Distribution of Auxin in Horizontal Woody Stems in Relation to Gravimorphism

Leach, R.; Wareing, P. F.

doi:10.1038/2141025a0

Cited by 37 publications

(13 citation statements)

References 9 publications

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“…Gravistimulation of primary roots and shoots resulting in differential growth responses and geotropism is mediated by auxin efflux regulators and a lateral transport of IAA toward the lower side in coleoptiles, shoots, and roots as demonstrated in maize coleoptiles, snapdragon, and Arabidopsis (Philippar et al, 1999;Philosoph-Hadas et al, 2001;Friml et al, 2002). In line with these findings, experiments where radiolabeled IAA was applied to woody horizontal shoots of both gymnosperms and angiosperms species have shown that transport is also favored toward the lower side (Leach and Wareing, 1967;Starbuck and Phelps, 1986;Lachaud, 1986). Endogenous levels of IAA were not measured in these experiments, but in the much older stems of pine and poplar used in our study, gravistimulation did not seem to induce any downward redistribution of IAA.…”

Section: Discussionsupporting

confidence: 65%

Patterns of Auxin Distribution during Gravitational Induction of Reaction Wood in Poplar and Pine

2004

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Gravistimulation of tree stems affects wood development by unilaterally inducing wood with modified properties, called reaction wood. Commonly, it also stimulates cambial growth on the reaction wood side. Numerous experiments involving applications of indole-3-acetic acid (IAA) or IAA-transport inhibitors have suggested that reaction wood is induced by a redistribution of IAA around the stem. However, in planta proof for this model is lacking. Therefore, we have mapped endogenous IAA distribution across the cambial region tissues in both aspen (Populus tremula, denoted poplar) and Scots pine (Pinus sylvestris) trees forming reaction wood, using tangential cryosectioning combined with sensitive gas chromatographymass spectrometry analysis. Moreover, we have documented the kinetics of IAA during reaction wood induction in these species. Our analysis of endogenous IAA demonstrates that reaction wood is formed without any obvious alterations in IAA balance. This is in contrast to gravitropic responses in roots and shoots where a redistribution of IAA has been documented. It is also of interest that cambial growth on the tension wood side was stimulated without an increase in IAA. Taken together, our results suggest a role for signals other than IAA in the reaction wood response, or that the gravitational stimulus interacts with the IAA signal transduction pathway.Displacement of stems and branches by wind or mechanical stress in woody species results in the formation of reaction wood. This response is unilateral and beneficial for the tree in that it creates physical strains in the wood that force the stem or branch back toward its original orientation in space (Scurfield, 1973;Wilson and Archer, 1977;Timell, 1986). Angiosperm and gymnosperm trees differ in their nature of reaction wood. In angiosperm trees, such as poplar, reaction wood is called tension wood and forms on the upper side of stems that have been displaced. Tension wood characteristically has few, small vessels, and fibers with an inner gelatinous cell wall layer (the G-layer) that consists of almost pure cellulose with microfibrils that are parallel to the long cell axis (Haygreen and Bowyer, 1996;Jourez et al., 2001). In gymnosperm trees, such as pine, reaction wood is called compression wood and forms at the lower side of displaced stems. Compression wood is characterized by short, rounded tracheids that have thick walls with increased lignin content and increased microfibril angles (Timell, 1969). The formation of reaction wood is often (but not always) accompanied by a stimulation of cambial cell division, whereas the cell division at the opposite side is more or less inhibited.The physiology and development of reaction wood formation has been extensively explored (particularly in gymnosperms) and reviewed in great detail by Timell (1986). The induction of reaction wood by gravistimuli rather than by mechanical stimulation has been deduced from a large number of bending, leaning, and clinostat experiments. Reaction wood can also be induced by int...

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Section: Discussionsupporting

confidence: 65%

Patterns of Auxin Distribution during Gravitational Induction of Reaction Wood in Poplar and Pine

2004

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“…Alternatively, graviperception could occur within the woody stem itself. In Populus, starch-filled plastids that could serve as statoliths were observed in some cells of the cortex of young stems (Leach and Wareing, 1967), but no function could be ascribed to these cells in graviperception. Likewise, the mechanisms responsible for the initial graviresponse of woody stems have not been established.…”

Section: Introductionmentioning

confidence: 99%

Transcriptional and Hormonal Regulation of Gravitropism of Woody Stems inPopulus

et al. 2015

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Angiosperm trees reorient their woody stems by asymmetrically producing a specialized xylem tissue, tension wood, which exerts a strong contractile force resulting in negative gravitropism of the stem. Here, we show, in Populus trees, that initial gravity perception and response occurs in specialized cells through sedimentation of starch-filled amyloplasts and relocalization of the auxin transport protein, PIN3. Gibberellic acid treatment stimulates the rate of tension wood formation and gravibending and enhances tissue-specific expression of an auxin-responsive reporter. Gravibending, maturation of contractile fibers, and gibberellic acid (GA) stimulation of tension wood formation are all sensitive to transcript levels of the Class I KNOX homeodomain transcription factor-encoding gene ARBORKNOX2 (ARK2). We generated genome-wide transcriptomes for trees in which gene expression was perturbed by gravistimulation, GA treatment, and modulation of ARK2 expression. These data were employed in computational analyses to model the transcriptional networks underlying wood formation, including identification and dissection of gene coexpression modules associated with wood phenotypes, GA response, and ARK2 binding to genes within modules. We propose a model for gravitropism in the woody stem in which the peripheral location of PIN3-expressing cells relative to the cambium results in auxin transport toward the cambium in the top of the stem, triggering tension wood formation, while transport away from the cambium in the bottom of the stem triggers opposite wood formation.

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“…Recently, GA has been found to exert a greater regulatory influence than auxins in controlling stem orientation of excised sunflower shoot tips (18). Although auxin distribution was not measured in this study, an asymmetric auxin ratio (60:40 increasing toward the lower side) has been found in naturally prostrate stems (12). Auxin gradients failed to straighten hooked pea hypocotyls (11), even though cellular elongation was enhanced in the absence of ethylene (1).…”

Section: Discussionmentioning

confidence: 67%

“…Increased levels of catechol and /3 inhibitor have been found in the lower half of horizontal woody Populus stems (12); an auxin gradient which increased toward the lower side was also demonstrated, but lateral shoot development and cambial activity were greater on the upper half of the stem. Rhizome orientation is also believed to be controlled by inhibitors (4), and /8 inhibitor was isolated from Aegopodiumn rhizomes (5).…”

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confidence: 94%