Effect of Asymmetric Auxin Application on Helianthus Hypocotyl Curvature

Abstract

lndole-3-acetic acid was applied asymmetrically to the hypocotyls of sunflower (Helianthus annuus L.) seedlings. After 5 hours on a clinostat, auxin gradients as small as 1 to 1.3 produced substantial (more than 60 degrees) hypocotyl curvature. This result suggests the asymmetric growth underlying hypocotyl gravitropism can be explained by lateral auxin redistribution.To the best of our knowledge neither proponents nor critics have systematically tested the effects of minor auxin gradients on gravitropism although the pioneering experiments ofBrauner and Diemter (2) come quite close to such an objective. In our opinion a critical test would involve monitoring the effects of small auxin gradients (similar to those observed following gravistimulation) on the curvature of shoots under conditions that would be expected to randomize gravity vectors (e.g. on a clinostat). Here we report on the results of such experiments.The Cholodyn-Went hypothesis (21) asserts that the gravitropic curvature of shoots results from gravity-induced lateral transport and accumulation of auxin on the lower side of horizontal shoots. It is assumed that this auxin gradient then initiates asymmetric cell extension which results in shoot curvature and eventual reorientation. Various lines of evidence support this hypothesis. For example, a large body of literature links auxin with plant cell extension, and the unilateral application of auxin to plant stems and coleoptiles leads to organ curvature (6,15,20,22). Inhibitors of auxin action and/or auxin transport inhibit gravitropism (14, 23). A mutant deficient in its ability to respond to auxin shows altered gravitropic behavior (1 1). Gravistimulation results in lateral auxin transport (2, 4, 8-10, 12, 14, 15, 17) and produces an asymmetrical distribution of auxin-regulated RNAs (13).In spite of the experimental support cited above for the role of auxin in shoot gravitropism, this has remained a controversial concept. The primary sources of concern are the relatively small auxin gradients that develop following gravistimulation. It is argued the auxin gradients that develop following gravistimulation are not sufficient to account for the observed differential growth (5, 7, 18). Indeed, excised plant segments generally exhibit a logarithmic dose-response curve in response to exogenous auxin, while gravistimulation generally produces an auxin gradient between the upper and lower halves of coleoptiles and stems of 1:3 or less (4,7,10,12,14,15). Thus, it is reasoned, the differential growth which occurs during gravitropism is unlikely to be mediated by IAA; extrapolation of straight growth data suggests gradients of 10-fold or more would be required for organ reorientation (5, 7). MATERIALS AND METHODSExperiments were performed using 5-d-old sunflower seedlings (Helianthus annuus cv Mammoth) germinated and grown in the light as previously described (23). Seedlings with hypocotyls 4 to 5 cm in length were gently removed from vermiculite and decapitated about 1 mm below the cotyledonary ...

Gravitropic Response of Inflorescence Stems in Arabidopsis thaliana

“…This result suggests that auxin is involved in the gravitropic response of inflorescence stems of Arabidopsis, as reported for many other plant shoots (Iwami and Masuda, 1974;Hatfield and LaMotte, 1984;Harrison and Pickard, 1989;Migliaccio and Rayle, 1989;Evans, 1991).…”

“…Many physiological and cytological studies of these three steps in both shoot and root gravitropism have been performed for more than 180 years, using many different plants. These studies have demonstrated that the amyloplast is involved in the gravity perception step (Iversen, 1969;Heathcote, 1981;Moore and Evans, 1986; reviewed by Sack, 1991) and that auxin (Gillespire and Thimann, 1963;Iwami and Masuda, 1976;Bandurski et al, 1984;Hatfield and LaMotte, 1984;Harrison and Pickard, 1989;Migliaccio and Rayle, 1989;Parker and Briggs, 1990; Young et al, 1990), calcium (Slocum andRoux, 1983;Poovaiah et al, 1987), calmodulin (Stinemetz et al, 1987, and protein phosphorylation (Friedmann and Poovaiah, 1991) are involved in the gravitropic response pathway. …”

Location of Transported Auxin in Etiolated Maize Shoots Using 5-Azidoindole-3-Acetic Acid

“…This work demonstrated that hormone concentration differences can occur between the epidermal and subtending cortical cell thus supporting the hypothesis that redistribution of auxin which leads to assymetrical growth may only need to occur between these two adjacent cells. The observed difference in the hormone concentration between the epidermis and cortex is well within the range necessary to obtain differential growth (24). This difference between two adjacent cell types may be important in growth control, particularly if adjacent cell types have different sensitivity to auxin (21).…”

A Mutation Altering Auxin Homeostasis and Plant Morphology in Arabidopsis.

“…These effects are reminiscent of the triple response (Neljubow, 1901) observed in wild-type Arabidopsis seedlings exposed to ethylene (Bleecker et al, 1988;Guzmán and Ecker, 1990), except that ethylene exaggerates apical hooks in the wild type. Formation of apical hooks has been attributed to asymmetric distributions of auxin (Migliaccio and Rayle, 1989;Evans, 1991;Li et al, 1991), ethylene (Schierle and Schwank, 1988), and a putative antagonist of ethylene action (Guzmán and Ecker, 1990). Hookless seedlings are characteristic of Arabidopsis mutants with reduced sensitivities to auxin (Mirza, 1987), ethylene (Bleecker et al, 1988;Guzmán and Ecker, 1990;Harpham et al, 1991), auxin and ethylene , or auxin, ethylene, and abscisic acid .…”