Abscission: the role of ethylene modification of auxin transport

The role of ethylene-mediated reduction of auxin transport in natural and ethylene-induced leaf abscission was studied in the cotton (Gossypium hirsutum L., cv. Stoneville 213) cotyledonary leaf system. The threshold level of ethylene required to cause abscission of intact leaves was between 0.08 and 1 μl/l with abscission generally occurring 12 to 24 hours following ethylene fumigation. The threshold level of ethylene required to reduce the auxin transport capacity in the cotyle-donary petiole paralleled that required for stimulation of abscission. In plants where cotyledons are allowed to senesce naturally there is a decline in auxin transport capacity of petioles and increase in ethylene synthesis of cotyledons. The visible senescence process which precedes abscission requires up to 11 days, and increases in ethylene production rates and internal levels were detected well before abscission. Ethylene production rates for entire cotyledons rose to 2.5 mμ1 g⁻¹ hr⁻¹ and internal levels of 0.7 μl/l were observed. These levels appear to be high enough to cause the observed decline in auxin transport capacity. These findings, along with those of others, indicate that ethylene has several roles in abscission control (e.g., transport modification, enzyme induction, enzyme secretion). The data indicate that ethylene modification of auxin transport participates in both natural abscission and abscission hastened by exogenous ethylene.     

Effect of auxin transport inhibitors and ethylene on the wood anatomy of poplar

The influence of the auxin transport inhibitors naphthylphthalamic acid (NPA) and methyl-2-chloro-9-hydroxyflurene-9-carboxylate (CF), as well as the gaseous hormone ethylene on cambial differentiation of poplar was determined. NPA treatment induced clustering of vessels and increased vessel length. CF caused a synchronized differentiation of cambial cells into either vessel elements or fibres. The vessels in CF-treated wood were significantly smaller and fibre area was increased compared with controls. Under the influence of ethylene, the cambium produced more parenchyma, shorter fibres and shorter vessels than in controls. Since poplar is the model tree for molecular biology of wood formation, the modulation of the cambial differentiation of poplar towards specific cell types opens an avenue to study genes important for the development of vessels or fibres.     

Time sequence of the effect of ethylene on transport, uptake and decarboxylation of auxin

The effect of ethylene on the uptake, decarboxylation and basipetaltransport of IAA-1-¹⁴C, IAA-2-¹⁴C and NAA-1-¹⁴C in cotton stemsections (Gossypium hirsutum L., var. Stoneville 213) was studied.A reduction in the capacity of cotton stem sections to transportauxin basipetally appears in sections excised from plants exposedto ethylene for only 3 hr and increases with fumigation time. In addition to reducing transport, increasing ethylene pretreatmentperiods from 3 to 15 hr also progressively reduced the uptakeof ¹⁴C and increased the release of ¹⁴C as ¹⁴CO₂ from IAA-1-¹⁴C.The effect of ethylene on the decarboxylation of IAA-1-¹⁴C wassignificant when expressed as either the cpm of ¹⁴C releasedper hr per mg dry weight or the cpm released per hr per mm²in contact with the IAA donor. Comparative experiments usingIAA-1-¹⁴C and IAA-2-¹⁴C demonstrated that the effect of ethyleneon the decarboxylation of IAA was primarily a cut surface effectwhich apparently contributes to the reduction of IAA uptakeby ethylene. Although ethylene significantly reduced the transport of NAA-1-¹⁴C,uptake was significantly increased rather than decreased aswith IAA-1-¹⁴C while decarboxylation was unaffected. Ethylene pretreatment caused no significant changes in the dryweight or the cross-sectional area of the absorbing surfaceof the transport tissue. ¹A contribution of the Texas Agricultural Experiment Station.Supported in part by Grant GB-5640, National Science Foundationand grants from the Cotton Producers Institute and the NationalCotton Council of America. ²Present address: Central Research Department, E. I. Du PontDe Nemours and Company, Wilmington, Delaware 19898, U. S. A. (Received May 29, 1969; )     

Effects of ethylene and auxin on polyamine levels in suspension-cultured tobacco cells

The effects of ethylene and auxin on polyamine levels were studied in suspension-cultured cells of tobacco ( Nicotiana tabacum . L). Treatment of 4-day-cultured cells with ethylene increased the levels of spermidine and spermine. The activities of arginine decarboxylase (ADC; EC 4.1.1.19), ornithine decarboxylase (ODC: EC 4.1.1.17), and S -adenosylmethionine decarboxylase (SAMDC: EC 4.1.1.50) rapidly increased between 3 and 12 h. An auxin, indole-3-acetic acid (IAA), increased polyamine levels and activities of ADC, ODC and SAMDC. The spermine level continued to increase significantly during a 24-h incubation with IAA. The increases in polyamine accumulation induced by ethylene were partially offset by an inhibitor of ethylene action, 2,5-norbornadiene. It is suggested that the auxin-induced polyamine accumulation occurred directly, without metabolic competition between ethylene and polyamine biosynthesis, and indirectly, through auxin-induced ethylene formation.     

Sites of auxin action: regulation of geotropism, growth, and ethylene production by inhibitors of auxin transport

The inhibitors of auxin transport-NPA (N-1-naphthylphthalamic acid), DPX1840 (3,3a-dihydro-2-(p-methoxyphenyl)-8H-pyrazolo[5,1-a] isoindol-8-one), and TIBA (2,3,5-triiodobenzoic acid)-inhibited geotropism in roots of intact Pisum sativum L. seedlings. NPA and DPX1840 also caused cellular swelling in the roots. The swelling was due to a greater inhibition of elongation than increase in weight and looked identical to the one caused by ethylene. However, ethylene did not act as an intermediate in the action of auxin transport inhibitors because all three failed to stimulate ethylene production and some of their growth-inhibiting effect was retained in the presence of saturating levels of ethylene. In the presence of 10 mum indoleacetic acid the growth-inhibiting effect of auxin transport inhibitors was lost after 18 hours. On the other hand, auxin transport inhibitors did not interfere with the ability of auxin to promote ethylene production. Growth inhibition caused by auxin transport inhibitors was reversible. Pea root sections resumed normal growth following flushing of treated sections with inhibitor-free solutions. Experiments with (14)C-2, 4-dichlorophenoxyacetic acid revealed that the herbicide and auxin transport inhibitors may have the same binding site. It was concluded that a class of structurally dissimilar compounds may share a similar physiological role since they all appear to compete with endogenous auxin for certain binding sites and they all have similar growth-regulating activities.     

The POLARIS peptide of Arabidopsis regulates auxin transport and root growth via effects on ethylene signaling

The rate and plane of cell division and anisotropic cell growth are critical for plant development and are regulated by diverse mechanisms involving several hormone signaling pathways. Little is known about peptide signaling in plant growth; however, Arabidopsis thaliana POLARIS (PLS), encoding a 36-amino acid peptide, is required for correct root growth and vascular development. Mutational analysis implicates a role for the peptide in hormone responses, but the basis of PLS action is obscure. Using the Arabidopsis root as a model to study PLS action in plant development, we discovered a link between PLS, ethylene signaling, auxin homeostasis, and microtubule cytoskeleton dynamics. Mutation of PLS results in an enhanced ethylene-response phenotype, defective auxin transport and homeostasis, and altered microtubule sensitivity to inhibitors. These defects, along with the short-root phenotype, are suppressed by genetic and pharmacological inhibition of ethylene action. PLS expression is repressed by ethylene and induced by auxin. Our results suggest a mechanism whereby PLS negatively regulates ethylene responses to modulate cell division and expansion via downstream effects on microtubule cytoskeleton dynamics and auxin signaling, thereby influencing root growth and lateral root development. This mechanism involves a regulatory loop of auxin-ethylene interactions.     

Effect of auxin on acropetal auxin transport in roots of corn

Acropetal [¹⁴C]indoleacetic acid (IAA) transport was investigated in roots of corn. At least 40 to 50% of this movement is dependent on activities in the root apex. Selective excision of various populations of cells comprising the root apex, e.g. the root cap, quiescent center, or proximal meristem show that the proximal meristem is the critical region in the apex with regard to influencing IAA movement. The quiescent center has no influence and the root cap has only a minor effect. Excision and replacement of the proximal meristem with an exogenous supply of 10⁻⁸ to 10⁻⁹ molar IAA prevents the reduction in acropetal IAA transport which would normally occur in the absence of this meristem. Substituting 10⁻⁹ molar IAA for the excised root cap brings about a significant increase in the amount of IAA moved acropetally, as compared to intact roots with the root cap still in place. From this and previous work, it is concluded that IAA synthesis occurring in the proximal meristem stimulates the movement of IAA from the basal to apical end of the root.     

Effects of osmotic stress on polar auxin transport in Avena mesocotyl sections

Segments of mesocotyls of Avena sativa L. transported [1-¹⁴C]indol-3yl-acetic acid (IAA) with strictly basipetal polarity. Treatment of the segments with solutions of sorbitol caused a striking increase in basipetal auxin transport, which was greatest at concentrations around 0.5 M. Similar effects were observed with mannitol or quebrachitol as osmotica, but with glucose or sucrose the increases were smaller. Polar transport was still detectable in segments treated with 1.2 M sorbitol. The effects of osmotic stress on the polar transport of auxin were reversible, but treatment with sorbital solutions more concentrated than 0.5 M reduced the subsequent ability of mesocotyl segments to grow in response to IAA. The increased transport of auxin in the osmotically stressed segments could not be explained in terms of an increased uptake from donor blocks. The velocity of transport declined with higher concentrations of osmoticum. The reasons for the enhancement of auxin transport by osmotic stress are not known.     

Two components of auxin transport

The transport of indoleacetic acid-1-¹⁴C out of sunflower stem sections has been analyzed by a compartmental analysis procedure in which the radioactivity moving out of the tissue (log per cent) is plotted against time. The analysis indicates that indoleacetic acid is transported via a fast transport system (t_½ of about 30 minutes) and a slow transport system (t_½ about 10 hours). While we do not know the sources of these two pools, by analogy with ion transport studies, the fast efflux is characteristic of transport from the cytoplasm across the plasmalemma and the slow efflux is characteristic of transport across the tonoplast and thus out of the vacuole. Both components of transport are inhibited by 2,3,5-triiodobenzoic acid.     

Kinetics of polar auxin transport

The movement of auxin in the basipetal and acropetal directions is compared for 4 types of tissue. It is observed that the transport may proceed in either a linear or a non-linear manner with time. The polarity of transport through any given type of tissue increases exponentially with increasing lengths of tissue traversed, suggesting that the polarity of transport is developed as a consequence of the repeated passage through cells. Using the mathematical model of Leopold and Hall, the extent of polarity for individual cells is estimated, and a very small polarity of individual cells is found to be capable of accounting for the marked polarity of whole tissues. It is suggested that transport polarity may be functionally a property of the multicellular structure, being amplified from very small differences in activities at the 2 ends of individual cells.     

Effects of brassinosteroid, auxin, and cytokinin on ethylene production in Arabidopsis thaliana plants

Inflorescence stalks produced the highest amount of ethylene in response to IAA as compared with other plant parts tested. Leaf age had an effect on IAA-induced ethylene with the youngest leaves showing the greatest stimulation. The highest amount of IAA-induced ethylene was produced in the root or inflorescence tip with regions below this producing less. Inflorescence stalks treated with IAA, 2,4-D, or NAA over a range of concentrations exhibited an increase in ethylene production starting at 1 microM with increasingly greater responses up to 100 microM, followed by a plateau at 500 microM and a significant decline at 1000 microM. Both 2,4-D and NAA elicited a greater response than IAA at all concentrations tested in inflorescence stalks. Inflorescence leaves treated with IAA, 2,4-D, or NAA exhibited the same trend as inflorescence stalks. However, they produced significantly less ethylene. Inflorescence stalks and leaves treated with 100 microM IAA exhibited a dramatic increase in ethylene production 2 h following treatment initiation. Inflorescence stalks showed a further increase 4 h following treatment initiation and no further increase at 6 h. However, there was a slight decline between 6 h and 24 h. Inflorescence leaves exhibited similar rates of IAA-induced ethylene between 2 h and 24 h. Light and high temperature caused a decrease in IAA-induced ethylene in both inflorescence stalks and leaves. Three auxin-insensitive mutants were evaluated for their inflorescence's responsiveness to IAA. aux2 did not produce ethylene in response to 100 microM IAA, while axr1-3 and axr1-12 showed reduced levels of IAA-induced ethylene as compared with Columbia wild type. Inflorescences treated with brassinolide alone had no effect on ethylene production. However, when brassinolide was used in combination with IAA there was a dramatic increase in ethylene production above the induction promoted by IAA alone.     

Subcellular trafficking of PIN auxin efflux carriers in auxin transport

The directional transport of the plant hormone auxin is a unique process mediating a wide variety of developmental processes. Auxin movement between cells depends on AUX1/LAX, PGP and PIN protein families that mediate auxin transport across the plasma membrane. The directionality of auxin flow within tissues is largely determined by polar, subcellular localization of PIN auxin efflux carriers. PIN proteins undergo rapid subcellular dynamics that is important for the process of auxin transport and its directionality. Furthermore, various environmental and endogenous signals can modulate trafficking and polarity of PIN proteins and by this mechanism change auxin distribution. Thus, the subcellular dynamics of auxin transport proteins represents an important interface between cellular processes and development of the whole plant. This review summarizes our recent contributions to the field of PIN trafficking and auxin transport regulation.     

Effect of ethylene on auxin transport and metabolism in midrib sections in relation to leaf abscission of woody plants

Abstract The relationship between ethylene-induced leaf abscission and ethylene-induced inhibition of auxin transport in midrib sections of the leaf blade of Citrus sinensis L. Osbeck, Populus deltoides Bart, and Eucalyptus camaldulensis Dehn. was studied. These species differed greatly in their abscission response to ethylene. The kinetic trend of abscission resembled that of the inhibition of auxin transport in all three species. It is suggested that one of the main actions of ethylene in the leaf blade is to inhibit auxin transport in the veinal tissues, thus reducing the amount of auxin transported from the leaf blade to the abscission zone. Ethylene inhibited transport of both IAA (indole-3-acetic acid) and NAA (α-naphthaleneacetic acid) in the midrib sections. However, while ethylene enhanced the conjugation of IAA with aspartic acid and glucose in the apical (absorbing) segment of the midrib sections, it had little effect on the conjugation of NAA. The data indicate that auxin destruction through conjugation does not play a major role in the inhibition of auxin transport by ethylene.     

The dependence of auxin transport on cell length

Abstract. Velocities of transport of IAA through long-celled (dark-grown) and short-celled (light-grown) coleoptile segments have been measured by the intercept method. Transport is faster through short-celled segments, and the difference is highly significant. Calculations show that this finding is consistent with a model for polar auxin transport which postulates a pumping mechanism between cells and movement through the cell by diffusion.     

Effects of morphactin and other auxin transport inhibitors on soybean senescence and pod development

Because triiodobenzoic acid increases pod number, albeit variably, in soybean (Glycine max), we tested other auxin-transport inhibitors. Morphactins, especially methylchlorflurenol (MCF), were found to be very active (optimal concentration 10 micromolar) when sprayed onto the foliage. Applications at 1 week after the start of flowering were most effective, producing a 40% increase in pod number with little inhibition (12%) of stem elongation. MCF increased the number of pods initiated (reaching 1 cm length) at least partially by prolonging the initiation period, while pod abortion (failure of pods > 1 cm long) remained low. Generally, MCF did not increase seed yield (dry weight/plant); more, but smaller seeds, were formed by the treated plants. The promotive effect of MCF on pod initiation seems to be independent of its inhibition of stem elongation, which is insignificant at 10 micromolar. MCF delayed pod maturation by 3 to 4 days, while foliar yellowing, blade abscission, and petiole abscission were retarded by 2, 4, and 2 days, respectively. MCF has only a small effect on senescence and that could be indirect, due to a delay in pod development. Other auxin-transport inhibitors tested, including N-1-naphthylphthalamic acid, produced little or no increase in pod number; however, 0.1 millimolar 5-[2′-carboxyphenyl]-3-phenylpyrazole caused a 27% increase. These results implicate auxin as a potential regulator of pod development, and they show that soybean seed yield is not simply sink limited.     

Mathematical model of polar auxin transport

Polar auxin transport can be simulated by a model which achieves polarity through the preferential secretion of more auxin from the lower end than from the upper end of each cell. Solution of the model using a computer provides a possible explanation of the differences between the polarity expressed by different tissues and the differences between pieces of different lengths, on the basis of small differences in the polarity of auxin secretion from individual cells. A method of estimating the polarity of individual cells is described.