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.     

Correction for non-linear relationships between root size and short term Pi uptake in genotype comparisons

Variation in phosphate (Pi) absorption rates, root length and root mass was investigated in 25 genotypes or inbred lines of Arabidopsis thaliana (L.) Heynh. All variables differed among lines (P<0.0001), and larger plants with larger roots and shoots had faster Pi uptake rates (influx) (P<0.0001). Within genotypes, influx was non-linearly related to root size. Increases in influx were not directly proportional to increases in root size; with a doubling in root length or biomass, there was only a 1.30 or 1.27 concomitant increase in influx, respectively. Because inbred lines are genetically homogeneous, this within-genotype pattern was random, and not genetically based.Influx has traditionally been standardized for variation in plant size by dividing by root length or biomass. This assumes a linear relationship between influx and root size. As uptake rate was non-linearly dependent on root size in A. thaliana, this resulted in a spurious negative correlation between influx per cm or mg root and root length or biomass. We present a general method that standardizes influx for variation in root size within genetically homogenous genotypes. This method determines and corrects for random non-linear relationships between influx and root size. In A. thaliana, influx per unit root length or biomass differed among lines (P<0.0001), but these differences did not correlate with root length or biomass. These data contradict the common interpretation in previous literature that species or cultivars with small roots have evolved greater influx per unit size of root.     

ABSCISIC ACID EFFECTS ON FRONDS AND ROOTS OF LEMNA MINOR L.

Plants of Lemna minor L. were grown in axenic culture in order to investigate the direct effect of abscisic acid (ABA) on individual fronds and roots. Both frond and root growth rates were inhibited by 60%, and total growth of both organs was reduced by 30% with ABA concentrations of 2 mg/l. Abscisic acid inhibited frond reproduction, and treated fronds tended to remain attached to parents. Reduction of root elongation and frond expansion occurred within 1–4 hr. Prolonged exposure to ABA inhibited both cell enlargement and cell division in the roots. The results are discussed in the light of current views on the control of growth with endogenous levels of promoter and inhibitor hormones.     

Effect of Fluoride on Nucleic Acids and Growth in Germinating Corn Seedling Roots

Corn seeds were treated with 0.01 M sodium fluoride for various time periods. The treated seeds were germinated and grown until the seedling roots reached a standard size of 12±3 mm. Analyses were made for RNA and DNA contents of 3-mm seedling root tips. Determinations also were made for growth rate, rate of cell elongation, cell multiplication, and tissue maturity of 12-mm roots. RNA contents of 3-mm root tips were found to be directly proportional to the growth rates of the entire seedling root of corn seeds treated with sodium fluoride for various periods of time. The RNA content was reduced on a cell basis and was independent of the root tip cell number. The amount of DNA was not related to the growth rate of the intact seedling roots. Since fluoride reduced the number of mitotic figures, it was likely that fluoride inhibited DNA synthesis during the interphase of the mitotic cycle. Growth by cell multiplication was inhibited more than that by cell elongation in the sample treated with fluoride for a shorter period. The two types of growth, however, showed a similar level of growth reduction in the sample treated with fluoride for a longer period. Fluoride seemed to reduce the rates of cellular elongation and multiplication not more than about 40 per cent of the control value in these tissues under present experimental conditions. Fluoride also induced maturity in the seedling roots in proportion to the periods of fluoride treatment.     

INFLUENCE OF THE TONIC EFFECT OF GRAVITATION AND AUXIN ON CELL ELONGATION AND POLARITY IN ROOTS

The influence of a longitudinal (tonic) gravitational force and of auxin on the pattern of growth and cell polarity has been studied on intact roots of wheat seedlings. A klinostat technique was used for controlling gravitation. Growth in length was evaluated as cell division activity, rate of cell elongation (μ/h) and duration of elongation (h). Exogenous auxin (1-NAA) increases the rate of cell elongation in all concentrations tested (10⁻⁸ — 3 × 10⁻⁷m ) and shortens the time of elongation with increasing concentration. It promotes rate of cell elongation in roots as it does in shoots. It also accentuates the polar insertion of root hairs and their growth. The tonic effect of gravitation resembles that of an increase in auxin both in light and darkness. The results are discussed in relation to plagiotropic growth of roots, root growth promotions by auxin, and the difference between root and shoot growth.     

Gradients in maize roots: local elongation and pH

The elongation rate, the gradient of the local elongation rate and the surface pH of maize roots were measured over 12 h. A data bank was constituted by storing these values. By sorting these results on the basis of different elongation rates, different classes of root were obtained. Two classes were chosen: the low-growth roots and the high-growth roots. The mean growth of these two root classes was stable with time and differed significantly from one another. The surface pH of the elongation zone was the same for the roots of these two classes, but the roots selected for their higher growth rate had a larger acid efflux in this zone.     

Mapping of QTLs for lateral and axile root growth of tropical maize

Maize genotypes may adapt to dry environments by avoiding desiccation by means of a deeper root system or by maintaining growth and water extraction at low water potentials. The aim of this study was to determine the quantitative genetic control of root growth and root morphology in a population of 236 recombinant inbred lines (RILs) from the cross between CML444 (high-yielding) × SC-Malawi (low-yielding), which segregates for the response to drought stress at flowering. The RILs and the parental lines were grown on blotting paper in growth pouches until the two-leaf stage under non-stressed conditions; the parents were additionally exposed to desiccation stress induced by polyethylene glycol with a molecular weight of 8000 Dalton (PEG-8000). The lengths of axile and lateral roots were measured non-destructively at 2, 5, 7 and 9 days after germination, by scanning with an A4 scanner followed by digital image analysis. CML444 had a lower rate constant of lateral root elongation (k_Lat) than SC-Malawi, but the two genotypes did not differ in their response to desiccation. QTLs affecting root vigor, as depicted by increments in k_Lat, the elongation rate of axile roots (ER_Ax) and the number of axile roots (No_Ax) were identified in bins 2.04 and 2.05. QTLs for No_Ax and ER_Ax collocated with QTLs for yield parameters in bins 1.03–1.04 and 7.03–04. The correspondence of QTLs for axile root traits in bins 1.02–1.03 and 1.08 and QTLs for lateral roots traits in bins 2.04–2.07 in several mapping populations suggests the presence of genes controlling root growth in a wide range of genetic backgrounds.     

Possible Involvement of Cytokinin in Nitrate-mediated Root Growth in Maize

Response of root system architecture to nutrient availability in soils is an essential way for plants to adapt to soil environments. Nitrate can affect root development either as a result of changes in the external concentration, or through changes in the internal nutrient status of the plant. Nevertheless, less is known about the physiological mechanisms. In the present study, two maize (Zea mays L.) inbred lines (478 and Wu312) were used to study a possible role of cytokinin in nitrate-mediated root growth in nutrient solutions. Root elongation of 478 was more sensitive to high nitrate supply than that of Wu312. Medium high nitrate (5 mM) inhibited root elongation in 478, while, root elongation in Wu312 was only inhibited at high NO ₃ ⁻ supply (20 mM). Under high nitrate supply, the root elongation zone in 478 became swollen and the site of lateral root elongation was close towards the root tip. Both of the phenomena are typical of root growth induced by exogenous cytokinin treatments. Correspondingly, zeatin and zeatin nucleotide (Z + ZR) concentrations were increased at higher nitrate supply in 478, whereas they were constant in Wu312. Furthermore, exogenous cytokinin 6-benzylaminopurine (6-BA) completely reversed the stimulatory effect of low nitrate on root elongation. Therefore, it is supposed that the inhibitory effect of high concentration of nitrate on root elongation is, at least in part, mediated by increased cytokinin level in roots. High nitrate supply may have negative influences on root apex activity by affecting cytokinin metabolism so that root apical dominance is weakened and, therefore, root elongation is suppressed and lateral roots grow closer to the root apex. Nitrate suppressed lateral root elongation in Wu312 at concentration higher than 5 mM. In 478, however, this phenomenon was not significant even at 20 mM nitrate. Although exogenous 6-BA (20 nM) could suppress lateral root elongation as well, the inhibitory effect of high NO ₃ ⁻ concentration of nitrate on lateral root growth cannot be explained by changes in endogenous cytokinin alone.     

Distribution and redistribution of extension growth along vertical and horizontal gravireacting maize roots

Horizontal primary roots of Zea mays L. were photographed during the course of their gravireaction and during a preceding growth period in the vertical orientation. The displacement, by root elongation, of marker particles on the root surface was recorded. The particle-displacement rates were used to estimate the distribution of elemental elongation rates along opposite sides of the growing root apex. In the temperature range 21–25°C there was a stimulation of local elongation rates along the upper side of a gravireacting root and a reduction (and sometimes a cessation) of elongation along the lower side. Elemental elongation rates have been related to the development of root curvature, and the magnitude of the differential growth between upper and lower sides required for a particular rate of bending has also been estimated. The results complement, and are compatible with, findings relating to the distribution of certain endogenous growth regulators believed to participate in the gravireaction.Abbreviation RELEL relative elemental rate of elongation     

QTLs for the elongation of axile and lateral roots of maize in response to low water potential

Changes in root architecture and the maintenance of root growth in drying soil are key traits for the adaptation of maize (Zea mays L.) to drought environments. The goal of this study was to map quantitative trait loci (QTLs) for root growth and its response to dehydration in a population of 208 recombinant inbred lines from the International Maize and Wheat Improvement Center (CIMMYT). The parents, Ac7643 and Ac7729/TZSRW, are known to be drought-tolerant and drought-sensitive, respectively. Roots were grown in pouches under well-watered conditions or at low water potential induced by the osmolyte polyethylene glycol (PEG 8000). Axile root length (L _Ax) increased linearly, while lateral root length (L _Lat) increased exponentially over time. Thirteen QTLs were identified for six seedling traits: elongation rates of axile roots (ER_Ax), the rate constant of lateral root elongation (k _Lat), the final respective lengths (L _Ax and L _Lat), and the ratios k _Lat/ER_Ax and L _Lat/L _Ax. While QTLs for lateral root traits were constitutively expressed, most QTLs for axile root traits responded to water stress. For axile roots, common QTLs existed for ER_Ax and L _Ax. Quantitative trait loci for the elongation rates of axile roots responded more clearly to water stress compared to root length. Two major QTLs were detected: a QTL for general vigor in bin 2.02, affecting most of the traits, and a QTL for the constitutive increase in k _Lat and k _Lat/ER_Ax in bins 6.04–6.05. The latter co-located with a major QTL for the anthesis-silking interval (ASI) reported in published field experiments, suggesting an involvement of root morphology in drought tolerance. Rapid seedling tests are feasible for elucidating the genetic response of root growth to low water potential. Some loci may even have pleiotropic effects on yield-related traits under drought stress.     

Primary and Lateral Root Development of Dark- and Light-Grown Cotton Seedlings under Salinity Stress*

Primary root growth dynamics and lateral root development of dark- and light grown cotton seedlings (Gossypium hirsutum L., cv. Acala SJ-2) were studied under control and salinity stress conditions. The seedlings were grown by two methods: A) in paper-lined, vermiculite-filled beakers with the plants growing between the paper and the glass wall (Gladish and Rost, 1993), and B) in hydroponics after germination and initial growth in germination paper rolls saturated with the treatment solutions (Kent and Läuchli, 1985). After germination, daily primary root elongation rate gradually incrased to a maximum, then gradually declined to close to zero for dark-grown seedlings, or to sustained rates of about 10 mm per day for light-grown control plants. Salinity stress delayed primary root growth and reduced peak elongation rates, without changing the general primary root growth pattern. These results suggest that salinity changed the time-scale, but did not modify the normal developmental sequence. Lateral root growth was more inhibited by salinity than primary root growth. In addition, elongation of lateral roots was more inhibited by salinity than their initiation and emergence. Light exposure of the shoot favored both sustained primary root growth from 7 days after planting, and lateral root emergence and growth. Salinity effects were more severe on seedlings germinated and grown in hydroponics (method B) than on vermiculite-grown plants (method A). These results emphasize the importance of growing conditions for the NaCl-induced effects on cotton root development. In addition, the differential effects of salinity on primary and lateral roots became evident, pointing to diverse control mechanisms for the development of these root types.     

Periodicity in the development of the root system of young rubber trees (Hevea brasiliensis Müell. Arg.): relationship with shoot development

The growth pattern of the root system of young rubber trees (Hevea brasiliensis) was studied in relation to shoot development over a period of 3 months. Temporal and spatial variations in elongation and branching processes were examined for the different root types, by means of root observation boxes. Shoot growth was typically rhythmic. Root development was periodic and related to leaf expansion. Root elongation was depressed during leaf growth, whereas branching was enhanced. Consequently, highly branched areas with vigorous secondary roots alternated along the taproot with poorly branched areas with shorter roots. Root types were not affected to the same degree by shoot competition: during leaf expansion, taproot growth was just depressed but remained continuous, the emergence and elongation rates of secondary roots were significantly affected and the elongation rates of tertiary roots fell to zero. These results were consistent with the hypothesis that root growth is related to competition for assimilates and to the sink strength of the different root types, whereas root branching appeared to be promoted by leaf development.     

Effect of auxin on mesocotyl elongation of dark-grown maize under different seeding depths

In order to elucidate the physiological mechanism of maize mesocotyl elongation induced by auxin under different seeding depths, seeds of five maize inbred lines, including 3681-4 line tolerant to deep seeding, were treated with IAA and triiodobenzoic acid (TIBA) under seeding depths of 20 or 2 cm. Under deep seeding conditions, maize mesocotyls grew by 1.5–2.0 times faster than under shallow seeding conditions. IAA (10⁻⁶ to 10⁻⁴ M) applied to roots stimulated mesocotyl elongation only of 3681-4 line and only under deep seeding conditions. TIBA (10⁻⁵ and 10⁻⁴ M) applied to roots inhibited mesocotyl elongation in all lines, but only 3681-4 was sensitive to 10⁻⁶ M TIBA. IAA promoted only cell elongation, and TIBA inhibited both cell elongation and cell division. After IAA and TIBA treatments, endogenous IAA content changed in parallel with the mesocotyl growth rate under different seeding depths. Furthermore, ABP1 gene expression changed in parallel with the mesocotyl growth rate under deep seeding conditions. Therefore, deep seeding tolerance of 3681-4 line was achieved due to auxin-regulated rapid mesocotyl elongation.     

Production of transgenic Aralia elata regenerated from Agrobacterium rhizogenes-mediated transformed roots

Transgenic hairy roots were induced from petiole and root segments of in vitro plant Aralia elata, a medicinal woody shrub, after co-cultivation with A. rhizogenes ATCC 15834. The percentage of putative hairy root induction from root segments was higher (26.7%) than petiole explants (10.0%). Hairy roots showed active production of lateral roots with vigorous elongation. Transgenic plants were regenerated from hairy roots via somatic embryogenesis. These plants had wrinkled leaves, short petioles and numerous lateral hairy roots. The RT-PCR analysis showed the expression of rol A, B, C, D, aux 1 and 2 genes differed between the transgenic lines. Endogenous IAA level was higher in transgenic than non-transgenic plants. Conclusively, transgenic hairy roots were developed for first time in A. elata and the transgenic hairy root lines showed distinct morphological growth pattern and gene expression.     

Root elongation rate is accounted for by intercepted PPFD and source-sink relations in field and laboratory-grown sunflower

The existence of relationships between intercepted photo-synthetic photon flux density (PPFD) and growth of individual organs is somewhat controversial. We have tested whether such relationships could account for the natural variability in elongation rates of taproot and secondary roots of sunflower (from 2 to 135 mm d⁻¹), in field and laboratory conditions. Elongation of taproot and secondary roots was recorded daily through windows in the field. A range of PPFD was obtained by following day-to-day natural fluctuation for three contrasting growing periods, and by shading part of the plants under study. A parallel experiment was carried out in a growth chamber with contrasting light intensities and with a ¹⁴CO₂ labelling experiment. After the two-leaf stage, i.e. when the contribution of photosynthetic carbon became appreciable in root growth, daily root elongation rate was closely linked to the PPFD intercepted from 36 to 12 h before the measurement of root elongation. Curvilinear relationships applied to plants grown in the field as well as in a growth chamber, and to shaded plants as well as to plants subjected to day-to-day changes in intercepted PPFD. For a given intercepted PPFD, the taproot elongated faster than secondary roots, and secondary roots originating near the base of the taproot elongated faster than those originating near the apex. The elongation rate of any secondary root apex was accounted for (r= 0.77) by the ratio of intercepted PPFD to the distance between the apex and the base of the taproot. No relationships between intercepted PPFD and elongation rate were observed before the two-leaf stage, when the CO₂ labelling experiment suggests that carbon essentially originates from the seed. Therefore, this study suggests a role for source-sink relations in the distribution of elongation between apices and a role for carbon nutrition in day-to-day variations of root elongation rate. Precise mechanisms explaining this behaviour remain to be investigated.     

Abscisic acid and auxin accumulation in Catasetum fimbriatum roots growing in vitro with high sucrose and mannitol content

Endogenous contents of indolyl-3-acetic acid (IAA) and abscisic acid (ABA) were quantified in excised roots of Catasetum fimbriatum (Orchidaceae) cultured in vitro on solidified Vacin and Went medium with 1, 2, 4, 6, 8 and 10 % sucrose, as well as 2 % sucrose plus mannitol. Maximum root growth was observed in media with 4 % sucrose and 2 % sucrose plus 2.2 % mannitol, suggesting that a moderate water or osmotic stress promotes orchid root growth. Contents of both ABA and IAA increased in parallel to increasing sucrose concentration and a correlation between root elongation and the ABA/IAA ratio was observed. Incubating isolated C. fimbriatum roots with radiolabeled tryptophan, we showed an accumulation of IAA and its conjugates.     

THE DEVELOPMENT OF ISOLATED ROOTS OF COMPTONIA PEREGRINA (MYRICACEAE) IN CULTURE

Seedling roots of the sweet fern Comptonia peregrina (L.) Coult. were excised aseptically and cultured in a modified Bonner-Devirian liquid nutrient medium. Root elongation was very slow in the basic medium which contained inorganic salts, B-vitamins, trace elements and 4 % sucrose. The addition of plant hormones including gibberellic acid, indoleacetic acid, and zeatin, alone or in combinations, had little effect on growth. Myoinositol at 10 or 100 ppm doubled the rate of elongation. The effect of this sugar alcohol could not be replaced by scyllitol, D-sorbitol, D-mannitol or by increasing the sucrose concentration. Subcultured root tips showed progressively less elongation in successive transfers. Secondary thickening of the roots, especially in the basal half, occurred in initial passages and in subcultured roots without added hormones. Root buds also occurred spontaneously especially in the basal portions of cultured roots, both in first and in successive passages. An anatomical analysis showed that these buds were endogenous, arising from a secondary cortex of pericyclic origin.     

Development of Linum flavum hairy root cultures for production of coniferin

Linum flavum hairy roots were initiated from leaf discs using Agrobacterium rhizogenes strains LBA9402 and TR105 though two other strains, 15834 and A4, were relatively ineffective for induction. Significant variation in coniferin accumulation was observed between hairy root lines originating from different L. flavum seedlings and/or A. rhizogenes strains. Coniferin reached 58 mg g^–1 dry wt by culturing the roots in Linsmaier and Skoog (LS) medium with 2,4-dichlorophenoxyacetic acid and naphthaleneacetic acid as growth regulators.     

Root growth dynamics of Aleppo pine (Pinus halepensis Mill.) seedlings in relation to shoot elongation, plant size and tissue nitrogen concentration

Large and high nitrogen (N) concentration seedlings frequently have higher survival and growth in Mediterranean forest plantations than seedlings with the opposite traits, which has been linked to the production of deeper and larger root systems in the former type of seedlings. This study assessed the influence of seedling size and N concentration on root growth dynamics and its relation to shoot elongation in Aleppo pine (Pinus halepensis Mill.) seedlings. We cultivated seedlings that differed in size and tissue N concentration that were subsequently transplanted into transparent methacrylate tubes in the field. The number of roots, root depth, and the root and shoot elongation rate (length increase per unit time) were periodically measured for 10 weeks. At the end of the study, we also measured the twig water potential (ψ) and the mass of plant organs. New root mass at the end of the study increased with seedling size, which was linked to the production of a greater number of new roots of lower specific length rather than to higher elongation rate of individual roots. Neither plant size nor N concentration affected root depth. New root mass per leaf mass unit, shoot elongation rate, and pre-dawn ψ were reduced with reduction in seedling size, while mid-day ψ and the root relative growth rate were not affected by seedling size. N concentration had an additive effect on plant size on root growth but its overall effect was less important than seedling size. Shoot and roots had an antagonistic elongation pattern through time in small seedlings, indicating that the growth of both organs depressed each other and that they competed for the same resources. Antagonism between shoot and root elongation decreased with plant size, disappearing in large and medium seedlings, and it was independent of seedling N concentration. We conclude that root and shoot growth but not rooting depth increased with plant size and tissue N concentration in Aleppo pine seedlings. Since production of new roots is critical for the establishment of planted seedlings, higher absolute root growth in large seedlings may increase their transplanting performance relative to small seedlings. The lack of antagonism between root and shoot growth in large seedlings suggests that these plants can provide resources to sustain simultaneous growth of both organs.     

In vitro loss of nucleic acids from cells of aseptically cultured excised pea roots

Summary Analysis of pea root tips taken from attached seedling roots and excised roots cultured in vitro has revealed major differences in cell constituents. The cells of cultured roots have only 40% and 13% of the protein and amino acid content of attached root cells. The nucleic acid content of cultured root cells was shown to be only 20% and 27% of the RNA and DNA respectively found in attached roots. It is suggested that there is excess nucleic acid in whole plant tissues above that required for transfer of genetic information necessary for normal growth and differentiation of root cells.