Induced Abscission Sites in Internodal Explants of Impatiens sultani: A New System for Studying Positional Control: with an Appendix: A Mathematical Model for Abscission Sites

Intemodes from Impatiens sultani shoots, explanted into sterileculture, often developed a transverse separation layer afterone to two weeks and the top then abscised from the bottom ofthe explant. Such abscission occurred more rapidly and in agreater proportion of explants when 00001 per cent auxin (IAA)was provided basally and when younger intemodes and shorterexplants were used. The distance of the separation layer fromthe base of the explant varied little with explant length, butincreased with the concentration of auxin applied basally. It seems that in this adventitious abscission the processesof positional definition and differentiation proceed withoutpause, whereas in normal abscission the position is definedearly in development but the final stage of differentiationof the separation layer is delayed until much later when theorgan senesces. To account for the results from the internodal explants andfrom surgical operations on shoots as well as for the characteristicposition of abscission sites of leaves and fruits, we suggestthat the position of abscission is controlled primarily by auxinacting as a morphogen: abscission sites occur at Y-junctionsjust above the base of the arm with the lower activity and auxinstatus, or in single axes above a region of higher auxin status.In both sites, the auxin concentration decreases in the apicaldirection. This hypothesis is supported by a mathematical model (see Appendix)of the interaction of diffusive and polar transport in controllingthe concentration gradient along intemodes with specified auxinconcentrations maintained basally. The model allows predictionsconcerning the site and timing of abscission which accord withobservations on intemodal explants. Impatiens sultani Hook., abscission, auxin, differentiation, diffusion coefficient, IAA, morphogen, polar transport coefficient, positional control, separation layer     

Effects of Applied IAA on the Position of Abscission Sites Induced in Wounded Explants from Impatiens sultani Internodes

Previous work established that if segments of Impatiens sultaniinternodes are explanted and incubated on a suitable medium,they tend to undergo abscission by a transverse separation layerthat differentiates a short distance above the explant base.The present study has shown that the position of the abscissionsite can be modified experimentally. When an explant was splitdown to midlength and auxin (IAA) was applied to the top ofone of the two arms, abscission often occurred at or near thebase of the other arm. Again, when IAA was applied to the explantlaterally midway along its length, abscission often occurredjust above the application point. These two modifications ofabscission sites had been predicted by a hypothesis statingthat separation layers tend to be positioned where auxin concentrationdecreases in the morphologically upward direction. Studies with[¹⁴C]IAA confirmed that the separation layers above the explantbase, and in the two experimentally modified sites, did indeedarise where the concentration decreased upwards. Also, woundingaltered the position of abscission in these explants in waysthat can be interpreted in terms of the above hypothesis coupledwith the destruction of auxin that occurs at wound surfaces.In this system, auxin is acting as a morphogen: its concentrationgradients provide positional information. Impatiens sullani Hook., abscission, auxin, IAA, morphogen, positional control, separation layer, wounding     

Morphogenetic, Inductive and Inhibitory Effects of IAA Conjugates on Abscission in Stem Explants of Impatiens sultani

IAA-L-alanine and IAA-DL-aspartate, when applied to internodalexplants of Impatiens sultani, mimic the effects of IAA bothin inhibiting abscission (when applied at the apical end) andin promoting abscission and rooting, and displacing the siteof induced abscission (when applied basally). These IAA conjugatesare less active than IAA at similar concentrations, and theirpromotory effects are slower. Their activity is interpretedin terms of their diffusion into the explants, where they become‘slow-release’ sources of free IAA through enzyme-catalyzedhydrolysis. Abscission, auxin, IAA, IAA conjugates, IAA-L-alanine, IAA-DL-aspartate, inhibition, morphogenesis, separation layer     

The Role of Cell Expansion in the Abscission of Impatiens sultani leaves

The histological events occurring during the latter stages ofabscission were followed continuously in longitudinal slicesthrough the petiole base of Impatiens sultani Hook. It appearsthat the middle lamella of the cortical parenchyma cells isdegraded first. This is followed by an expansion of these cellsand a concomitant stretching and separation of the collenchymaand vascular trace. The parenchyma cells continue to enlargeuntil they become virtually spherical, a process which finallyruptures the last restraining xylem vessels. The increased volumeof the parenchyma cells appears to be principally due to a conformationalchange in cell shape from a near regular hexagonal prism toa sphere of similar surface area. The dimensions of the prismaticcells are such that most of them change into spheres whose diametersare the same as the transverse distance between the oppositesides of the six axially orientated faces of the prisms. Cellularexpansion is thus entirely directed along the axis of the petiole.The significance of these observations to the general anatomyand mechanism of fracture of abscission zones is discussed. Impatiens sultani Hook., abscission, cell expansion, cell wall degradation, cell shape     

The Formation of Structural Abnormalities in Karelian Birch Wood is Associated with Auxin Inactivation and Disrupted Basipetal Auxin Transport

Journal of Plant Growth Regulation - Figured wood of Karelian birch (Betula pendula Roth var. carelica (Merckl.) Hämet-Ahti) is highly appraised for its ornamental properties. The reasons for...     

Role of Ethylene Biosynthesis and Auxin Content and Transport in High Temperature-Induced Abscission of Pepper Reproductive Organs

High temperatures induced abscission of pepper (Capsicum annuum L. cv. Maor) reproductive organs at various developmental stages. The role of ethylene biosynthesis and auxin economy in high temperature-induced abscission is described. High temperatures somewhat increased ethylene production in the reproductive organs, but the highest temperature treatment, which was the most active in inducing reproductive organ abscission, decreased it. In contrast to ethylene, 1-aminocyclopropane-1-carboxylic acid levels increased significantly in response to high temperatures and correlated positively with the increase in temperature. High temperatures reduced indole-3-acetic acid levels and particularly auxin transport capacity in the reproductive organs. The data suggest that the reduction of auxin transport capacity is the major mechanism by which high temperatures induce reproductive organ abscission in pepper. Received September 27, 1996; accepted March 13, 1997     

A Role for Auxin and Auxin Transport Inhibitors on the Ca Content of Artificially Induced Parthenocarpic Fruits

Artificially induced parthenocarpic fruits of apples, pears and tomatoes, as well as seeded fruits treated with 2,3,5-triiodobenzoic acid, frequently show symptoms of Ca deficiency and a low Ca content. It was concluded that auxins, probably produced by the seeds, play a significant role in Ca translocation into fruits. Exogenous indoleacetic acid but not 4-chlorophenoxyacetic acid applications could replace the effect of seeds in this respect. Auxin transport, rather than auxin accumulation, seems to be necessary for Ca transport, as can be concluded from the effect of auxin transport inhibitors.     

PINOID Kinase Regulates Root Gravitropism through Modulation of PIN2-Dependent Basipetal Auxin Transport in Arabidopsis

Reversible protein phosphorylation is a key regulatory mechanism governing polar auxin transport. We characterized the auxin transport and gravitropic phenotypes of the pinoid-9 (pid-9) mutant of Arabidopsis (Arabidopsis thaliana) and tested the hypothesis that phosphorylation mediated by PID kinase and dephosphorylation regulated by the ROOTS CURL IN NAPHTHYLPHTHALAMIC ACID1 (RCN1) protein might antagonistically regulate root auxin transport and gravity response. Basipetal indole-3-acetic acid transport and gravitropism are reduced in pid-9 seedlings, while acropetal transport and lateral root development are unchanged. Treatment of wild-type seedlings with the protein kinase inhibitor staurosporine phenocopies the reduced auxin transport and gravity response of pid-9, while pid-9 is resistant to inhibition by staurosporine. Staurosporine and the phosphatase inhibitor, cantharidin, delay the asymmetric expression of DR5∷revGFP (green fluorescent protein) at the root tip after gravistimulation. Gravity response defects of rcn1 and pid-9 are partially rescued by treatment with staurosporine and cantharidin, respectively. The pid-9 rcn1 double mutant has a more rapid gravitropic response than rcn1. These data are consistent with a reciprocal regulation of gravitropism by RCN1 and PID. Furthermore, the effect of staurosporine is lost in pinformed2 (pin2). Our data suggest that reduced PID kinase function inhibits gravitropism and basipetal indole-3-acetic acid transport. However, in contrast to PID overexpression studies, we observed wild-type asymmetric membrane distribution of the PIN2 protein in both pid-9 and wild-type root tips, although PIN2 accumulates in endomembrane structures in pid-9 roots. Similarly, staurosporine-treated plants expressing a PIN2∷GFP fusion exhibit endomembrane accumulation of PIN2∷GFP, but no changes in membrane asymmetries were detected. Our data suggest that PID plays a limited role in root development; loss of PID activity alters auxin transport and gravitropism without causing an obvious change in cellular polarity.A variety of important growth and developmental processes, including gravity response, embryo and vascular development, and the branching of roots and shoots, are controlled by the directional and regulated transport of auxin in higher plants. Reversible protein phosphorylation is an important regulatory strategy that may modulate auxin transport and dependent processes such as root gravitropism, perhaps through action of the PINOID (PID) kinase (for review, see DeLong et al., 2002; Galvan-Ampudia and Offringa, 2007). PID is an AGC family Ser/Thr kinase (Christensen et al., 2000) and belongs to an AGC kinase clade containing WAG1, WAG2, AGC3-4, and D6PK/AGC1-1 (Santner and Watson, 2006; Galvan-Ampudia and Offringa, 2007; Zourelidou et al., 2009). PID activity has been demonstrated in vitro and in vivo (Christensen et al., 2000; Michniewicz et al., 2007), and several pid mutant alleles exhibit altered auxin transport in the inflorescence and a floral development defect resembling that of auxin transport mutants (Bennett et al., 1995). Overexpression of the PID gene results in profound alterations in root development and responses to auxin transport inhibitors, reduced gravitropism and auxin accumulation at the root tip (Christensen et al., 2000; Benjamins et al., 2001; Michniewicz et al., 2007), as well as enhanced indole-3-acetic acid (IAA) efflux in tobacco (Nicotiana tabacum) cell cultures (Lee and Cho, 2006) and altered PINFORMED1 (PIN1), PIN2, and PIN4 localization patterns (Friml et al., 2004; Michniewicz et al., 2007), consistent with PID being a positive regulator of IAA efflux. However, the effects of pid loss-of-function mutations on auxin transport activities and gravitropic responses in roots have not yet been reported (Robert and Offringa, 2008).In contrast, auxin transport and gravitropism defects of a mutant with reduced protein phosphatase activity have been characterized in detail. The roots curl in naphthylphthalamic acid1 (rcn1) mutation, which ablates the function of a protein phosphatase 2A regulatory subunit, causes reduced PP2A activity in vivo and in vitro (Deruère et al., 1999). Roots and hypocotyls of rcn1 seedlings have elevated basipetal auxin transport (Deruère et al., 1999; Rashotte et al., 2001; Muday et al., 2006), and rcn1 roots exhibit a significant delay in gravitropism, consistent with altered auxin transport (Rashotte et al., 2001; Shin et al., 2005). These data indicate that PP2A is a negative regulator of basipetal transport and suggest that if PID-dependent phosphorylation regulates root auxin transport and gravitropism, then it may act in opposition to PP2A-dependent dephosphorylation.In roots, auxin transport is complex, with distinct sets of influx and efflux carriers that define tissue-specific and opposing directional polarities (for review, see Leyser, 2006). IAA moves acropetally, from the shoot toward the root apex, through the central cylinder (Tsurumi and Ohwaki, 1978), and basipetally, from the root apex toward the base, through the outer layer of cells (for review, see Muday and DeLong, 2001). When plants are reoriented relative to the gravity vector, auxin becomes asymmetrically distributed across the root tip, as a result of a process termed lateral auxin transport (for review, see Muday and Rahman, 2008). Several carriers that mediate root basipetal IAA transport have been clearly defined and include the influx carrier AUXIN-INSENSITIVE1 (AUX1; Marchant et al., 1999; Swarup et al., 2004; Yang et al., 2006) and efflux carriers of two classes, PIN2 (Chen et al., 1998; Müller et al., 1998; Rashotte et al., 2000) and ATP-BINDING CASSETTE TYPE B TRANSPORTER4/MULTIDRUG-RESISTANT4/P-GLYCOPROTEIN4 (ABCB4/MDR4/PGP4; Geisler et al., 2005; Terasaka et al., 2005; Lewis et al., 2007). Lateral transport at the root tip may be mediated by PIN3, an efflux carrier with a gravity-dependent localization pattern (Friml et al., 2002; Harrison and Masson, 2007).Gravitropic curvature of Arabidopsis (Arabidopsis thaliana) roots requires changes in IAA transport at the root tip (for review, see Muday and Rahman, 2008). Auxin transport inhibitors (Rashotte et al., 2000) and mutations in genes encoding basipetal transporters, including aux1 (Bennett et al., 1996), pin2/agr1 (Chen et al., 1998; Müller et al., 1998), and abcb4/mdr4/pgp4 (Lin and Wang, 2005; Lewis et al., 2007), alter gravitropism. Auxin-inducible reporters exhibit asymmetric expression across the root tip prior to differential growth, and this asymmetry is abolished by treatment with auxin transport inhibitors that prevent gravitropic curvature (Rashotte et al., 2001; Ottenschläger et al., 2003). Additionally, the pin3 mutant exhibits slightly reduced rates of gravitropic curvature (Harrison and Masson, 2007), and PIN3 is expressed in the columella cells, which are the site of gravity perception (Blancaflor et al., 1998; Friml et al., 2002). The PIN3 protein relocates to membranes on the lower side of columella cells after gravitropic reorientation, consistent with a role in facilitating asymmetric IAA transport at the root tip (Friml et al., 2002; Harrison and Masson, 2007).The available data suggest a model in which PID and RCN1 antagonistically regulate basipetal transport and gravitropic response in root tips (Fig. 1). In this model, the regions with the highest IAA concentrations in the epidermal and cortical cell layers are indicated by shading, and the arrows indicate the direction and relative amounts of basipetal auxin transport. Our previous work suggests that elevated basipetal IAA transport in rcn1 roots impairs gravitropic response, presumably due to the inability of roots either to form or to perceive a lateral auxin gradient in the context of a stronger polar IAA transport stream (Rashotte et al., 2001). Enhanced basipetal transport may increase the initial auxin concentration along the upper side of the root, impeding the establishment or perception of a gradient in rcn1 and cantharidin-treated wild-type roots (Fig. 1, right). Based on the published pid inflorescence transport data (Bennett et al., 1995), we hypothesize that pid seedling roots and staurosporine-treated wild-type roots have reduced basipetal auxin transport (Fig. 1, left). Upon reorientation of roots relative to the gravity vector, the reduced basipetal IAA transport in pid may lead to slower establishment of an auxin gradient across the root. This model then predicts that cantharidin treatment of pid-9 or staurosporine treatment of rcn1 seedlings would enhance or restore gravitropism in these mutants. Similarly, a double mutant might be expected to exhibit a corrected gravitropic response relative to the single mutants.Open in a separate windowFigure 1.Auxin transport defects in pid-9 and rcn1 mutants alter auxin redistribution after reorientation relative to the gravity vector. This model predicts that differences in basipetal auxin transport activities of wild-type, pid-9, and rcn1 roots will affect the formation of lateral auxin gradients. The shaded area in each root represents the region of highest IAA concentration in epidermal and cortical cells, with darker shading in the central columella cells, believed to be the auxin maxima. The direction and amount of basipetal IAA transport are indicated by arrows. The region of differential growth during gravitropic bending is indicated by the shaded rectangle. If auxin transport is reduced (as shown in the pid-9 mutant or in staurosporine-treated seedlings), this would lead to a slower formation of an auxin gradient in root tips. The rcn1 mutation (or treatment with cantharidin) has already been shown to lead to increased basipetal transport and a reduced rate of gravitropic bending, consistent with altered formation or perception of an auxin gradient. The antagonistic effects of kinase and phosphatase inhibition are predicted to lead to normal gravity responses in the pid-9 rcn1 double mutant as well as in pid-9 and rcn1 single mutants treated with the “reciprocal” inhibitor.The experiments described here were designed to test this model by examining gravitropism and root basipetal IAA transport in pid and staurosporine-treated seedlings. We investigated the regulation of gravity response by PID kinase and RCN1-dependent PP2A activities and observed antagonistic interactions between the rcn1 and pid-9 loss-of-function phenotypes that are consistent with reciprocal kinase/phosphatase regulation. We found that loss of kinase activity in the pid mutant and in staurosporine-treated wild-type plants inhibits basipetal auxin transport and the dependent physiological process of root gravitropism. Our results suggest that staurosporine acts to regulate these processes through inhibition of PID kinase and that PID effects are PIN2 dependent. In both wild-type and pid-9 roots, we observed polar membrane distribution of the PIN2 protein; unlike wild-type roots, though, pid-9 roots exhibited modest accumulation of PIN2 in endomembrane structures. Similarly, we detected asymmetric distribution and endomembrane accumulation of PIN2∷GFP in staurosporine-treated roots. Our data suggest that PID plays a limited role in root development; loss of PID activity alters PIN2 trafficking, auxin transport, and gravitropism without causing an obvious loss of cellular polarity. Together, these experiments provide insight into phosphorylation-mediated control of the gravity response and auxin transport in Arabidopsis roots.     

Auxin Stimulation of Cambial Activity in Pinus silvestris II. Dependence upon Basipetal Transport

Natural auxin content has been determined in the cambial region of large Pinus silvestris L. trees at various dates during the year. The tissue was collected from the stem of intact or ring-barked trees and from stumps remaining after the trees were cut down at breast height in early summer or late autumn. No seasonal decrease of concentration of the extractable auxin in the cambial region could be detected. Decapitation or ring-barking produced severe reduction in auxin content and arrested cambial division. In the next season the auxin level and the cambial activity remained completely depressed. It is concluded that without tissue continuity in the region external to xylem and without basipetal supply of substances, no mechanism operated by roots or remaining stem tissue near the tree base can ensure a high level of auxin in the cambial region or activate and maintain the cambial division. The activity of extracted pine auxin was found not to be identical with the stimulatory potential of authentic IAA determined by standard bioassays. The possibility of interaction with other extracted substances is discussed.     

Some ultrastructural observations on the nature of foliar abscission in Impatiens sultani

Summary Both scanning and transmission electron microscopes have been used to study the anatomy of the abscission zone of Impatiens sultani Hook. Evidence is presented to show that the fracture line follows the middle lamella in all the living cells of the abscission zone including those in the vascular traces. The separation of these cells is preceded by a breakdown of the middle lamellar region of the wall. The characteristics of this process vary in different cell types. Accompanying this breakdown is an enlargement of inner cortex cells mainly in a direction parallel to the axis of the petiole. It is suggested that this expansion of cells is necessary to produce the tensions which rupture the cuticle and xylem vessels prior to separation. The occurrence of transfer cells and tyloses in the abscission zone is also described and the physiological implications of their presence discussed.     

PIN蛋白在生长素极性运输中的作用

PIN蛋白是生长素流出栽体,它在细胞中的不对称分布决定细胞间生长素流方向.PIN蛋白网络系统决定生长素的极性运输,为植物体各部位的细胞提供了特异的位置和方向信息.从细胞水平上介绍PIN蛋白在生长素极性运输中的作用及对PIN蛋白功能调节的研究进展.     

Role of IAA-Oxidase in Abscission Control in Cotton

The potential role of indoleactic acid (IAA)-oxidase as an in vivo abscission regulating system in the cotton (Gossypium hirsutum L.) cotyledonary explant was investigated. Phenols (usually monophenols), which are cofactors of cotton IAA-oxidase in vitro, accelerated abscission. Phenols (usually orthodihydroxyphenols), which inhibit cotton IAA-oxidase in vitro, inhibited abscission. Inhibition or stimulation of abscission was accomplished by phenols both with and without IAA. Results were similar when treatments were applied as lanolin pastes to the cut petiole ends or as solutions in which explants were submerged. An abscission accelerating phenol stimulated the decarboxylation of IAA-1-¹⁴C by explants and an abscission inhibiting phenol inhibited the decarboxylation of IAA-1-¹⁴C.     

The Role of Auxin in the Apical Regulation of Leaf Abscission in Cotton (Gossypium hirsutum L.)

The petiole abscission induced by deblading cotyledonary leavesof cotton (Gossypium hirsutum L. cv. Delta Pine) was acceleratedby the presence of the intact shoot apex or, in decapitatedplants and explants, by application to the stem (proximal application)of indol-3yl-acetic acid (IAA) or 1-aminocyclopropane-l-carboxylicacid (ACC). IAA and ACC accelerated the abscission of debladedpetioles whether applied above or below the cotyledonary node.Transport of IAA to the node was not required for the responseto proximal IAA. [2,3-¹⁴C]ACC was readily transported to thenodal region whether applied to the stem above or below thenode. Application of IAA or ACC to the stem did not induce theabscission of intact leaves or of debladed petioles treateddistally with IAA The acceleration of abscission by proximal IAA, but not thatcaused by ACC, was prevented if explants were treated with a-aminooxyaceticacid (AOA), an inhibitor of ACC-synthase. AOA also preventedthe acceleration of abscission caused by the shoot apex. Theprogress of abscission in debladed explants was greatly delayedby silver thiosulphate (STS—an inhibitor of ethylene action),whether or not the explants were treated with IAA or ACC. Itis suggested that the speeding effects of the shoot apex andof proximal auxin on the abscission of debladed petioles requiresauxin-induced ACC synthesis. The possibility is discussed thatACC may function as a mobile abscission promoter Key words: Abscission, ACC, ACC-synthase, cotton (Gossypium), proximal auxin     

On the Question of Stem Polarity with Respect to Auxin Transport

The effect of section length and number of longitudinally contiguous cells upon polar transport of natural auxin from the pine stem cambial region was investigated with oat coleoptile curvature tests. Basipetal and acropetal efflux of auxin to agar declines with increasing length of the sections, but the polarity quotient varies little and is similar to the polarity of individual cells. An integrated system of cells produces a wave along the stem in the efflux of auxin from consecutive segments. The possible role of such waves in development of polarity gradients and of the morphogenic maps of orientation of cells in the stem cambial region is discussed.     

Rhythmicity in the Basipetal Transport of Indoleacetic Acid through Coleoptiles

¹⁴C-Indoleacetic acid was applied to coleoptiles of corn (Zea mays) and oat (Avena sativa). The coleoptiles were detached from the endosperms at 6-minute intervals after indoleacetic acid application, and the radioactivity was determined in successive 2-millimeter regions. The rate (per cent per minute) of basipetal transport of indoleacetic acid is periodic in various regions of the coleoptile, with a period of about 20 minutes. The possible relation of this cyclic phenomenon to other rhythmic processes of similar periodicities is discussed. A distinct acropetal transport (against the concentration gradient) from the subapical region to the apical 2-millimeter region of the coleoptile was detected.     

Auxin-Induced Changes in Protein Synthesis in the Abscission Zone of Bean Explants

Treatment of bean pulvinar explants with auxin significantlydelayed abscission. The pattern of protein synthesis in beanexplants that were treated with and without auxin was investigatedby labelling the pulvinar segments with [³⁵S]-methionine andanalyzing the polypeptides by one and two dimensional gel electrophoresis.One dimensional gel electrophoresis of labelled proteins revealedan increased synthesis of 63, 54 and 29 kDa polypeptides anddecreased synthesis of 60, 49, 30 and 23 kDa polypeptides inthe presence of auxin. Further analysis of proteins on two dimensionalgels revealed several differences in polypeptides between controland auxin-treated explants. These results provide evidence forthe alteration of protein synthesis by auxin in bean explantsand suggest that auxin delays abscission by regulating the synthesisof specific polypeptides. ¹Scientific Paper No. 7934, College of Agriculture and HomeEconomics Research Center, Washington State University, Pullman,Washington, Project 0321. ²Supported by National Science Foundation grant DCB-8502215to BWP (Received September 11, 1987; Accepted November 12, 1987)