Differences in Auxin Level Detected Across Geostimulated Dandelion Peduncles: Evidence Supporting a Role for Auxin in Geotropism

Clifford, P. E.; Mousdale, David M.; Lynd, S. J.; Oxlade, E. L.

doi:10.1093/oxfordjournals.aob.a086905

Cited by 11 publications

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“…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).…”

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“…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 (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).…”

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Effect of Asymmetric Auxin Application on Helianthus Hypocotyl Curvature

Migliaccio

Rayle²

1989

Plant Physiol.

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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 ...

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Effect of Asymmetric Auxin Application on Helianthus Hypocotyl Curvature

Migliaccio

Rayle²

1989

Plant Physiol.

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“…Asymmetric growth is thought to be controlled by changing hormone levels in different parts of graviresponding organs. Even now, asymmetric distribution of auxin, as already described by the Cholodny-Went theory (Went and Thimann 1937), is still considered to be the main causative factor for differential growth although the observed changes in free auxin levels are often rather small, non-existent or transient (Mertens and Weiler 1983;Clifford et al 1985;Schwark and Bopp 1993;Philosoph-Hadas et al 2001). Recent findings regarding the lateral relocation of the auxin efflux regulator PIN3 upon gravistimulation and the observed differential expression of a synthetic DR5::GUS auxin reporter element, however, strongly support the auxinredistribution theory (Friml et al 2002;Ottenschla¨ger et al 2003).…”

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An auxin-responsive 1-aminocyclopropane-1-carboxylate synthase is responsible for differential ethylene production in gravistimulated Antirrhinum majus L. flower stems

Woltering

Балк

Nijenhuis-deVries

et al. 2004

Planta

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The regulation of gravistimulation-induced ethylene production and its role in gravitropic bending was studied in Antirrhinum majus L. cut flower stems. Gravistimulation increased ethylene production in both lower and upper halves of the stems with much higher levels observed in the lower half. Expression patterns of three different 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) genes, an ACC oxidase (ACO) and an ethylene receptor (ETR/ERS homolog) gene were studied in the bending zone of gravistimulated stems and in excised stem sections following treatment with different chemicals. One of the ACS genes (Am-ACS3) was abundantly expressed in the bending zone cortex at the lower side of the stems within 2 h of gravistimulation. Am-ACS3 was not expressed in vertical stems or in other parts of (gravistimulated) stems, leaves or flowers. Am-ACS3 was strongly induced by indole-3-acetic acid (IAA) but not responsive to ethylene. The Am-ACS3 expression pattern strongly suggests that Am-ACS3 is responsible for the observed differential ethylene production in gravistimulated stems; its responsiveness to IAA suggests that Am-ACS3 expression reflects changes in auxin signalling. Am-ACS1 also showed increased expression in gravistimulated and IAA-treated stems although to a much lesser extent than Am-ACS3. In contrast to Am-ACS3, Am-ACS1 was also expressed in non-bending regions of vertical and gravistimulated stems and in leaves, and Am-ACS1 expression was not confined to the lower side cortex but evenly distributed over the diameter of the stem. Am-ACO and Am-ETR/ERS expression was increased in both the lower and upper halves of gravistimulated stems. Expression of both Am-ACO and Am-ETR/ERS was responsive to ethylene, suggesting regulation by IAAdependent differential ethylene production. Am-ACO expression and in vivo ACO activity, in addition, were induced by IAA, independent of the IAA-induced ethylene. IAA-induced growth of vertical stem sections and bending of gravistimulated flowering stems were little affected by ethylene or 1-methylcyclopropene treatments, indicating that the differential ethylene production plays no pivotal role in the kinetics of gravitropic bending.

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Gravi- and Phototropism of Higher Plants

Hensel

1986

Progress in Botany

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Differences in Auxin Level Detected Across Geostimulated Dandelion Peduncles: Evidence Supporting a Role for Auxin in Geotropism

Cited by 11 publications

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Effect of Asymmetric Auxin Application on Helianthus Hypocotyl Curvature

Effect of Asymmetric Auxin Application on Helianthus Hypocotyl Curvature

An auxin-responsive 1-aminocyclopropane-1-carboxylate synthase is responsible for differential ethylene production in gravistimulated Antirrhinum majus L. flower stems

Gravi- and Phototropism of Higher Plants

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