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.     

Inhibited polar auxin transport results in aberrant embryo development in Norway spruce

Current hypotheses concerning the role of polar auxin transport in embryo development are entirely based on studies of angiosperms, while little is known about how auxin regulates pattern formation in gymnosperms. In this study, different developmental stages of somatic embryos of Norway spruce (Picea abies) were treated with the polar auxin transport inhibitor 1-N-naphtylphthalamic acid (NPA). Effects of the treatments on auxin content, embryo differentiation and programmed cell death (PCD) were analysed. During early embryo development, NPA-treatment led to increased indole-3-acetic acid (IAA) content, abnormal cell divisions and decreased PCD, resulting in aberrant development of embryonal tube cells and suspensors. Mature embryos that had been treated with NPA showed both apical and basal abnormalities. Typically the embryos had abnormal cotyledon formation and irregular cell divisions in the area of the root meristem. Our results show that polar auxin transport is essential for the correct patterning of both apical and basal parts of conifer embryos throughout the whole developmental process. Furthermore, the aberrant morhologies of NPA-treated spruce embryos are comparable with several auxin response and transport mutants in Arabidopsis. This suggests that the role of polar auxin transport is conserved between angiosperms and gymnosperms.     

On polar auxin transport in plant cells

We present here explicit mathematical formulas for calculating the concentration, mass, and velocity of movement of the center of mass of the plant growth regulator auxin during its polar movement through a linear file of cells. The results of numerical computations for two cases, (a) the conservative, in which the mass in the system remains constant and (b) the non-conservative, in which the system acquires mass at one end and loses it at the other, are graphically presented. Our approach differs from that of Mitchison's (Mitchison 1980) in considering both initial effects of loading and end effects of substance leaving the file of cells. We find the velocity varies greatly as mass is entering or leaving the file of cells but remains constant as long as most of the mass is within the cells. This is also the time for which Mitchison's formula for the velocity, which neglects end effects, reflects the true velocity of auxin movement. Finally, the predictions of the model are compared with two sets of experimental data. Movement of a pulse of auxin through corn coleoptiles is well described by the theory. Movement of auxin through zucchini shoots, however, shows the need to take into account immobilization of auxin by this tissue during the course of transport.     

生长素极性运输研究进展

生长素极性运输与植物生长发育密切相关并受许多因素调控,生长素极性运输机理方面已取得较大进展,但仍有一些亟待解决的问题.研究植物生长素极性运输的生理机制及其调控具有十分重要的意义.通过了解生长素在植物生长发育中的作用,进而阐述生长素极性运输机理方面的研究进展.     

Modulation of the oscillatory system involved in polar transport of auxin by other phytohormones

A wave-like pattern of the basipetal efflux of natural auxin from the cambial region of a series of consecutive short sections of stems of Larix decidua Mill., Acer pseudoplatanus L. and Picea abies (L.) Karst. has been demonstrated as it was earlier reported for Pinus silvestris L. Apical application of ABA suppressed the IAA-stimulated increase of the auxin-wave amplitude, and zeatin or GA₃ prevented this repression in stem segments of Pinus silvestris . All the exogenously applied substances were highly effective in physiological concentrations. Already 20-min of exposure to IAA or ABA at the apical end produced modulations of the auxin-wave along the whole 6.6 cm long stem segment. Application of 2, 3, 5-triiodobenzoic acid (TIBA) caused suppression of the wave-like pattern of auxin efflux similarly as ABA, supporting the association of the modulatory effects of ABA with the phenomena involved in polar transport of auxin. Abscisic acid applied to the basal end of the stem segment also reduced the auxin-wave amplification caused by simultaneous supply of IAA to the apical end. This finding additionally confirms the hypothesis that: 1) the supracellular auxin-wave generation is associated with the functioning of a system of oscillators coupled at the cellular level and 2) the auxin-wave modulations can be propagated acropetally, that is against the main direction of the auxin molecular transport.     

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.     

Shedding light on auxin movement: Light-regulation of polar auxin transport in the photocontrol of plant development

By being sessile, plants have evolved a remarkable capacity to perceive and respond to changes in environmental conditions throughout their life cycle. Light represents probably the most important environmental factor that impinge on plant development because, other than supplying the energy source for photosynthesis, it also provides seasonal and positional information that are essential for the plant survival and fitness. Changes in the light environment can dramatically alter plant morphogenesis, especially during the early phases of plant life, and a compelling amount of evidence indicates that light-mediated changes in auxin homeostasis are central in these processes. Auxin exerts its morphogenetic action through instructive hormone gradients that drive developmental programs of plants. Such gradients are formed and maintained via an accurate control on directional auxin transport. This review summarizes the recent advances in understanding the influence of the light environment on polar auxin transport.     

A role for polar auxin transport in rhizogenesis

Internodal shoot sections of the easy-to-root Forsythia×intermedia cv. Lynwood, and the difficult-to-root Syringa vulgaris cv. Madame Lemoine were used in vitro to investigate the role of polar auxin transport (PAT) in rhizogenesis. Syringa internodes required the distal application of indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) or naphthaleneacetic acid to induce rooting, while 2,4-dichlorophenoxyacetic acid was ineffective. In contrast, Forsythia internodes rooted equally well when IBA was applied at either end of the internode. Using [³H]IAA showed transport of exogenous auxin was basipetal, and that despite similar transport velocities, the intensity of auxin transport in Syringa was greater than in Forsythia. Basipetal transport of exogenous auxin was blocked using the PAT inhibitors 2,3,5-triiodobenzoic acid (TIBA) and naringenin (Nar); where Forsythia proved more sensitive to TIBA, but less so to Nar, in comparison with Syringa. In both species, percentage rooting and the number of roots formed were greater in 5-mm-long internodes than in shorter internodes. The results demonstrate the importance of PAT for root initiation in Syringa, whereas Forsythia tissue appears to be more sensitive to the direct application of auxin.     

Jasmonic acid modulates xylem development by controlling polar auxin transport in vascular tissues

Development of xylem cells is affected by environmental stresses such as drought and oxidative stress, and recent findings suggested that jasmonic acid (JA) mediates this process through interaction with other phytohormones such as cytokinin. In this study, we showed that polar auxin transport regulated by PIN3 and PIN7 is involved in the JA-mediated xylem development in vascular tissues. The mutant plants that lack the activity of PIN3 and PIN7 responsible for the auxin transport developed extra xylems in vascular tissues such as the JA-treated wild-type plants. Visualization of auxin response and xylem development in the roots treated with NPA, an inhibitor of polar auxin transport, suggested that disruption of polar auxin transport is involved in the xylem phenotype of pin3 pin7 double mutants. We also found that cytokinin increases expressions of PIN3 and PIN7 responsible for the auxin transport while JA decreases only PIN7. These suggested that PIN7-mediated polar auxin transport system modulates xylem development in response to JA. The finding that JA affects auxin distribution in root vascular tissues further supported this. Collectively, these suggest that JA promotes xylem development by disrupting auxin transport in vascular tissues, and the auxin efflux genes, more especially PIN7 whose expression is suppressed by JA mediates this process.     

Narciclasine modulates polar auxin transport in Arabidopsis roots

Plant development displays an exceptional plasticity and adaptability that involves the dynamic, asymmetric distribution of the phytohormone auxin. Polar auxin flow, which requires transport facilitators of the PIN family, largely contributes to the establishment and maintenance of auxin gradients and mediates multiple developmental processes. Here, we report the effects of narciclasine (NCS), an Amaryllidaceae alkaloid isolated from Narcissus tazetta bulbs, on postembryonic development of Arabidopsis roots. Arabidopsis seedlings grown on NCS showed defects in root gravitropism which correlates with a reduction in auxin transport in roots. Expressions of auxin transport genes were affected and the polar localization of PIN2 protein was altered under NCS treatment. Taken together, we propose that NCS modulates auxin transport gene expression and PIN2 localization, and thus affects auxin transport and auxin distribution necessary for postembryonic development of Arabidopsis roots.     

The PINOID protein kinase regulates organ development in Arabidopsis by enhancing polar auxin transport

Arabidopsis pinoid mutants show a strong phenotypic resemblance to the pin-formed mutant that is disrupted in polar auxin transport. The PINOID gene was recently cloned and found to encode a protein-serine/threonine kinase. Here we show that the PINOID gene is inducible by auxin and that the protein kinase is present in the primordia of cotyledons, leaves and floral organs and in vascular tissue in developing organs or proximal to meristems. Overexpression of PINOID under the control of the constitutive CaMV 35S promoter (35S::PID) resulted in phenotypes also observed in mutants with altered sensitivity to or transport of auxin. A remarkable characteristic of high expressing 35S::PID seedlings was a frequent collapse of the primary root meristem. This event triggered lateral root formation, a process that was initially inhibited in these seedlings. Both meristem organisation and growth of the primary root were rescued when seedlings were grown in the presence of polar auxin transport inhibitors, such as naphthylphtalamic acid (NPA). Moreover, ectopic expression of PINOID cDNA under control of the epidermis-specific LTP1 promoter provided further evidence for the NPA-sensitive action of PINOID. The results presented here indicate that PINOID functions as a positive regulator of polar auxin transport. We propose that PINOID is involved in the fine-tuning of polar auxin transport during organ formation in response to local auxin concentrations.     

Nature of cell-to-cell transfer of auxin in polar transport

Summary By application of agar blocks (side blocks) against the inner and outer epidermis of maize (Zea mays L.) coleoptiles whose cuticle has been abraded it is found that radioactive auxin in the polar transport stream exchanges rapidly with the tissue's free space and therefore does not move confined within the symplast. Polar transport of IAA is demonstrable in Avena coleoptile segments plasmolyzed in 0.5 and 0.7 M mannitol, in which most of the plasmodesmatal connections between successive cells in the polar transport pathway appear to have been broken. We conclude that during polar transport IAA probably moves from cell to cell by crossing the plasmalemmas and the free space between successive cells, rather than via plasmodesmata. Auxin in the polar transport stream exchanges rapidly with side blocks by a cyanide-and azide-insensitive, presumably passive, process. A similarly passive uptake takes place into the cells from an external donor. NPA almost completely inhibits efflux from the polar transport stream even though it does not inhibit uptake; its inhibition of efflux is completely reversed by azide or cyanide. These findings are compatible either with the traditional model of polar transport as passive uptake combined with an active basal efflux pump for IAA, or with the model of purely passive polar transport driven by pH and/or potential differences across the plasma membrane, provided certain ad hoc assumptions are made about the characteristics of the IAA anion carrier that would be operating in either model.Abbreviations IAA indoleacetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid     

Vascular transport and soybean nodule function: nodule xylem is a blind alley, not a throughway

Abstract. The only published consideration of product removal from the soybean root nodule hypothesizes that the peripheral xylem circuit of this determinate nodule structure is flushed by the transpiration stream. However, dyes fed to the transpiration stream through a cut root distal to the nodule do not enter the nodule, and the observed movement of radio-tracers from the root into the nodule can be explained by simple diffusion, Also, there are few xylem elements in the nodule, and these elements are of a small diameter, such that this path can not act as a functional loop of the root system. Further, in this study, nodule vascular strands were never observed to be continuous about the nodule, but were observed to end at the nodule tip in a loop within an intact, closed endodermal sac. Nodule vascular tissue was invested in a pericycle of at least three cell layers. These cells are suggested to be active in the loading of the xylem apoplast with ureides, such that the xylem of the nodule always operates in an export role. Nodule water requirements may be supplied via the phloem or the root cortex apoplasm.     

Difluoromethylornithine counteracts effects of auxins and inhibitors of polar auxin transport on plant development

Difluoromethylornithine (DFMO) counteracted several processes that are promoted or inhibited by auxins or inhibitors of polar auxin transport: inhibition of asymmetric gene expression in carrot, stimulation of gametic embryogenesis inBrassica, inhibition of root elongation in tobacco, inhibition of the development of lateral roots in pea and adventitious roots in apple, and inhibition of floral bud formation inArabidopsis     

Inhibition of the polar auxin transport by a morphactin

Summary IAA- and gravity-induced curvatures in coleoptiles are altered by the morphactin methyl-2-chloro-9-hydroxyfluorene-(9)-carboxylate (CFM), the length of the curved part of the coleoptile being greatly reduced. A ring of CFM-containing paste blocks the bud-inhibiting effect of IAA when placed between the bud and the site of IAA application. The IAA-transporting capacity of Helianthus hypocotyl sections, as determined by the classical transport method (agar donors and receivers), is greatly reduced by a pretreatment of the sections with CFM.     

Disruption of auxin transport is associated with aberrant leaf development in maize

Despite recent progress, the mechanisms governing shoot morphogenesis in higher plants are only partially understood. Classical physiological studies have suggested that gradients of the plant growth regulator auxin may play a role in controlling tissue differentiation in shoots. More recent molecular genetic studies have also identified knotted1 like homeobox (knox) genes as important regulators of shoot development. The maize (Zea mays L.) mutant rough sheath2 (rs2) displays ectopic expression of at least three knox genes and consequently conditions a range of shoot and leaf phenotypes, including aberrant vascular development, ligular displacements, and dwarfism (R. Schneeberger, M. Tsiantis, M. Freeling, J.A. Langdale [1998] Development 125: 2857–2865). In this report, we show that rs2 mutants also display decreased polar auxin transport in the shoot. We also demonstrate that germination of wild-type maize seedlings on agents known to inhibit polar auxin transport mimics aspects of the rs2 mutant phenotype. The phenotype elaborated in inhibitor-treated plants is not correlated with ectopic KNOX protein accumulation.