Auxin Transport in Avena

Abstract

MIeasurements have been made on the initial stages of the transport of carbon-14-labeled indoleacetic acid in the coleoptile of Avena sativa L. Concentrations of mobile and immobilized indoleacetic acid are related to both distance and time during the first 2 hours after application of the indoleacetic acid at several concentrations to the top of the decapitated coleoptile.At the lowest concentration of indoleacetic acid applied (0.3 Among the recent investigations of the elusive and intriguing phenomenon of auxin transport, there is a dearth of detailed measurements of the distribution and physiological state of the auxin in the plant. The excellent work of Goldsmith and Thimann (6) on the distribution of carbon-14-labeled indoleacetic acid in the Avena coleoptile is often quoted, but that work dealt mainly with IAA distribution and movement at least 2 hr after application to the cut top. It is desirable also to know the distribution of IAA in the coleoptile at earlier times. At these times, the IAA concentration in the coleoptile is rising as the hormone stream moves down from the source at the top. These studies, while not of the normal transport of endogenous auxin, may illuminate some aspects of normal transport.To aid in the understanding of the transport process, it is also necessary to distinguish experimentally between IAA bile in the tissue and that which has been immobilized. Goldsmith and Thimann (6) did not attempt to make this distinction, whereas a complication noted by Scott and Briggs (22) is the presence of some free (not irreversibly immobilized) but nonmoving IAA. This paper is an attempt to fill some gaps in our knowledge of distributions at short times of both mobile and immobilized IAA.It is recognized (5) that the basipetal movement of auxin in the plant is generally an active process, as is implied by the name "transport," and requires metabolic energy from cellular activity. Models of the transport process often include the simple assumption that auxin is transported from cell to cell in amounts that are proportional to the (mobile) concentration difference between the adjacent cells (15). (A transfer under a concentration difference does not need to be an active process, but it could be.) The necessary consequence of this assumption is that the concentration of IAA, after application to the top, must be roughly a decreasing exponential function of distance. (In some literature it is called a "logarithmic" function.) The predicted "profile", or graph of concentration against distance, is steepest at the top and becomes progressively less steep lower down. This kind of profile was found by Goldsmith and Thimann (6) under the conditions of their experiments.Canny (1) has pointed out the difficulties of defining a front to the stream of IAA moving with such a profile. Goldsmith and Thimann (6), being aware of this, did not make detailed measurements in the region of the coleoptile where a front might have been found. Several lines of evidence, however, suggest that a clear fr...

Evidence for Compartmentalization of Conjugates of 2,4-Dichlorophenoxyacetic Acid in Soybean Callus Tissue

“…It was concluded that IAA passively entered cells of the green alga Hydrodictyon and during a 2-h period of efflux more than 90%Yo of the auxin appeared in the medium (15). The efflux of auxin from higher plant tissues is a more complex process, but it has been calculated that it takes 30 min to deplete a cell of auxin (5,13). Metabolites of exogenously supplied IAA appeared in tissue but only unaltered IAA was capable of efflux from the tissue (9).…”

The Decrease in Auxin Polar Transport Down the Lupin Hypocotyl Could Produce the Indole-3-Acetic Acid Distribution Responsible for the Elongation Growth Pattern

“…This picture of polar transport has been challenged by much evidence that points to a considerable flexibility of the auxin transport system. Most of this evidence is derived from studies that showed variations in the transport characteristics along the axes of corn and oat coleoptiles (3,11,23,24), pea seedlings (22), sunflower hypocotyls (25), and prefloral flower stalks of Fritillaria (5, 6). Recently, Parker and Briggs (13) showed that the transport of tritiated IAA down intact corn coleoptiles proceeds at a velocity that declines from 20 mm h-l at the apex to 12 mm h'1 near the mesocotyl node.…”

Effect of TIBA on the action of 2, 4-D on plants

“…Therefore the "absorbed" and "unabsorbed" fractions, in many cases, appear poorly defined in practice. (Newman, 1970). Autoradiography is another important tool to study the distribution of a labeled molecule into a plant.…”

“…Buds and roots were formed in any part of the tissue when TIBA was added. Newman (1970) reported an apparently detailed and accurate study on the kinetic aspects of auxin 14 transport as afrected by TIBA. The use of a C detector with an anti coincidence circuit, and counting times in che order of several hours allowed him to detect, as significant, differences as small as 0-1 CPM.…”