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Collection Geotropism an4 Auxin Transport inthe Corn Cole op tile folder 7- modified first leaf)is responsible for the h of the shoot of seedlings out of the soil. organ elongates fast and reacts strongly t o-Tk light and gravity. The slop g ation is controlled by the plant hormone auxin which is produced i n and transported down from the tip of the cole op tile. By virtue of these features,the cole op tile has become a classical experimental tool in plant growth physiology. This project is concerned with the geotropism of the cole op tiles of represents a simple, but nevertheless interesting sensory reaction chain, involving only a small number of steps from the stimulus tothe response, Hopefully, it is assumed that this response to gravity - that is,the upward bending of the organ. After a gravitational stimulus, i.e. when the organ is transferred to the horizontal position (t = 0 minutes), trhe following events can be observed: (1) displacement of starch plastids toward the lower side in each cell(0 - 15 minutes); (2) deflection of auxin flow toward the lower half of the cole op tile (from less than 10 minutes on); (3) increase of water permeability at the lower side of the organ (ca. 2 - 5 minutes); electric effect (after 10 - 15 minutes). In order to find out about any causal relations mainly of (1) with (2) and (4), and eventually also(3),the role of starch grains is reinvestigated and experiments are performed on the mechanism of auxin (a) PoscibZe Rote of Starch Plastids i n the Geotropic Reaction A special centrifuge has been designed t o expose excised cole op tile ; t o centrifugal forces of 0to 20 x g at 2' and at room temperature (25 1. The question being investigated is whether the displacement of starch in cells of the tip region is always correlated with the subsequent bending (and with the lateral auxin transport). The displacement of starch plastids from the bottom of the cell to the 2' within 30 minutes; its time course can be registered in samples by standard microscopic techniques. While inthe cold (2'), these short stimuli are practically not perceived by the tissue. Curvature, however, can be developed subsequently when the cole op tiles are rotated at 25 a cli nost at. _I and the distribution of starch at the time of transfer from the cold 0centrifuge to the warm cli nost at. fv The transport of auxin from a donor (agar.block with added auxin) through cole op tile sections into a receptor(initially empty agar blcok) is fast, act%--eand polar, presumably consisting of a series of specific secretion steps repeated at each cell. The auxin, either indole-3-acetic acid (IAA) o r naphthalene-1-acetic acid (NAA, which is transported ina similar manner) is applied as free acid in the donor and i t reappears as free acid,15th no lower" aide wall o r from one side to the other is achieved by 8xga t11 Preliminary data indicate a correlation between bending
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changes inthe C-skeleton, in the receptor. active transport does occur, implies a specific,transient chemical these reactions,the following results suggest that the auxin-transport "pumps" are acting by non-covalent forces, and that the auxin is trans - ported as the free molecule. physiological action which is connected with the "transported" auxin. (i) After short exposure (5to 20 minutes) of the tissue to NAA-14C o r LAA-14C, no auxin complex could b e found which could b e chased as required for any transport - carrier intermediate. w a s formed with both NAA and IAA; ilowever, any turnover of conjugate produced in the tissue could be ruled out, although this compound was split by the cole op tiles efficiently when applied externally. The fact that a specific, The same may be said also of the primary - (ii) When XAA was labeled In its carboxyl group with the heavy Isotope l80 and sent through the cole op tile ' s transport system and the transported NAA was then analysed for l8O, of NAA during the transport through the tissue. no l80 was lost from the carboxyl group After applying IAA t o auxin-depleted tissue,transport does not proceed .ata constant rate. Rather, oscillations with a period length of ca. 25 minutes between high and low exit rate were registered from single cole op tile sections. It is reasonable t o assume that auxin transport "pumps" are located in the plasma membrane and that the pumps may be o px a ted by transformation i n membrzne structure.Indirect evidence for such a mechanism comes from experiments which indicate cooper at iv it yIn auxin transport (by a reasoning similar to that used in describing al lost eric transitions). It was observed accelerated if the segments are treated vith IAA. ..A rei-ated finding involves the effect of IAA pretreatment on IAA transport. It was found that the rate of 14C-IAA transport could be increased approximately twofold by a brief pretreatment with cold IAA. concentration of IAA used for the pretreatment is critical, but very low concentrations appear to be effective.