Rôle des polyphénols dans la croissance. Définition ďun modèle expérimental chez Lycopersicum esculentum

Using young plants of Lycopersicum esculentum Mill., we attempt to define a system for a new evaluation of the physiological role of polyphenols, particularly of their regulating properties on growth and development, Exogenously supplied qoinic acid causes an important increase of the phenolic content and a reduced growth of the plants. The decrease in size is proportional to the stimulation of the phenolic pool and is equally obtained using other precursors of aromatic compounds like phenylalanine and cinnamic acid. Non-precursors of polyphenols structurally related to quinic acid are ineffective. It appears from these data that polyphenol levels and growth could be related in tomato plant and that this material would be convenient for studies on interactions between growth and polyphenols.     

The putative auxin efflux carrier OsPIN3t is involved in the drought stress response and drought tolerance

The phytohormone auxin plays a critical role in plant growth and development, and its spatial distribution largely depends on the polar localization of the PIN‐FORMED (PIN) auxin efflux carrier family members. In this study, we identify a putative auxin efflux carrier gene in rice, OsPIN3t, which acts in auxin polar transport but is also involved in the drought stress response in rice. We show that OsPIN3t–GFP fusion proteins are localized in plasma membranes, and this subcellular localization changes under 1‐N‐naphthylphthalamic acid (NPA) treatment. The tissue‐specific expression patterns of OsPIN3t were also investigated using a β‐glucuronidase (GUS) reporter, which showed that OsPIN3t was mainly expressed in vascular tissue. The GUS activity in OsPIN3tpro::GUS plants increased by NAA treatment and decreased by NPA treatment. Moreover, knockdown of OsPIN3t caused crown root abnormalities in the seedling stage that could be phenocopied by treatment of wild‐type plants with NPA, which indicated that OsPIN3t is involved in the control of polar auxin transport. Overexpression of OsPIN3t led to improved drought tolerance, and GUS activity significantly increased when OsPIN3tpro::GUS plants were subjected to 20% polyethylene glycol stress. Taken together, these results suggest that OsPIN3t is involved in auxin transport and the drought stress response, which suggests that a polar auxin transport pathway is involved in the regulation of the response to water stress in plants.     

The specificity of auxin transport in intact pea seedlings (Pisum sativum L.)

Summary When eight ¹⁴C-labelled auxin and non-auxin compounds were applied to the apical buds of intact dwarf pea seedlings (Pisum sativum L.), only [1-¹⁴C]indoleacetic acid ([¹⁴C]IAA) and -[1-¹⁴C] naphthaleneacetic acid ([¹⁴C]NAA) underwent appreciable basipetal transport during the first 24 h; over a longer period (72 h) considerable basipetal transport of the auxin [1-¹⁴C]2,4-dichlorophenoxyacetic acid ([¹⁴C]2,4-D) also occurred, but at a very much lower velocity (ca. 1.4–2.2 mm·h^-1). The movement of 2,4-D possessed many of the characteristics of a typical auxin transport. During uptake and transport IAA and NAA were extensively metabolised to the corresponding aspartates, and to ethanol-insoluble/NaOH-soluble compounds; little metabolism of 2,4-D was observed. None of the non-auxin compounds applied (sorbose, sucrose, leucine, adenine and kinetin) underwent appreciable basipetal transport from the apical bud. All but sorbose were extensively metabolised by the apical tissues. Little metabolism of sorbose itself was detected.The results suggest that the long-distance basipetal auxin transport system from the apical bud of intact plants is specific for auxins; the specificity may result from the affinity of auxins for specific transport sites.     

Inhibitors of the carrier-mediated influx of auxin in suspension-cultured tobacco cells

 Active auxin transport in plant cells is catalyzed by two carriers working in opposite directions at the plasma membrane, the influx and efflux carriers. A role for the efflux carrier in polar auxin transport (PAT) in plants has been shown from studies using phytotropins. Phytotropins have been invaluable in demonstrating that PAT is essential to ensure polarized and coordinated growth and to provide plants with the capacity to respond to environmental stimuli. However, the function of the influx carrier at the whole-plant level is unknown. Our work aims to identify new auxin-transport inhibitors which could be employed to investigate its function. Thirty-five aryl and aryloxyalkylcarboxylic acids were assayed for their ability to perturb the accumulation of 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene-1-acetic acid (1-NAA) in suspension-cultured tobacco (Nicotiana tabacum L.) cells. As 2,4-D and 1-NAA are preferentially transported by the influx and efflux carriers, respectively, accumulation experiments utilizing synthetic auxins provide independant information on the activities of both carriers. The majority (60%) of compounds half-inhibited the carrier-mediated influx of [¹⁴C]2,4-D at concentrations of less than 10 μM. Most failed to interfere with [³H]NAA efflux, at least in the short term. Even though they increasingly perturbed auxin efflux when given a prolonged treatment, several compounds were much better at discriminating between influx and efflux carrier activities than naphthalene-2-acetic acid which is commonly employed to investigate influx-carrier properties. Structure-activity relationships and factors influencing ligand specificity with regard to auxin carriers are discussed. Received: 28 June 1999 / Accepted: 28 August 1999     

Auxin carriers in Cucurbita vesicles

Carrier-mediated uptake of indole-3-acetic acid (IAA) by microsomal vesicles from Cucurbita pepo L. hypocotyls was strongly inhibited by 2,4-dichlorophenoxyacetic acid (2,4-D; i ₅₀= 0.3 M) but only weakly by 1-naphthylacetic acid (NAA). The fully ionised auxin indol-3-yl methanesulphonic acid also inhibited (i ₅₀=3 M). The same affinity ranking of these auxins for the uptake carrier, an electroimpelled auxin anion-H⁺ symport, is demonstrable in hypocotyl segments. The specificity of the auxin-anion eflux carrier was tested by the ability of different nonradioactive auxins to compete with [³H]IAA and reduce the stimulation of net radioactive uptake by N-1-naphthylphthalamic acid (NPA), a noncompetitive inhibitor of this carrier. By this criterion, NAA and IAA had comparable affinities, with 2,4-D interaction more weakly. Stimulation of [³H]IAA uptake by NAA, as a result of competition for the efflux carrier, could also be demonstrated when a suitable concentration of 2,4-D was used selectively to inhibit the uptake carrier. However, when [³H]NAA was used, no stimulation of its association with vesicles by NPA, 2,3,5-triiodobenzoic acid, or nonradioactive NAA was found. In hypocotyl segments, [³H]NAA net uptake was much less sensitive to NPA stimulation than was [¹⁴C]IAA uptake. The apparent contradictions concerning NAA could be explained by carrier-mediated auxin efflux making a smaller relative contribution to the overall transport of NAA than of IAA. The relationship between carrier specificity as manifested in vitro and the specificity of polar auxin transport is discussed.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - ION3 mixture of 4 M carbonylcyanide m-chlorophenylhydrazone, nigericin and valinomycin - IMS indol-3-yl methanesulphonic acid - NAA 1-naphthylacetic aci - NPA N-1-naphthylphthalamic acid     

Response of cytokinin concentration in the xylem exudate of bean (Phaseolus vulgaris L.) plants to decapitation and auxin treatment,and relationship to apical dominance

When xylem exudate of previously untreated Phaseolus vulgaris plants was analysed for cytokinins by radioimmunoassay, a low concentration (about 5 ng · ml^–1) was found. However, when the plants were decapitated about 16 h before the xylem exudate was collected, an almost 25-fold increase in cytokinin concentration was observed. Twenty-four hours after decapitation this increase even reached 4000 compared to control plants. Applying naphthaleneacetic acid (NAA) to the shoot of decapitated plants almost eliminated the effect of shoot tip removal on cytokinin concentration, suggesting that cytokinins in the xylem exudate of intact plants are under the control of the polar auxin transport system. Other xylem constituents, such as potassium or free amino acids did not show this strong increase after decapitation and did not respond to NAA application. It is concluded that the observed auxin/cytokinin interaction has an important regulatory role to play, not only in apical dominance but in many other correlative events as well.Abbreviations AD apical dominance - CKs cytokinin(s) - iAde/iAdo isopentenyladenine/iospentenyladenosine - NAA naphthaleneacetic acid - Z/ZR zeatin/zeatin riboside     

Transport and Degradation of Auxin in Relation to Geotropism in Roots of Phaseolus vulgaris

The movement of auxin in Phaseolus vulgaris roots has been examined after injection of IAA^?3H into the basal root/hypocotyl region of intact, dark-grown seedlings. Only a portion of the applied IAA^?3H was transported unchanged to the root tip. The major part of the chromatographed, labelled compounds translocated to the roots was indole-3-acetylaspartic acid (IAAsp) and an unidentified compound running near the front in isopropanol, ammonia, water. The velocity of the auxin transport (7.2 mm per hour) was calculated from scintillation countings of methanol extracts from serial sections of the root. An accumulation of radioactive compounds in the extreme root tip, was observed 5 h after the injection of IAA. The influence of exogenous IAA on the geotropical behaviour of the bean seedling roots was examined. Pretreated roots were stimulated for 5 min in the horizontal position and then rotated parallel to the horizontal axis of the klinostat for 60 or 90 min. The resulting geotropic curvature of IAA-injected and control roots showed significantly different patterns of development. When the stimulation was started 5 h after application of the auxin, the geotropic curvature became larger in roots of the injected plants than in the controls. If, however, the translocation period was extended to 20 h the geotropic curvature was significantly smaller in the roots of the injected plants. The auxin injection did not significally affect the rate of root elongation. The change in geotropical behaviour of the roots is interpreted as a result of the influence of the conversion products of the applied IAA on the geotropical responsiveness.     

Indole-3-butyric acid in plants: occurrence, synthesis, metabolism and transport

Indole-3-butyric acid (IBA) was recently identified by GC/MS analysis as an endogenous constituent of various plants. Plant tissues contained 9 ng g^?1 fresh weight of free IBA and 37 ng g^?1 fresh weight of total IBA, compared to 26 ng g^?1 and 52 ng g^?1 fresh weight of free and total indole-3-acetic acid (IAA), respectively. IBA level was found to increase during plant development, but never reached the level of IAA. It is generally assumed that the greater ability of IBA as compared with IAA to promote rooting is due to its relatively higher stability. Indeed, the concentrations of IAA and IBA in autoclaved medium were reduced by 40% and 20%, respectively, compared with filter sterilized controls. In liquid medium, IAA was more sensitive than IBA to non-biological degradation. However, in all plant tissues tested, both auxins were found to be metabolized rapidly and conjugated at the same rate with amino acids or sugar. Studies of auxin transport showed that IAA was transported faster than IBA. The velocities of some of the auxins tested were 7. 5 mm h^?1 for IAA, 6. 7 mm h^?1 for naphthaleneacetic acid (NAA) and only 3. 2 mm h^?1 for IBA. Like IAA, IBA was transported predominantly in a basipetal direction (polar transport). After application of ³H-IBA to cuttings of various plants, most of the label remained in the bases of the cuttings. Easy-to-root cultivars were found to absorb more of the auxin and transport more of it to the leaves. It has been postulated that easy-to-root, as opposed to the difficult-to-root cultivars, have the ability to hydrolyze auxin conjugates at the appropriate time to release free auxin which may promote root initiation. This theory is supported by reports on increased levels of free auxin in the bases of cuttings prior to rooting. The auxin conjugate probably acts as a ‘slow-release’ hormone in the tissues. Easy-to-root cultivars were also able to convert IBA to IAA which accumulated in the cutting bases prior to rooting. IAA conjugates, but not IBA conjugates, were subject to oxidation, and thus deactivation. The efficiency of the two auxins in root induction therefore seems to depend on the stability of their conjugates. The higher rooting promotion of IBA was also ascribed to the fact that its level remained elevated longer than that of IAA, even though IBA was metabolized in the tissue. IAA was converted to IBA by seedlings of corn and Arabidopsis. The K_m value for IBA formation was low (approximately 20 μM), indicating high affinity for the substrate. That means that small amounts of IAA (only a fraction of the total IAA in the plant tissues) can be converted to IBA. It was suggested that IBA is formed by the acetylation of IAA with acetyl-CoA in the carboxyl position via a biosynthetic pathway analogous to the primary steps of fatty acid biosynthesis, where acetyl moieties are transferred to an acceptor molecule. Incubation of the soluble enzyme fraction from Arabidopsis with ³H-IBA, IBA and UDP-glucose resulted in a product that was identified tentatively as IBA glucose (IBGIc). IBGIc was detected only during the first 30 min of incubation, showing that it might be converted rapidly to another conjugate.     

Effects of Flavonoids on the Polar Transport of Auxins

The effect of some flavonoids on the polar transport of auxins was investigated in hypocotyl sections of dark grown seedlings of cucumber, Cucumis sativus L., by means of ¹⁴C labelled auxins. In experiments of 4–6 h duration quercitrin, morin, dihydroquercetin, naringin, sulfuretin and ferulic acid increased the polarity of the transport of indol-3yl-acetic acid (stimulation of basipetal, inhibition of acropetal transport). Naringenin, genistein and pinobanksin, on the other hand, decreased the polarity of this transport. For NAA no increase in the polarity of the transport could be observed, but all the substances tested inhibited the basipetal transport. There was no simple correlation between the effects on the polar transport and the effects on IAA oxidase.     

Characteristics of adventitious root formation in cotyledon segments of mango (Mangifera indica L. cv. Zihua): two induction patterns, histological origins and the relationship with polar auxin transport

Cotyledon segments derived from zygote embryos of mango (Mangifera indica L. cv. Zihua) were cultured on agar medium for 28 days. Depending on different pre-treatments with plant growth regulators, two distinct patterns of adventitious roots were observed. A first pattern of adventitious roots was seen at the proximal cut surface, whereas no roots were formed on the opposite, distal cut surface. The rooting ability depended on the segment length and was significantly promoted by pre-treatment of embryos with indol-3-acetic acid (IAA) or indole-3-butyric acid (IBA) for 1 h. A pre-treatment with the auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) completely inhibited adventitious root formation on proximal cut surfaces. A second pattern of roots was observed on abaxial surfaces of cotyledon segments when embryos were pre-treated with 2,700 μM 1-naphthalenacetic acid (NAA) for 1 h. Histological observations indicated that both patterns of adventitious roots originated from parenchymal cells, but developmental directions of the root primordia were different. A polar auxin transport assay was used to demonstrate transport of [³H] indole-3-acetic acid (IAA) in cotyledon segments from the distal to the proximal cut surface. In conclusion, we suggest that polar auxin transport plays a role in adventitious root formation at the proximal cut surface, whereas NAA levels (influx by diffusion; carrier mediated efflux) seem to control development of adventitious roots on the abaxial surface of cotyledon segments.     

Interaction of Auxin-TIBA in Lycopersicon esculentum

[5-³H] indoleacetic acid (IAA) was applied to the apical budof young tomato plants (Lycopersicon esculentum Mill.) treatedor not with 2,3,5-triiodobenzoic acid (TIBA). Chromatographicanalysis showed that treatment with TIBA increased the quantityof breakdown products in the apical 1.5 mm segment of the stem.Autoradiographs of ultra-thin sections of this segment wereprepared after treatment with glutaraldehyde. The inhibitionof polar auxin transport by TIBA caused an increase in the densityof labelling of all the cellular types in the shoot apex. TheTIBA treatment provoked a modification of the structure of theshoot apex in which the greatest increases in density of labellingwere detected in the superficial tunical layer and in the pithmeristem. The increase in radioactivity of the superficial tunicallayer could be explained by the presence of polyphenols in thevacuoles. For the pith meristem, the TIBA treatment intensifiedthe high labelling previously observed at this level in thecontrol plants. This could indicate a role for auxin in cellularelongation. It could also point to the importance of the pithmeristem in the functioning of the shoot apex. Key words: [³H] IAA, Lycopersicon esculentum, TIBA     

1-N-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid

Summary Auxin transport in corn coleoptile sections was inhibited by 2,3,5-triiodobenzoic acid (TIBA) as well as by 1-N-naphthylphthalamic acid (NPA); this inhibition was effected within 1 min of application.A particulate cell fraction-presumably plasma-membrane vesicles-specifically binds NPA and properties of these binding sites were studied using ³H-NPA and a pelletting technique. The saturation kinetics of the physiological NPA effect, i.e. the inhibition of auxin transport, is similar to that of the specific in-vitro NPA binding. Half saturation of the inhibitory effect was found with about 5×10^-7 M TIBA and with 10^-7 M NPA. Both substances also decreased the speed of movement of auxin pulses within coleoptile sections.NPA dissociates from its binding site when the particulate cell material is centrifuged through an NPA-free cushion. The NPA that is washed from its binding site can be used in another binding test without any apparent change and is chromatographically unaltered. Therefore, the NPA binding is probably reversible and non-covalent. Inhibition of auxin transport by TIBA or NPA could also be reversed when the coleoptile sections were washed in buffer.The movement of ¹³¹I-TIBA in corn coleoptiles appears to be polar in a basipetal direction. Higher concentrations of indoleacetic acid or TIBA inhibited this polar movement, suggesting that TIBA moves in the same channels as auxin. With ³H-NPA, however, no polar transport could be detected. Together with the in-vitro binding results, these data indicate that TIBA acts directly at the auxin receptor while NPA has a different receptor site.The effect of TIBA and NPA on elongation, with or without auxin, is neglegible in comparison to their effects on auxin transport.     

The Effect of Environmental Conditions on the Expression of Disease in Cultured Tissues of Brassica napus ssp. oldfera Infected with Leptosphaeria maculans

As a basis for devising an in vitro screening programme, culture conditions were optimized so that tissue cultures from two resistant cultivars of Brassica napus ssp. oleifera (Mikado, Bienvenu) and two susceptible cultivars (Lesira, Ceres) could be differentiated using a disease scoring scheme, when inoculated with Leptosphaeria maculans. Tissues inoculated included thin cell layer explants from soil-grown plants and in vitro-grown shoot cultures and callus tissue formed on such explants. The period of incubation and the incubation temperature were of importance in the development of differential disease reactions. Increasing temperature generally resulted in an increase in infection and too great an incubation period resulted in total overgrowth of the tissue. Increasing concentrations (1 × 10^?6 M-1 ×10^?4 M) of the auxins 1-naphthylacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D) and mdole-3-acetic acid (IAA) in the culture medium, resulted in a decrease in disease score of the thin cell layer (TCL) explants from soil-grown plants. The cytokinins examined 6-benzyl-aminopurine (BAP) and 6-4-hydroxy-3-methyl-2-enylaminopurine (zeatin), reduced the extent of infection of the TCL explants when used in combination with the auxin NAA. Medium containing NAA at a concentration of 1 × 10^?6 M in combination with BAP at a concentration of 1× 10^?6 or 1 × 10^?4 M allowed differentiation of the disease reactions of the resistant and susceptible cultivars, when the explants were incubated for 10 days at 20 °C after inoculation. Similar conditions of incubation and the addition of NAA (1 × 10^?6 M) combined with BAP (1 × 10^?6 M) to the medium also allowed the differentiation of the disease reactions on TCL explants from stems of in vitro shoot cultures of the cultivars Mikado and Lesira. Increasing concentrations of the auxin NAA and the cytokinin BAP resulted in a reduction in the mean disease score of the callus tissue produced on TCL explants from soil-grown plants, and NAA (1 × 10^?5 M) combined with BAP (1 × 10^?6 or 1 × 10^?5 M) allowed differentiation of resistance and susceptibility in callus tissues when incubated for 5 days at 20 °C. 2,4-D did not allow differentiation of the cultivars. This was in contrast to the inoculation of callus tissue attached to TCL explants of in vitro shoot cultures, where combinations of 2,4-D and BAP at concentrations of 1 × 10^?6 M allowed differentiation of the resistant and susceptible cultivars. These findings provide a basis for designing selection protocols of value in both traditional as well as in vitro breeding programmes to select lines of oilseed rape with resistance/novel resistance to L. maculans.     

Time course study on accumulation of cell wall-bound phenolics and activities of defense enzymes in tomato roots in relation to <Emphasis Type="Italic">Fusarium</Emphasis> wilt

Major cell wall-bound phenolic compounds were detected and identified in roots of tomato at different stages of growth. Alkaline hydrolysis of the cell wall material of the root tissues yielded ferulic acid as the major bulk of the phenolic compounds. Other phenolic compounds identified were 4-hydroxybenzoic acid, vanillic acid, 4-hydroxybenzaldehyde, vanillin and 4-coumaric acid. All the six phenolic acids were higher in very early stage of plant growth. Ferulic acid, 4-hydroxybenzoic acid and 4-coumaric acid exhibited a decreasing trend up to 60 days and then the content of these phenolic acids increased somewhat steadily towards the later stage of growth. Total phenolics, phenylalanine ammonia-lyase (PAL) activity and peroxidase (POD) activity were in tandem match with the occurrence pattern of the phenolic acids. Ferulic acid showed highest antifungal activity against tomato wilt pathogen Fusarium oxysporum f. sp. lycopersici. The results of this study may be interpreted to seek an explanation for high susceptibility of tomato plants at flowering stage to Fusarium wilt. It may also be concluded that greater amounts of ferulic acid in combination with other phenolics and higher level of PAL and POD activities after 60 days of growth may have a role in imparting resistance against Fusarium wilt at a late stage of plant growth.     

The Role of Auxin Metabolism in Root Meristem Regeneration by Pinus lambertiana Embryo Cuttings

Since the existence of root promoting substances that consist of a complex between auxin and another molecule has been suggested, we have examined the role of auxin conversion products in root regeneration by Pinus lambertiana embryo cuttings. Auxin conversion products were detected using radioactive forms of the auxins IAA (indoIe-3-acetic acid), NAA (a-napthaleneacetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid). 10^?7M NAA was more effective than 10^?6M IAA at promoting rooting, yet it formed conversion products much less rapidly. Also continuous exposure to IAA was necessary for optimum root formation. Based on these and other findings, we conclude that free auxin, and not the conversion products we detected, is essential to root meristem formation.     

Naphthylphthalamic acid-binding sites in cultured cells from Nicotiana tabacum

Naphthylphthalamic acid (NPA), an inhibitor of polar auxin transport, binds with high affinity to membrane preparations from callus and cell suspension cultures derived from Nicotiana tabacum (K _d approx. 2·10^–9 M). The concentration of membrane-bound binding sites is higher in cell suspension than in callus cultures. The binding of NPA to these sites seems to be a simple process, in contrast to the binding of the synthetic auxin naphthylacetic acid (1-NAA) to membrane preparations from callus cultures, which is more complex (A.C. Maan et al., 1983, Planta 158, 10–15). Naphthylacetic acid, a number of structurally related compounds and the auxin-transport inhibitor triiodobenzoic acid were all able to compete with NPA for the same binding site with K _d values ranging from 10^–6 to 10^–4 M. On the other hand, NPA was not able to displace detectable amounts of NAA from the NAA-binding site. A possible explantation is the existence of two different membrane-bound binding sites, one exclusively for auxins and one for NPA as well as auxins, that differ in concentration. The NPA-binding site is probably an auxin carrier.Abbreviations 1-NAA 1-Naphthylacetic acid - 2-NAA 2-Naphthylacetic acid - NPA N-1-Naphthylphthalamic acid     

Activities of auxin polar transport in inflorescence axes of flower mutants ofArabidopsis thaliana: Relevance to flower formation and growth

Relationships between the activity of auxin polar transport and flower formation were studied using several flower mutants ofArabidopsis thaliana. The activity of auxin polar transport in the upper portion of inflorescence axis of wildtype plants ofArabidopsis thaliana was significantly lower than that of the basal part. The activities of auxin polar transport in the upper portion of inflorescence axes ofap1 andclv1 mutants were significantly higher than that of wild-type plant. However, those of other flower mutants tested,ap3-1, ag, pi, Fl-40, Fl-54, Fl-89 andpin-formed, were extremely low as compared with that of wild one. We got some evidence that the reduction of the activity of auxin polar transport is concerned with the growth and development of plants. We could mimic it by the removal of all flowers and pods including mature or immature seeds. Moreover, artificial pollination inap3-1 andpi mutants, in which no seeds are found naturally, resulted in the partial recovery of the activity of auxin polar transport in inflorescence axis. Considering these results in this study together with the fact that inhibitors of auxin polar transport generated almost same disruptions ofpin-formed orpinoid mutants which normally had no flowers in inflorescence axis (Okadaet al. 1991, Uedaet al. 1992, Bennettet al. 1995), the systern of auxin polar transport and its activity in inflorescence axis seems to be essential for the development of flower bud in early stage ofArabidopsis thaliana, and the activity of auxin polar transport is also regulated by the formation of flowers and seeds in inflorescence axis.     

Is Cell Elongation Regulated by Extracellular Auxin?

Experiments with isolated epidermal strips of maize coleoptiles, pretreated with auxin and further incubated on sucrose agar containing different concentrations of auxin (indole-3-acetic acid, IAA or naphthalene-1-acetic acid, NAA) and/or naphthylphthalamic acid (NPA), are described. Preincubation for 2h with 2 . 10^?4M IAA or 10^?5M NAA in buffer, followed by 30 min wash in buffer results in measurable cell elongation during a subsequent incubation for 6 h on sucrose agar. Addition of 10^?4M NPA inhibited the response to auxin and this inhibition could be reversed by providing IAA in addition to NPA. Inner tissue fragments (without outer epidermis) did not respond to external IAA. These results lead to the conclusion that auxin secretion at the outer epidermis may be an essential step in auxin-regulated coleoptile growth.