segments were harvested from the incubation medium, they were fixed immediately in a cold glutaraldehyde-acrolein cacodylate buffer (-4 C, pH 7.4) overnight. The segments were rinsed and fixed again in 1% OSO4, dehydrated in a graded series of ethanols, and embedded in an Araldite (6005)-Epon (812) mixture and crosssectioned (8). The tissues were blind coded during EM examinations to eliminate bias.…”

“…In this paper we present data showing the effects of IAA on the quantity of other organelles, mitochondria, microbodies, and plastids, in the apical and the subapical expanding cells of oat coleoptiles. The quantitative tabulation of organelles, an approach we had used in several earlier studies (8,27,28), is more meaningful in the understanding of organelle function than qualitative descriptions based on a few electron micrographs. …”