Auxin gradient along the root of the maize seedling

[5-³H]Indol-3yl-acetic acid (IAA) applied to the shoot apices of intact 6-day-old maize (Zea mays L.) plants moved into the primary root and accumulated at the root apex. IAA from the shoot could partially satisfy the requirement of the primary root for IAA for growth.Abbreviation IAA indol-3yl-acetic acid     

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.     

Acid growth effects in maize roots: Evidence for a link between auxin-economy and proton extrusion in the control of root growth

The role of proton excretion in the growth of apical segments of maize roots has been examined. Growth is stimulated by acidic buffers and inhibited by neutral buffers. Organic buffers such as 2[N-morpholino] ethane sulphonic acid (MES) — 2-amino-2-(hydroxymethyl)propane-1,3 diol (Tris) are more effective than phosphate buffers in inhibiting growth. Fusicoccin(FC)-induced growth is also inhibited by neutral buffers. The antiauxins 4-chlorophenoxyisobutyric acid (PCIB) and 2-(naphthylmethylthio) propionic acid (NMSP) promote growth and H⁺-excretion over short time periods; this growth is also inhibited by neutral buffers. We conclude that growth of maize roots requires proton extrusion and that regulation of root growth by indol-3yl-acetic acid (IAA) may be mediated by control of this proton extrusion.Abbreviations IAA indol-3yl-acetic acid - ABA abscisic acid - FC fusicoccin - PCIB 4-chlorophenoxy-isobutyric acid - MES 2(N-morpholino)ethane sulphonic acid - Tris 2-amino-2-(hydroxymethyl) propane-1,3-diol - NMSP 2-(naphthylmethylthio)propionic acid     

The influence of small direct electric currents on the transport of auxin in intact plants

When a d.c. potential of 9.0 V was applied to the stem of intact pea seedlings (Pisum sativum L. cv. Meteor and cv. Alderman) via 10 mM KCl-soaked filter paper electrodes placed ca. 50 mm apart the stem passed a steady current of 15–20 A (resistance ca. 100 k cm^-1). The basipetal transport of [1-¹⁴C]IAA applied to the apical bud was completely inhibited over the portion of the stem through which current flowed and ¹⁴C-labelled compounds accumulated in the vicinity of the upper electrode. The inhibition of transport was independent of the polarity of the applied potential. The basipetal transport of IAA in the stem above the electrode was not affected.Labelled auxin accumulated at the upper electrode both as unchanged IAA and as a compound tentatively identified as indol-3yl-acetyl aspartic acid (IAAsp). These compounds were only slowly remobilised when the current was interrupted. However, the ability of the transport system to move freshly-applied IAA was rapidly and fully restored when the potential was removed. No injury to the plant was detected after maintaining a current flow for up to 72 h. No leakage of ¹⁴C-labelled compounds into the KCl solution bathing the electrodes was detected.Abbreviations IAA indol-3yl-acetic acid - IAAsp indol-3yl-acetyl aspartic acid     

Auxin binding in roots: A comparison between maize roots and coleoptiles

Auxin binding onto membrane fractions of primary roots of maize seedlings has been demonstrated using naphth-1yl-acetic acid (NAA) and indol-3yl-acetic acid (IAA) as ligands. This binding is compared with the already well characterized interaction between auxins and coleoptile membranes. The results indicate that while kinetic parameters are of the same order for root and coleoptile binding, a number of differences occur with respect to location in cells and relative affinity. The possible significance of the existence of such binding sites in root cells is discussed in relation to auxin action.Abbreviations 4-Cl-PA 4-chlorophenoxyacetic acid - EDTA ethylene diamine tetracetic acid - IAA indol-3yl-acetic acid - MCPA 2-methyl-4-chlorophenoxyacetic acid - NAA naphth-1yl-acetic acid - 2-NAA naphth-2yl-acetic acid - Tris 2-amino-2-(hydroxymethyl) propane-1,3 diol - TIBA 2,3,5 triiodobenzoic acid - NPA naphthylphthalamic acid - PCIB 4-chlorophenoxyisobutyric acid - PCPP 4-chlorophenoxyisopropionic acid - 2,4-D 2,4-dichlorophenoxyacetic acid     

Reactive oxygen species localization in roots of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> seedlings grown under phosphate deficiency

Arabidopsis plants responding to phosphorus (P) deficiency increase lateral root formation and reduce primary root elongation. In addition the number and length of root hairs increases in response to P deficiency. Here we studied the patterns of radical oxygen species (ROS) in the roots of Arabidopsis seedlings cultured on media supplemented with high or low P concentration. We found that P availability affected ROS distribution in the apical part of roots. If plants were grown on high P medium, ROS were located in the root elongation zone and quiescent centre. At low P ROS were absent in the elongation zone, however, their synthesis was detected in the primary root meristem. The proximal part of roots was characterized by ROS production in the lateral root primordia and in elongation zones of young lateral roots irrespective of P concentration in the medium. On the other hand, plants grown at high or low P differed in the pattern of ROS distribution in older lateral roots. At high P, the elongation zone was the primary site of ROS production. At low P, ROS were not detected in the elongation zone. However, they were present in the proximal part of the lateral root meristem. These results suggest that P deficiency affects ROS distribution in distal parts of Arabidopsis roots. Under P-sufficiency ROS maximum was observed in the elongation zone, under low P, ROS were not synthesized in this segment of the root, however, they were detected in the apical root meristem.     

The Control of Lateral Root Development in Cultured Pea Seedlings. III. Spacing Intervals

The spacing of lateral root primordia in the primary root of Pisum sativum (cv. Alaska) seedlings is influenced by both predetermined lateral root initiation sites in the embryonic radicle and by factors present during seedling growth. When pea seeds were germinated in the presence of the mitotic inhibitor, colchicine, the triarch radicle produced three ranks of primordiomorphs indicating sites of embryonic lateral root primordia. The number of primordiomorphs was not the same along the three xylem strands in the radicle. Normally germinated seedling roots (5 days old) also showed a different number of lateral root primordia associated with the three strands. In both cases, the strand with the greatest number of primordia (or primordiomorphs) was associated with a cotyledonary trace. This indicated a possible role for the cotyledons in setting the pattern of lateral root distribution during radicle development. The spacing of lateral root primordia could be altered by the application of growth regulators. Seedling root tips (2 mm) were removed (? rt) and replaced with indoleacetic acid (+IAA), and in some instances seedlings were also treated with the auxin transport inhibitor, 3,3a-dihydro-2-(p-methoxyphenyl)-8H-pyrazolo[5, 1-α]isoindol-8-one (+DPX). In the growth regulator treatments, primary root elongation was inhibited, a greater number of lateral root primordia were initiated compared to controls, and the spacing intervals between primordia were greatly reduced. The — rt, +IAA, +DPX-treatment resulted in the closest possible spacing intervals (av. 0.4 ? 0.6 mm), but resulted in fused or fasciated laterals. The — rt, + IAA-treatment produced the shortest spacing intervals which resulted in “normal” lateral roots (0.8 ? 1.1 mm).     

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     

Applicability of the chemiosmotic polar diffusion theory to the transport of indol-3yl-acetic acid in the intact pea (Pisum sativum L.)

The transport of exogenous indol-3yl-acetic acid (IAA) from the apical tissues of intact, light-grown pea (Pisum sativum L. cv. Alderman) shoots exhibited properties identical to those associated with polar transport in isolated shoot segments. Transport in the stem of apically applied [1-¹⁴C]-or [5-³H]IAA occurred at velocities (approx. 8–15 mm·h^-1) characteristic of polar transport. Following pulse-labelling, IAA drained from distal tissues after passage of a pulse and the rate characteristics of a pulse were not affected by chases of unlabelled IAA. However, transport of [1-¹⁴C]IAA was inhibited through a localised region of the stem pretreated with a high concentration of unlabelled IAA or with the synthetic auxins 1-napthaleneacetic acid and 2,4-dichlorophenoxyacetic acid, and label accumulated in more distal tissues. Transport of [1-¹⁴C]IAA was also completely prevented through regions of the intact stem treated with N-1-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid.Export of IAA from the apical bud into the stem increased with total concentration of IAA applied (labelled+unlabelled) but approached saturation at high concentrations (834 mmol·m^-3). Transport velocity increased with concentration up to 83 mmol·m^-3 IAA but fell again with further increase in concentration.Stem segments (2 mm) cut from intact plants transporting apically applied [1-¹⁴C]IAA effluxed 93% of their initial radioactivity into buffer (pH 7.0) in 90 min. The half-time for efflux increased from 32.5 to 103.9 min when 3 mmol·m^-3 NPA was included in the efflux medium. Long (30 mm) stem sections cut from immediately below an apical bud 3.0 h after the apical application of [1-¹⁴C]IAA effluxed IAA when their basal ends, but not their apical ends, were immersed in buffer (pH 7.0). Addition of 3 mmol·m^-3 NPA to the external medium completely prevented this basal efflux.These results support the view that the slow long-distance transport of IAA from the intact shoot apex occurs by polar cell-to-cell transport and that it is mediated by the components of IAA transmembrane transport predicted by the chemiosmotic polar diffusion theory.Abbreviations IAA indol-3yl-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid     

The involvement of the root apex and cytokinins in the control of lateral root emergence in lettuce seedlings

Removal of the apical 3 mm of the primary root of hydroponically-grown lettuce seedlings 3 or 5 days after sowing, prevented further elongation of the root and increased both the number and total length of lateral roots. The length of the lateral zone, i.e. the distance from the base of the parent root to the lateral nearest the tip, except on one occasion, remained the same as the control in both 3 and 5 day treatments, until the length of the decapitated root (which had ceased elongating) became limiting.Zeatin applied via the roots, at a concentration range from 3 × 10^–10 M to 10^–8 M reduced tap root extension growth at all concentrations. Lateral root emergence was enhanced by low zeatin concentrations and retarded by higher ones. In general, the lateral zone length was the same in cytokinin-treated plants as in untreated controls.     

The effect of cycloheximide on IAA-stimulated transport of 14C-ABA and 14C-sucrose in long pea epicotyl segments

The effect of cycloheximide (CH) on the indol-3yl-acetic acid (IAA)-stimulated transport of ¹⁴C-labelled abscisic acid (ABA) and ¹⁴C-labelled sucrose was studied in 110 mm long pea epicotyl segments. IAA application resulted in elongation growth of the segments. This effect was decreased by CH treatment which also reduced [¹⁴C] ABA and [¹⁴C] sucrose accumulation in the growing apical part of the segments. A reduction in [¹⁴C] IAA uptake and in protein synthesis in this part of the segments was also observed. The simultaneous inhibition of protein synthesis and reduction of [¹⁴C] ABA and [¹⁴C] sucrose transport suggests that IAA can stimulate the transport of ABA and sucrose through a protein synthesis-based elongation growth.     

The Effect of IAA on Sugar Accumulation and Basipetal Transport of 14C-labelled Assimilates in Relation to Root Formation in Phaseolus vulgaris Cuttings

The effect of indol-3yl-acetic acid on root formation, accumulation of 80% ethanol-soluble sugars and basipetal transport of ¹⁴C-labelled assimilates has been investigated in Phaseolus vulgaris (cv. Canadian Wonder) hypocotyl cuttings. The removal of leaves reduced root formation in the hypocotyl, while excision of the apical bud was less detrimental. The expression of the IAA effect in inducing more roots was dependent on the area of leaves, and was found to be better when all leaves were present. Sugars accumulated slowly at the base of cuttings during a four-day period after excision, and IAA greatly enhanced this accumulation. By comparing sugar content at the base of green and starved cuttings it was established that IAA greatly increased it concurrently with root formation. IAA applied in solution to the hypocotyl greatly enhanced the basipetal transport of ¹⁴C-labelled assimilates and their accumulation at the hypocotyl during a 24-h period. The IAA-induced accumulation was found to be connected with a greater mobilization of labelled assimilates from upper parts of the cutting. Experiments involving pretreatment with IAA and transport in cuttings already possessing root primordia, suggest a dual effect of IAA: (I) a direct effect on transport, and (2) an increase in the root-“sink”. It is concluded that both may be operating in inducing basipetal accumulation of labelled assimilates. It is suggested that one of the roles of IAA in promoting rooting of cuttings is to increase sugar availability at the site of root formation.     

Root primordia and endogenous auxin in maize roots cultured in vitro

Quantitative analyses of indol-3yl-acetic acid (I aa ) in Zea mays L. (cv. LG 11) root segments cultured in vitro were performed by gas chromatography-mass spectrometry with selected ion monitoring. The root extracts were first purified by highperformance liquid chromatography. Root primordia initiation in intact and decapitated roots showed different patterns: decapitation strongly enhanced primordia initiation in their first 10 mm. During the culture (5 days), I aa content decreased in both intact and decapitated roots. No correlation was found between the level of endogenous auxin and the numher of root primordia initiated from either intact or decapitated maize root segments.     

The transport and metabolism of 14C-labelled indoleacetic acid in intact pea seedlings

Summary Part of the IAA-I- or IAA-2-¹⁴C applied at low concentrations to the apices of intact, light-grown dwarf pea seedling was transported unchanged to the root system The calculated velocity of transport in the stem was 11 mm per hour. In the root the label accumulated in the developing lateral root primordia.A large proportion of the applied IAA was converted by tissues of the apical bud, stem and root to indole-3-acetyl-aspartic acid (IAAsp). This compound was not transported. In addition evidence was obtained for the formation of IAA-protein complexes in the apex and roots, but not in the fully-expanded internodes.Large quantities of a decarboxylation product of IAA, tentatively indentified as indole-3-aldehyde (IAld), and several minor metabolites of IAA, were detected in extracts of the roots and first internodes, but not in the above-ground organs exposed to light. These compounds were readily transported through stem and root tissues. Together, the decarboxylation of IAA and the formation of IAAsp operated to maintain a relatively constant level of free IAA-¹⁴C in the root system.     

Lateral root formation in pine seedlings

The role of assimilates in lateral root development was studied in Pinus pinea seedlings grown in a nutrient solution. Seedlings were treated with ¹⁴CO₂ for 2 h following removal of the tap root tip at various times prior to the application of ¹⁴CO₂ or removal of a different number of cotyledons at one time. In seedlings with intact root systems most of the radioactivity accumulated in the lower section of the root containing the tap root apex. When the tap root tip was removed, the pattern of radioactivity accumulation along the root was affected by the presence and the stage of lateral root development. Removing the tap root tip of young seedlings (with no lateral roots) resulted in an almost equal distribution of radioactivity along the root. About 50% of the total radioactivity was found in the section showing the highest lateral root growth. Removing the tap root tip of mature seedlings (with lateral roots in the upper section) resulted in an immediate increase in the radioactivity accumulation in the upper section. When lateral roots appeared in the middle section, the pattern of radioactivity distribution was similar to that found in root decapitated young seedlings. Removal of cotyledons of mature seedlings somewhat increased the transport of radioactivity to the lower root section at the expense of the radioactivity in the lateral roots of the upper section. The present study suggests that competition within the root system between the tap root apex and the lateral roots may play an important role in determining the morphology of the root system.     

The Control of Lateral Root Development in Cultured Pea Seedlings. II. Root Fasciation Induced by Auxin Inhibitors

Lateral root development in cultured seedlings of Pisum sativum (cv. Alaska) was modified by the application of auxin transport inhibitors or antagonists. When applied either to replace the root tip or beneath the cotyledonary node, two auxin transport inhibitors, 2,3,5-triiodobenzoic acid (TIBA) and 3,3a-dihydro-2-(p-methoxyphenyl)-8H-pyrazolo[5,1-α]isoindol-8-one (DPX-1840), increased cell division activity opposite the protoxylem poles. This resulted in the formation of masses of cells, which we are calling root primordial masses (RPMs), 2 to 3 days after treatment. RPMs differed from lateral root primordia in that they lacked apical organization. Some roots however developed both RPMs and lateral roots indicating that both structures were similar in terms of the timing and location of cell division in the pericycle and endodermis leading to their initiation. Removal of the auxin transport inhibitors allowed many of the RPMs to organize later into lateral root primordia and to emerge in clusters. When the auxin, indoleacetic acid (IAA) was added to the growth medium along with DPX-1840, 3 ranks of RPMs now in the form of fasciated lateral roots emerged from the primary root. The auxin antagonist, p-chlorophenoxy-isobutyric acid (PCIB), also induced RPM formation. In contrast to DPX-1840 treatment, the addition of IAA during PCIB treatment caused normal lateral root development.     

Lateral Root Development in Attached and Excised Roots of Zea mays L. Cultivated in the Presence or Absence of Indol-3-yl Acetic Acid

The initiation of lateral root primordia and their subsequentemergence as secondary roots have been examined in attachedand excised roots of Zea mays grown in the presence or absenceof indol-3-yl acetic acid (IAA). Exposure to IAA enhanced anlageinception in both batches of roots. In the attached roots, theIAA-induced stimulation of primordium initiation was followedby a similar increase in lateral emergence. IAA treatment, however,had no effect on the number of laterals produced, per centimetreof root, in the excised primaries. Thus, exposure to IAA didnot directly enhance lateral emergence in the attached rootsnor did it stimulate such emergence in the excised ones. Nocorrelation was found between proliferative activity in themeristem at the apex of the primary or the rate of root elongationon the one hand, and either the number of primordia initiated,or the number of laterals produced, per centimetre of primary,on the other. Zea mays, maize, root, primordium, lateral, indol-3-yl acetic acid, meristematic activity     

Lead distribution in corn seedlings (Zea mays L.) and its effect on growth and the concentrations of potassium and calcium

It was observed that dry weight yield is not a sensitive parameter withwhich to assess lead toxicity to plants. Elongation growth of corn seedlingroots was more sensitive to lead than shoot growth and was inhibited by allconcentrations tested (10^–5, 10^–4, and 10^–3 M).It was positively correlated with potassium concentration and negativelycorrelated with lead concentration in the roots. Negative correlation also wasobserved between lead concentration and potassium concentration in roots. It ispostulated that inhibition of corn root growth is connected with potassiumleakage from root cells. The toxic action of lead on corn seedling mesocotylandcoleoptile growth was not correlated with potassium concentration in planttissue and correlation between growth and lead concentration was low. Inseedlings treated with 10^–4 and 10^–3 M lead the growthof mesocotyl and coleoptile was affected similarly, although the concentrationof lead was threefold higher in mesocotyl tissue than in coleoptile tissue. It isproposed that depression of corn seedlings shoot growth is not an effect ofpotassium leakage or lead accumulation but of an unknown signal induced inroots, as a response to exposure to lead, which is transmitted to shoots. Thepositive correlation between lead and calcium concentrations found in seedlingroots might be connected with high constitutional tolerance of corn to lead.Since the first 8 mm of an apical root accounts for 50% of thelead accumulated by the whole root, it is postulated that rhizofiltration oflead contaminated waters should be more efficient when plant species withhighly branched root systems are used.     

Regulation of auxin transport in pea (Pisum sativum L.) by phenylacetic acid: inhibition of polar auxin transport in intact plants and stem segments

The transport of [¹⁴C]phenylacetic acid (PAA) in intact plants and stem segments of light-grown pea (Pisum sativum L. cv. Alderman) plants was investigated and compared with the transport of [¹⁴C]indiol-3yl-acetic acid (IAA). Although PAA was readily taken up by apical tissues, unlike IAA it did not undergo long-distance transport in the stem. The absence of PAA export from the apex was shown not to be the consequence of its failure to be taken up or of its metabolism. Only a weak diffusive movement of PAA was observed in isolated stem segments which readily transported IAA. When [1-¹⁴C]PAA was applied to a mature foliage leaf in light, only 5.4% of the ¹⁴C recovered in ethanol extracts (89.6% of applied ¹⁴C) had been exported from the leaf after 6.0 h. When applied to the corresponding leaf, [¹⁴C]sucrose was readily exported (46.4% of the total recovered ethanol-soluble ¹⁴C after 6.0 h). [1-¹⁴C]phenylacetic acid applied to the root system was readily taken up but, after 5.0 h, 99.3% of the recovered ¹⁴C was still in the root system.When applied to the stem of intact plants (either in lanolin at 10 mg·g^-1, or as a 10^-4 M solution), unlabelled PAA blocked the transport through the stem of [1-¹⁴C]IAA applied to the apical bud, and caused IAA to accumulate in the PAA-treated region of the stem. Applications of PAA to the stem also inhibited the basipetal polar transport of [1-¹⁴C]IAA in isolated stem segments. These results are consistent with recent observations (C.F. Johnson and D.A. Morris, 1987, Planta 172, 400–407) that no carriers for PAA occur in the plasma membrane of the light-grown pea stem, but that PAA can inhibit the carrier-mediated efflux of IAA from cells. The possible functions of endogenous PAA are discussed and its is suggested that an important role of the compound may be to modulate the polar transport and-or accumulation by cells of IAA.Abbreviations IAA indol-3yl-acetic acid - NPA N-1-naphthylphthalamic acid - PAA phenylacetic acid - IIBA 2,3,5-triiodobenzoic acid     

Influence of rocks on soil temperature,soil water potential,and rooting patterns for desert succulents

Summary At a site in the Sonoran Desert, subterranean rocks and exposed boulders affected soil water potential as well as root morphology and distribution. For Agave deserti, the number of lateral roots per unit length of main root was 11 times higher under rocks and six times higher alongside rocks than in rock-free regions. Total root length per unit soil volume for Echinocereus engelmannii averaged 3-fold higher within 1 cm of boulders than 5 cm away, where the soil was drier. The total length of lateral roots per unit length of main root for Ferocactus acanthodes was 4.2 m m^–1 under rocks but only 0.8 m m^–1 in rock-free regions. The number of lateral roots per unit length of main root for Opuntia acanthocarpa was 7-fold higher alongside rocks than in rock-free regions and even higher under rocks. For transplanted and watered A. deserti, the number of new main roots produced per 1–2 month interval averaged 13 for five plants on the north side of boulders, 8 on the south side, 11 for five plants with half of their roots under rocks, 2 for those with half of their roots over rocks, and 3 for the control plants without rocks. Laboratory experiments showed that the soil water potential under rocks for 10 and 30 mm waterings stayed above –0.5 MPa for 13 and 19 d longer, respectively, than for regions away from rocks. The shortwave absorptance of granitic rocks from the field site was 0.82, the thermal conductivity coefficient was 1.50 W m^–1 °C^–1, and the volumetric heat capacity was 1.75 MJ m^–3 °C^–1. Field measurements indicated that 5-cm-thick buried rocks decreased the diel variation in soil temperatures on their undersurface by only 0.4° C compared with soil. Thus, the primary influence of rocks at the field site on root proliferation and branching for the four species was apparently caused by influences on soil water content.