Endogenous and exogenous auxin in the control of root growth

The endogenous indol-3yl-acetic acid (IAA) of detipped apical segments from roots of maize (cv ORLA) was greatly reduced by an exodiffusion technique which depended upon the preferential acropetal transport of the phytohormone into buffered agar. When IAA was applied to the basal cut ends of freshly prepared root segments only growth inhibitions were demonstrable but after the endogenous auxin concentration had been reduced by the exodiffusion technique it became possible to stimulate growth by IAA application. The implications of the interaction between exogenous and endogenous IAA in the control of root segment growth are discussed with special reference to the role of endogenous IAA in the regulation of root growth and geotropism.Abbreviations IAA indol-3yl-acetic acid - GC-MS gas chromatography-mass spectrometry     

Nutation of growing and georsacting roots

Abstract. Growth and geocurvature of the primary roots of maize (cv. LG 11) were analyzed by a filming technique allowing continuous recording, root by root. Typical nutations, in vertical growth and following the downward curvature of the roots, have been observed. The possible relations between these nutations, the elongation, the georeaction and the hormonal mechanisms which control georeaction, are discussed.     

Effects of Light on the Georeaction and Growth Inhibitor Content of Roots

The positive geotropic response of the apical segments prepared from the primary roots of Zea mays depends upon at least one growth inhibitor, produced by the root cap, moving basipetally into the extending zone of the root in which it accumulates in the lower part. Anjou maize reacts in both darkness and light while Kelvedon maize is, for the first few hours, geotropic only in light. The production (or activity) of the growth-inhibiting substance — tested by using vertical half-decapitated root segments — is quite similar to the georeaction. This finding provides strong evidence that, in the case of Kelvedon maize roots, the inhibitory substance may depend on light. Observations related to the root segment of Anjou and Kelvedon maizes of which the tips are exchanged, are in agreement with the above results.     

Effect of the light on adenine nucleotide content of georeacting maize roots

Apical segments from maize roots of LG 11 and Orla 264 varietiesgeoreacted, at least for the first few hours, only in light,while those of the Anjou 210 variety were georeactive both inlight and darkness. The energy charge in the apical end of thetwo light geo-sensitive (LG and Orla) maize roots significantlyincreased in the light. In contrast, in Anjou root tips, theenergy charge was already high in the dark and did not changesignificantly after light exposure. The time course of light-inducedchanges in adenine nucleotide contents in root tips was comparedwith varietal differences in georeactivity in light and darkness.The present data corroborate the hypothesis that a high energystate in the root tip is a prerequisite for the expression ofthe root reaction to gravity. (Received October 17, 1978; )     

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.     

Ontogenetic Changes in the Transport of Indol-3yl-acetic Acid into Maize Roots from the Shoot and Caryopsis

The quantities of endogenous indol-3yl-acetic acid (IAA) in endosperms and scutella of 6-day-old maize seedlings (Zea mays L. cv Giant White Horsetooth) were determined by a fluorimetric method. Endosperms were found to contain 33.4 nanograms IAA per plant, and scutella 7.5 nanograms IAA per plant. [5-³H]IAA applied to endosperms of 6-day-old seedlings moved into the roots and radioactivity accumulated at the apex of the primary root within 8 hours. Two to 7-day-old seedlings were treated simultaneously with [5-³H]IAA in the endosperm and [2-¹⁴C] IAA on the shoot apex. The patterns of transport into the root were found to change during ontogeny: in successively older plants, transport from the shoot into the roots increased relative to transport from the endosperm into the roots. The auxin required for the growth of maize roots could, therefore, partially be contributed by the shoot and endosperm. Ontogenetic changes in the relative importance of these two supplies could be of significance for the integration of growth and development between shoot and root.     

Translocation of Shoot-Applied Indolylacetic Acid into the Roots of Populus tremula

Application of 10 to 100 μg indol-3-ylacetic acid to the leaves of rooted cuttings of aspen caused inhibition of root growth after three hours. Root growth recovered within 24 hours after IAA treatment. Swelling of the root tips occurred during the period of inhibition. The roots responded in the same way if IAA was applied in solution to the cut stem surface above the mature leaves. IAA-1-¹⁴C applied through a cut stem surface or to mature leaves was translocated downwards in the plants and labelled IAA could be isolated from the roots 3 to 24 hours after application. The ethanol-soluble activity decreased rapidly indicating a rapid metabolism or binding of IAA. IAA-1-¹⁴C applied to growing leaves was not translocated. From the rapid response of root growth it was concluded that IAA was translocated into the roots at a rate of about 7 cm per hour. This rate of translocation indicates that the sieve tubes are involved in the translocation. Implications of the results for the translocation of endogenous auxin into the roots are discussed.     

Differential growth of georeacting maize roots

The growth rate of the two sides of 10-mm apical segments prepared from primary roots and of intact primary roots of maize has been analyzed in both vertical and horizontal positions, using a filming method allowing continuous growth recording. The data showed that the georeaction began by a decrease in the overall elongation rate of the roots. This inhibition is effective on the lower side of the bending zone, where the growth is practically stopped during the period of maximum rate of geocurvature. In contrast, the growth is slightly enhanced on the upper part of the elongating zone.     

Saturated humidity accelerates lateral root development in rice (Oryza sativa L.) seedlings by increasing phloem-based auxin transport

Auxin transport plays a significant role modifying plant growth and development in response to environmental signals such as light and gravity. However, the effect of humidity on auxin transport is rarely documented. It is shown here that the transport of labelled indole-3-acetic acid (IAA) from the shoot to the root is accelerated in rice (Oryza sativa L. ssp. indica cv. IR8) seedlings grown under saturated humidity (SH-seedlings) compared with plants grown under normal humidity (NH-seedlings). The development of lateral roots in SH-seedlings was greatly enhanced compared with NH-seedlings. Removal of the shoot from SH-seedlings reduced the density of lateral roots, and the application of IAA to the cut stem restored the lateral root density, while the decapitation of NH-seedlings did not alter lateral root development. Phloem-based auxin transport appeared responsible for enhanced lateral root formation in SH-seedlings since (i) the rate of IAA transport from the shoot to the root tip was greater than 3.5 cm h-1 and (ii) naphthylphthalamic acid (NPA)-induced reduction of polar auxin transport in the shoot did not influence the number of lateral roots in SH-seedlings. It is proposed that high humidity conditions accelerate the phloem-based transport of IAA from the leaf to the root, resulting in an increase in the number of lateral roots.     

The light effect on the growth inhibitors produced by the root cap

Summary Under light, some growth inhibiting substances were produced in the root cap of maize; they moved basipetally from the tip to the extending zone. An asymmetrical uptake by the root stumps of these inhibitors induced a curvature of the root segments. Evidence was given that these growth regulators, formed in the root cap on exposure to light, can cause root curvature in darkness. Assays with two varieties of maize (Anjou which is georeactive both in dark and in light, and Kelvedon georeacting in light only) and with IAA—applied on the basal cut end of the root segments—were discussed in relation to the light effect on the formation of the cap growth factors. Experiments involving use of ABA-which has some growth properties identical to those of these inhibitors—lead to the conclusion that the light was only acting on the formation, in the root, of the growth-inhibiting substances. But light seems not to have an effect on the transport of these inhibitors from the cap to the stump or on their action on the elongating part of the roots.Abbreviations IAA indolyl-3-acetic acid - ABA abscisic acid     

Effect on Root Growth of Endogenous and Applied IAA and ABA: A Critical Reexamination

Applications of indole-3yl-acetic acid (IAA) and abscisic acid (ABA) were done on two-day-old intact maize (cv LG 11) roots. The effect of the treatment on the root growth depends on their initial elongation rate. The slow growing roots were all inhibited by exogenous IAA and ABA at any concentrations used whereas for the fast growing roots their elongation was promoted by these two hormones at low concentrations. Quantitative analyses of endogenous IAA and ABA were performed using the gas chromatography-mass spectrometry technique. Detection and quantification of endogenous IAA and ABA were done on the zone of the root implicated in elongation. These techniques were achieved by electron impact on the IAA-Me-heptafluorobutyryl derivative and by negative ion chemical ionization with NH₃ on the ABA-Me ester derivative. A negative correlation between the growth and the endogenous content of these two hormones was obtained. ABA presented a larger range of endogenous level than IAA on the whole population of roots tested. When using applied IAA and ABA at different concentrations the same differentiating effect on the growth was observed. This allowed us to conclude that for identical concentrations, IAA has a more powerful effect on root elongation than ABA. Present results are discussed in relation to previous data related to the role of IAA and ABA in the growth and gravireaction of maize roots.     

Hormonal characterization of a nonrooting naphthalene-acetic Acid tolerant tobacco mutant by an immunoenzymic method

The comparative analysis of plant hormones was undertaken on a 1-naphthaleneacetic acid tolerant mutant and normal tobacco (Nicotiana tabacum cv Xanthi) plantlets. The mutant plantlet was scrubby and impaired in its root morphogenesis. Degeneration of the root meristem was studied on tissue sections; it appeared very fast (as early as the 3rd or 4th day after sowing), after which the root was further transformed into a callus. Indoleacetic acid (IAA), abscisic acid (ABA), and the isopentenyladenine (iP)- and trans-zeatin(Z)-type cytokinin levels were measured in terminal buds and root tips 13 days after sowing, by enzyme linked immunosorbent assay of high performance liquid chromatography fractions. Some differences appeared between the apical buds of the two genotypes, but the mutant tobacco differed from the wild type mainly by the presence of higher levels of IAA, ABA, and iP + isopentenyladenosine (iPA) in its small root. Thus, the IAA, ABA, and iP + iPA contents were increased by a factor of 15, 7, and 24 times, respectively, in mutant root compared to wild-type tobacco roots. Previous work has shown that the mutation impairs membrane polarization effects induced by auxin at the cell level. The present results would favor the hypothesis that the mutation has also affected the control of growth regulator accumulation in tissues.     

Comparison of growth,exogenous auxin sensitivity,and endogenous indole-3-acetic acid content in roots ofHordeum vulgare L. and an agravitropic mutant

The chemically induced barley (Hordeum vulgare L.) mutation, agr, was found to be a simple recessive trait resulting in agravitropic roots and normal gravitropic shoots. The total seedling root growth was similar for mutant and wild-type roots, although the mutant had fewer roots per seed and greater elongation per root. Although the concentration of exogenous indole-3-acetic acid (IAA) required to reduce root growth by 50% (GR50) was 12 times greater for the agravitropic mutant, agravitropic and gravitropic roots were equally sensitive to exogenous applications of 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene acetic acid (NAA). Root IAA contents, determined by high-pressure liquid chromatography (HPLC), were not different for gravitropes and agravitropes. The greater root elongation rates, lack of sensitivity to exogenous IAA, and normal endogenous IAA levels indicate that auxin-controlled growth regulation may be altered in the mutant.     

Abscisic acid as a root growth inhibitor: Physiological analyses

Summary Abscisic acid (ABA) moves basipetally and laterally in maize (Zea mays L.) root segments placed horizontally; its transport properties are thus similar to those of the growth-inhibiting substances produced by the root cap. The two opposite flows af ABA and of indolyl-3-acetic acid (IAA) — substances both present in the cap — may control elongation and georeaction of the root.     

Hormonal Effects on Growth and Morphology of Normal and Hairy Roots of Hyoscyamus muticus

Treatment of normal and Agrobacterium rhizogenes-transformed root cultures of Hyoscyamus muticus with three different auxins, indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and naphthaleneacetic acid (NAA), revealed that the response varied considerably among auxins, between transformed and normal roots, and depending on the parameter. In normal roots all three auxins provoked abundant branching, with IBA and NAA being the most effective at 2.5 and 0.5 μm, respectively, whereas IAA was most effective at low concentrations (0.05 and 0.1 μm). In transformed roots exogenously supplied auxins were generally inhibitory or, at best, without effect on growth and branching. Only 0.01 μm IAA significantly enhanced lateral root number, whereas at the higher concentrations IBA, although inhibitory, was the least effective auxin. In both root types IBA had little effect on primary root growth, but normal roots were more sensitive to IAA and NAA. These results suggest a different sensitivity to auxins of normal and transformed roots since there was no significant difference in endogenous free and conjugated IAA content nor in IAA uptake capacity. Ethylene production and biosynthesis were approximately threefold higher in hairy roots, but production could be stimulated up to tenfold that of control levels in normal roots by supplying NAA or 1-aminocyclopropane-1-carboxylic acid (ACC). Treatment with 2.5 μm NAA, but not IAA or IBA, also enhanced ethylene biosynthesis in normal roots but not in transformed ones. ACC and malonyl-1-aminocyclopropane-1-carboxylic acid accumulated to detectable levels only after treatment with an auxin (NAA). Received March 3, 1997; accepted May 28, 1997     

Effects of Dimethyl Sulfoxide and pH on Indoleacetic Acid-Induced Lateral Root Formation in the Radish Seedling Root

Segments (2.5 cm) cut from 3-day-old seedling roots of radish (Raphanus sativus L. `Scarlet Globe') were cultured in medium with or without indoleacetic acid (IAA). Lateral root primordia frequency, determined for the central centimeter of segments, was dependent on IAA concentration and on conditions affecting IAA uptake. Dimethyl sulfoxide treatment, or a relatively low medium pH, greatly enhanced the response to exogenous IAA. It was concluded that a permeation barrier exists between the external medium and the hormone responsive sites within the radish seedling root.     

Effect of applied and endogenous indol-3-yl-acetic acid on maize root growth

The level of endogenous Indol-3-yl-acetic acid (IAA) measured by gas chromatography-mass spectrometry in the elongating zone of intact primary roots of Zea mays showed a good linear correlation with the growth rate of these roots. When they were treated with IAA, their relative elongation decreased; this indicates a supraoptimal content of endogenous IAA. However, the growth of some of the relatively rapidly extending roots was enhanced by such treatment. Interactions between endogenous and applied IAA in the control of root growth are discussed.Abbreviations GC-MS gas chromatography-mass spectrometry - IAA Indol-3-yl-acetic acid     

Variation in structure and plant regeneration of Agrobacterium rhizogenes transformed and control roots of the potato cv. Bintje

Agrobacterium rhizogenes transformed and control roots of the tetraploid potato cv. Bintje were compared. Transformed roots were obtained after infection by A. rhizogenes 15834 or 1855. Both in leaf and stem segments, more roots were formed at the basal side of the segments, indicative for a polarity in root formation. As compared to control roots the transformed roots are characterized by smaller and more densely stained cells, a zone of cell division, and smaller statoliths. These characteristics are correlated with vigorous growth, high branching incidence and diminished geotropism. The plant regeneration procedure according to Ooms et al. [1] was modified. The transformed roots required less 2,4-D than control roots for the induction of shoot-competent calli. The callus and shoot induction phases were reduced from 8 and 6 weeks to 3 and 3 weeks, respectively. Upon induction, 25%, 58% and 61% of the root clones originating from tuber, stem and leaf, respectively, produced shoots, whereas all of the control roots produced shoots. Shoot outgrowth occurred on liquid MS medium in the absence of hormones.Abbreviations Ri-root Agrobacterium rhizogenes transformed root - BAP benzylaminopurine - IAA indoleacetic acid - GA₃ gibberellic acid - NAA naphthaleneacetic acid - 2,4-D 2,4 dichlorophenoxyacetic acid     

生长素和模拟微重力效应对大白菜不定根形态发生的影响

Under the induction of indole-3-acetic acid (IAA), adventitious roots were differentiated on hypocotyl segments derived from seedlings of Chinese cabbage (Brassica campestris spp. pekinensis). IAA at concentration of 0.4-1.0 mg/L in solid MS medium incited many adventitious roots on hypocotyl segments. The earliest anatomic changes were observed on cut surface of hypocotyl segments under optical microscope 24 hours after IAA treatment: cytoplasmic and nuclear density became higher in a few of parenchytmatous cells adjacent to phloem in tissue of pericycle, followed by cell divisions. Lately, the dividing cells expanded and developed into root primordium from which root cap was differentiated. After five days, most roots protruded through hypocotyl cortex and appeared just below the cut surface. The rooting capacity of the segments derived from three regions of each hypocotyl was different. High level of IAA modified the polarity of root formation on segment inserted upside down and sucrose increased the function of IAA. Additionally, microgravity did not significantly change the rooting polarity under the condition of stimulated microgravity, but it increased the competence of explants to IAA treatment. The results presented here provided an experimental system for further investigation of molecular events associated with adventitious root initiation.     

The Relationship between Levels of Endogenous Gibberellins and the Response of Vigna Hypocotyls to Exogenous Indole-3-Acetic Acid

Segments excised from the upper and the lower parts of cowpea(Vigna unguiculata L.) hypocotyls were compared in terms oftheir responses to exogenous indole-3-acetic acid (IAA) in relationto their endogenous levels of gibberellin. Growth of the segmentswas measured continuously during xylem perfusion with a lineardifferential transformer. IAA induced a burst of elongationin the upper segments but only slight promotion of growth inthe lower segments. Treatment with uniconazole, a potent inhibitorof the biosynthesis of gibberellins, reduced the responsivenessof the upper segments to exogenous IAA to about one half ofthe control value. Pre-perfusion with GA₃ of such segments fortwo hours prior to application of IAA, partially restored theresponsiveness to IAA. Analysis by GC/MS identified GA₁, GA₄,GA₉ GA₂₀ and GA₅₁ as native gibberellins in the hypocotyls ofcowpea seedlings. Analysis by GC/SIM also showed that the physiologicallyactive gibberellins (GA₁ and GA₄) were located mainly in theupper part of the hypocotyl and the treatment with uniconazolemarketly reduced the endogenous level of gibberellins thereto less than 11% of the control level. These results suggestthat levels of endogenous gibberellins possibly control theresponse to IAA in these segments. (Received May 12, 1994; Accepted November 15, 1994)