Phloem differentiation in Coleus pith explants

A simple tissue culture protocol was used to examine phloem differentiation. Sieve tube elements differentiated from Coleus blumei Benth. pith parenchyma when pith blocks were explanted on to an agar medium containing sucrose and indole acetic acid. Growth regulator concentration had surprisingly little influence on the production of phloem cells. Increasing the availability of sucrose, however, resulted in an enhancement of phloem differentiation. Evidently phloem and xylem differentiation began at the same time in this system, and the concentration of sucrose determined the relative production of phloem cells.     

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.     

A role of auxin in phloem regeneration in Coleus internodes

The physiology of regenerative phloem formation in the number 5 internode of Coleus blumei Benth. was studied by estimating quantitatively its extent in plants or plant parts wounded so as to sever one or more of the existing phloem bundles and then allowed to regenerate for a short period (usually 5 days).Removal of all leaves and buds from plants resulted in a marked reduction (to approximately one-quarter of the value of the intact controls) in phloem regeneration. Primary leaf removal also resulted in some, though a smaller, reduction, as did the one other treatment—removal of all distal organs—which involved removal of young primary leaves. The other patterns of shoot organ removal tried—all buds removed and all proximal leaves and buds removed—had no effect on phloem regeneration.Indoleacetic acid (IAA) applied in aqueous solution (2 mg/l) to plants from which all leaves and buds had been removed completely substituted for them in phloem regeneration. Applied in lanolin paste (0.1% and 1%), it evoked more phloem regeneration than attached organs themselves. Sucrose (2%) applied alone or together with IAA in aqueous solution had no effect.IAA (0.1%) applied in lanolin to excised internodes increased phloem regeneration in these, but the anomalous appearance of some of the wound sieve tubes indicated that sugar or other material may have become limiting for full differentiation.A relationship between the number of phloem bundles cut in wounding and the extent of phloem regeneration was shown. This and other observations were interpreted as indicating the involvement in phloem regeneration of a substance or substances leaking from the cut phloem.Existing evidence is interpreted to mean that auxin is limiting for the cell divisions which immediately precede phloem differentiation and that some other factor, presently unknown, may be limiting for differentiation when auxin is in sufficient supply for cell division. However, present evidence neither implies nor excludes the possible participation of auxin in differentiation itself.     

Ethylene and auxin-induced cell growth in relation to auxin transport and metabolism and ethylene production in the semi-aquatic plant,Regnellidium diphyllum

Cell elongation in the rachis of the semiaquatic fern Regnellidium diphyllum is induced by the addition of ethylene or indoleacetic acid (IAA). Experiments with whole plants or rachis segments have shown that ethylene-induced growth requires the presence of auxin. Ethylene does not cause a modification in either endogenous auxin levels or in the extent of auxin metabolism but auxin transport is reduced. Rates of ethylene production in Regnellidium are not altered by either mechanical excitation or by the addition of auxin. A two-hormone control of cell expansion is proposed in which an initial, auxin-dependent growth event pre-conditions the cells to a further subsequent (or synchronous) ethylene-dependent growth event.Abbreviation IAA indole-3yl-acetic acid     

The Relationship between auxin transport and maize branching

Maize (Zea mays) plants make different types of vegetative or reproductive branches during development. Branches develop from axillary meristems produced on the flanks of the vegetative or inflorescence shoot apical meristem. Among these branches are the spikelets, short grass-specific structures, produced by determinate axillary spikelet-pair and spikelet meristems. We investigated the mechanism of branching in maize by making transgenic plants expressing a native expressed endogenous auxin efflux transporter (ZmPIN1a) fused to yellow fluorescent protein and a synthetic auxin-responsive promoter (DR5rev) driving red fluorescent protein. By imaging these plants, we found that all maize branching events during vegetative and reproductive development appear to be regulated by the creation of auxin response maxima through the activity of polar auxin transporters. We also found that the auxin transporter ZmPIN1a is functional, as it can rescue the polar auxin transport defects of the Arabidopsis (Arabidopsis thaliana) pin1-3 mutant. Based on this and on the groundbreaking analysis in Arabidopsis and other species, we conclude that branching mechanisms are conserved and can, in addition, explain the formation of axillary meristems (spikelet-pair and spikelet meristems) that are unique to grasses. We also found that BARREN STALK1 is required for the creation of auxin response maxima at the flanks of the inflorescence meristem, suggesting a role in the initiation of polar auxin transport for axillary meristem formation. Based on our results, we propose a general model for branching during maize inflorescence development.     

Carbohydrates stimulate ethylene production in tobacco leaf discs : I. Interaction with auxin and the relation to auxin metabolism

Various naturally occurring carbohydrates, applied at a concentration range of 1 to 100 mm, stimulated ethylene production for several days in indoleacetic acid (IAA)-treated or untreated tobacco (Nicotiana tabacum L. cv `Xanthi') leaf discs. The lag period for this sugar-stimulated ethylene production was 8 to 12 hours after excision in the untreated leaf discs, but less than 2 hours in the IAA-treated ones. Among the tested carbohydrates, 12 were found to increase synergistically ethylene production, with d-galactose, sucrose, and lactose being the most active; mannitol and l-glucose had no effect. The extent and duration of the increased ethylene production was dependent upon the type of sugar applied, the tissue's age, and the existence of both exogenous IAA and sugar in the medium. Sucrose appeared to elicit a continuous IAA effect for 48 hours, as expressed by increased ethylene production, even when IAA was removed from the medium after a 4-hour pulse. Sucrose stimulated both the uptake and decarboxylation of [1-¹⁴C]IAA, as well as the hydrolysis of the esteric and amide IAA conjugates formed in the tissue after application of free IAA. This gradual hydrolysis was accompanied by a further accumulation of a third IAA metabolite. Moreover, synthetic indole-3-acetyl-l-alanine increased ethylene production mainly with sucrose, and this effect was accompanied by its increased decarboxylation and turnover pattern suggesting that release of free IAA was involved. An esteric IAA conjugate, tentatively identified by GC retention time was found to be the major component (84%) of the naturally occurring IAA conjugates in tobacco leaves. Accordingly the sucrose-stimulated ethylene production in tobacco leaves can be ascribed mainly to the sucrose-stimulated hydrolysis of the esteric IAA conjugate.     

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.     

On the relation between haloperidol-induced alterations in DA release and DA metabolism in rat striatum

Haloperidol (1 mg/kg) was administered to rats pretreated with α-methyl-para-tyrosine-methylester. HCl (αMpT) and the levels of dopamine (DA) as well as HVA and DOPAC were measured in the striatum. While the release of DA was stimulated by haloperidol for at least 60 min, HVA and DOPAC levels were markedly increased only at 30 min, but not at 60, 90, or 120 min, after haloperidol administration. In rats not pretreated with αMpT, on the other hand, a strong increase in metabolite levels was observed between 60 and 120 min after haloperidol administration. It is concluded that a direct relation between DA release and metabolite levels does not exist in the present experiments. DA biosynthesis and processes involved with the clearance of metabolites appear to be important factors in the haloperidol-induced increase in metabolite levels. The relative importance of these three processes remains to be clarified.     

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.     

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.     

Auxin transport in explants of coleus

α-Naphthaleneacetic acid-C¹⁴, labeled in the carboxyl group, was applied in blocks of agar to the distal and to the proximal (either apical or basal) ends of explants of Coleus. The radioactivity in receiver blocks at the opposite ends was measured. Acropetal transport was slight, only 4% of the basipetal transport.

Translocation of NAA-C¹⁴ was polar in basipetal direction. Only 1.4% of the radioactivity lost from donor blocks at the apical position reached the receiver blocks; the greatest part remained in the tissue and was immobilized there. All activity found in receiver blocks at the basal end appeared to be still in the form of NAA. There were no differences between petiole tissue and stem tissue, so far as the transport of NAA is concerned.

     

The specificity of the auxin transport system

Summary In an effort to examine the specificity of the auxin transport system, the movement of a variety of growth substances and of auxin analogues through corn coleoptile sections was measured in both the basipetal and acropetal directions. In contrast to the basipetal, polar transport of the auxins indoleacetic acid (IAA) and 2,4-dichlorophenoxyacetic acid, no such movement was found for benzoic acid or for gibberellin A₁. A comparison of the - and -isomers of naphthaleneacetic acid showed that the growth-active -form is transported, but not the inactive -analogue. Both the dextro (+) and leavo (-) isomer of 3-indole-2-methylacetic acid showed the basipetal movement characteristic of IAA, the dextro isomer being more readily transported than the (-)-form. In this instance, too, the transport was roughtly proportional to the growth promoting activity. The antiauxin p-chlorophenoxyisobutyric acid inhibited auxin transport as it inhibited auxin-induced growth. These results agree with the hypothesis that processes involved in auxin transport are closely linked to or even identical with the primary auxin action.     

Patterning during somatic embryogenesis in Scots pine in relation to polar auxin transport and programmed cell death

Somatic embryogenesis is a useful tool to propagate conifers vegetatively. However, a major limitation in many pine species is the low quality of cotyledonary somatic embryos. The aim of this study has been to elucidate the developmental pathway of somatic embryos in Scots pine (Pinus sylvestris), to identify deviations from the normal pathway and to identify processes that might disturb normal development. Initially we compared the developmental pathway of somatic embryogenesis in representative cell lines yielding cotyledonary embryos with normal and abnormal morphology. Early embryos carrying suspensor cells in excess of the normal number (supernumerary) were more frequent in cell lines giving rise to abnormal cotyledonary embryos. In this study we show that the frequency of early somatic embryos with supernumerary suspensor cells increased after treatment with the auxin transport inhibitor 1-N-naphtylphthalamic acid (NPA). Furthermore, the yield of developing embryos increased significantly after treatment with the antiauxin 2-(4-chlorophenoxy)-2-methylpropionic acid (PCIB), but the morphology of the embryos was not affected. The number of cells undergoing PCD was analyzed using a TUNEL-assay. The frequency of TUNEL-positive cells was high both in proliferating cultures and during differentiation of early somatic embryos. However, the pattern of TUNEL-positive cells was similar in normal somatic embryos and in embryos with supernumerary suspensor cells. Together our results suggest that the presence of supernumerary suspensor cells in early somatic embryos of Scots pine is caused by disturbed polar auxin transport and results in aberrant embryo development.     

Uptake and metabolism of NAA and BAP in explants of tobacco in relation toin vitro flower bud formation

In vitro flower bud initiation and development depend on the presence of two hormones in the culture medium—auxin (NAA) and cytokinin (BAP). The uptake of both NAA and BAP by the explants was shown to be proportional to the concentrations supplied in the medium over a period of 4 days after the onset of culture. However, when supplied at equal concentrations for 24 h, the NAA uptake was up to 10-fold higher than the BAP uptake. Both hormones are rapidly metabolized by the explants. Nevertheless, the concentrations of free hormones inside the explants appeared to be high and in the case of NAA exceeded the concentration in the medium by more than 1 order of magnitude within 24 h. Apparently flower bud initiation in tobacco explants requires relatively high concentrations free NAA and BAP in the tissue maintained by a continuous supply in the medium. There are at present no indications that the products of hormone metabolism are directly involved in bud formation.     

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.     

Dynamic integration of auxin transport and signalling

Recent years have seen rapid progress in our understanding of the mechanism of action of the plant hormone auxin. A major emerging theme is the central importance of the interplay between auxin signalling and the active transport of auxin through the plant to create dynamic patterns of auxin accumulation. Even in tissues where auxin distribution patterns appear stable, they are the product of standing waves, with auxin flowing through the tissue, maintaining local pockets of high and low concentration. The auxin distribution patterns result in changes in gene expression to trigger diverse, context-dependent growth and differentiation responses. Multi-level feedback loops between the signal transduction network and the auxin transport network provide self-stabilising patterns that remain sensitive to the external environment and to the developmental progression of the plant. The full biological implications of the behaviour of this system are only just beginning to be understood through a combination of experimental manipulation and mathematical modelling.     

Morphogenesis in selaginella: auxin transport in the stem

Selaginella willdenovii Baker is a prostrate vascular cryptogam with a dorsiventral stem. At each major branching of the stem apex a dorsal and a ventral angle meristem is formed. The ventral meristem becomes determined as a root, and the dorsal meristem as a shoot. The present investigation examined the distribution and transport of ¹⁴C-indoleacetic acid through stem tissues as a basis for the pattern of meristem determination. Externally applied indoleacetic acid is transported into receiver blocks with a velocity of 12 millimeters per hour. Much of the auxin becomes immobilized in the tissue and is not transported. The polar ratio of auxin transport is approximately 2. Auxin is transported equally on the dorsal and the ventral sides of the stem axis, and the auxin flux in vascular tissue is twice that in the cortex. In the branch junctions twice as much auxin is transported on the dorsal side as on the ventral side, and this is held to be the consequence of the lateral branch vascular tissue connecting with the dorsal and median, but not with the ventral vascular strand of the stem axis.