Promoted Root Elongation Research Articles

Interactions between magnesium, calcium, and aluminum on soybean root elongation

Alleviation of Al rhizotoxicity by Ca and Mg can differ among species and genotypes. Root elongation of soybean [Glycine max (L.) Merr.] line N93-S-179 and cvs. Young and Ransom exposed to varying concentrations of Al, Ca and Mg were compared in two experiments using a vertically split root system. Roots extending from a surface compartment with limed soil grew for 12 days into a subsurface compartment with nutrient solution treatments maintained at pH 4.6 with either 0 or 15 µmol L-1 Al. Calcium and Mg concentrations in treatments ranging from 0 to 20 mmol L-1. Although an adequate supply of Mg was provided in the surface soil compartment for soybean top growth, an inclusion of Mg was necessary in the subsurface solutions to promote root elongation in both the presence and absence of Al. In the absence of Al in the subsurface solution, tap root length increased by 74 % and lateral root length tripled when Mg in the solutions was increased from 0 to either 2 or 10 mmol L-1. In the presence of 15 µmol L-1 Al, additions of 2 or 10 mmol L-1 Mg increased tap root length fourfold and lateral root length by a factor of 65. This high efficacy of Mg may have masked differences in Al tolerance between genotypes N93 and Young. Magnesium was more effective than Ca in alleviating Al rhizotoxicity, and its ameliorative properties could not be accounted for by estimated electrostatic changes in root membrane potential and Al3+ activity at the root surface. The physiological mechanisms of Mg alleviation of Al injury in roots, however, are not known.

Polyamines promote root elongation and growth by increasing root cell division in regenerated Virginia pine (Pinus virginiana Mill.) plantlets

Polyamines have been demonstrated to play an important role in adventitious root formation and development in plants. Here, we present a detailed analysis of influence of exogenously added polyamines on adventitious root development and its relationship to cold tolerance in Virginia pine (Pinus virginia Mill.). Our results demonstrated that polyamines putrescine (Put), spermidine (Spd), and spermine (Spm) at 0.001 mM improve rooting frequency and promote root elongation. Put, Spd, and Spm at 0.01-1 mM decrease rooting frequency and reduce root elongation root elongation. Measurements of diamine oxidase (DAO, EC 1.4.3.6) and polyamine oxidase (PAO, EC 1.4.3.4) activities showed that higher DAO and PAO enzyme activities were obtained when high concentrations of polyamines were applied and when plantlets were treated for 5-7 week at 4 degrees C and 16 degrees C. Survival rate of plantlets increased with the treatment of polyamines at low temperature. Polyamines increased mitotic index of cells in root tips of regenerated plantlet cultured on medium containing 0.001 microM Put, Spd, or Spm, but did not increase mitotic index in tissues of needle tips of the same plantlets. These results demonstrated that polyamines promote root elongation and growth by increasing root cell division in regenerated Virginia pine plantlets.

Seedling growth of Acacia tortilis and Faidherbia albida in response to simulated groundwater tables

The growth and survival of woody seedlings in arid and semi-arid riverine forests is partly determined by the ability of their roots to maintain contact with the alluvial water table. After a flood event, the water table declines in conjunction with the river stage and the rate of decline may have species-specific effects on the regeneration of riverine trees. In this study, a growth experiment involving seedlings of Acacia tortilis and Faidherbia albida, two keystone tree species along the regulated Turkwel River in northern Kenya, was conducted to assess the effects of simulated pre- and post-dam water table dynamics. Seedlings were grown in rhizopods with constantly shallow water tables (0 cm day −1 treatment), slow rates of water table decline (5 cm day −1 treatment), rapid rates of water table decline (10 cm day −1 treatment) and no subsurface water (rainfall treatment). The last treatment corresponded to the dam-induced paucity of floodplain inundation but allowed early seedling growth in response to rainfall. Results showed that F. albida attained larger shoot growth but shorter root lengths than A. tortilis, while water table decline promoted root elongation in both species. However, F. albida seedlings were adversely affected by moisture deficits under the rapid rate of water table decline and rainfall treatments. In contrast, the growth of A. tortilis seedlings was sustained under all treatments, suggesting that A. tortilis is a drought tolerant facultative phreatophyte, while F. albida behaves more like a near obligate phreatophyte. For F. albida, the ideal stage recession rate following a flood event is 5 cm day −1 or less, which is also favourable for A. tortilis. Although more research is still needed on the hydrological preferences of woody seedlings in the Turkwel River floodplain, it is evident that the regeneration of strictly riverine trees such as F. albida depends on slow rates of water table decline in the post-flood period.

Brassinosteroids Promote Root Growth in Arabidopsis

Although brassinosteroids (BRs) are known to regulate shoot growth, their role in the regulation of root growth is less clear. We show that low concentrations of BRs such as 24-epicastasterone and 24-epibrassinolide promote root elongation in Arabidopsis wild-type plants up to 50% and in BR-deficient mutants such as dwf1-6 (cbb1) and cbb3 (which is allelic to cpd) up to 150%. The growth-stimulating effect of exogenous BRs is not reduced by the auxin transport inhibitor 2,3,5-triidobenzoic acid. BR-deficient mutants show normal gravitropism, and 2,3,5-triidobenzoic acid or higher concentrations of 2,4-dichlorophenoxyacetic acid and naphtaleneacetic acid inhibit root growth in the mutants to the same extent as in wild-type plants. Simultaneous administration of 24-epibrassinolide and 2,4-dichlorophenoxyacetic acid results in largely additive effects. Exogenous gibberellins do not promote root elongation in the BR-deficient mutants, and the sensitivity to the ethylene precursor 1-aminocyclopropane-1-carboxylic acid is not altered. Thus, the root growth-stimulating effect of BRs appears to be largely independent of auxin and gibberellin action. Furthermore, we analyzed BR interactions with other phytohormones on the gene expression level. Only a limited set of auxin- and ethylene-related genes showed altered expression levels. Genes related to other phytohormones barely showed changes, providing further evidence for an autonomous stimulatory effect of BR on root growth.

Silicon-induced changes in viscoelastic properties of sorghum root cell walls.

Silicon is deposited in the endodermal tissue in sorghum (Sorghum bicolor L. Moench) roots. Its deposition is thought to protect vascular tissues in the stele against invasion by parasites and drying soil via hardening of endodermal cells. We studied the silicon-induced changes in mechanical properties of cell walls to clarify the role of silicon in sorghum root. Sorghum seedlings were grown in nutrient solution with or without silicon. The mechanical properties of cell walls were measured in three separated root zones: basal, apical and subapical. Silicon treatment decreased cell-wall extensibility in the basal zone of isolated stele tissues covered by endodermal inner tangential walls. The silicon-induced hardening of cell walls was also measured with increases in elastic moduli (E) and viscosity coefficients (eta). These results provided new evidence that silicon deposition might protect the stele as a mechanical barrier by hardening the cell walls of stele and endodermal tissues. In contrast to the basal zone, silicon treatment increased cell-wall extensibility in the apical and subapical zones with concomitant decrease in E and eta. Simultaneously, silicon promoted root elongation. When root elongation is promoted by silicon, one of the causal factors maybe the silicon-enhanced extensibility of cell walls in the growing zone.

4,4,4-trifluoro-3-(indole-3-)butyric acid promotes root elongation in Lactuca sativa independent of ethylene synthesis and pH.

We studied the mode of action of 4,4,4-trifluoro-3- (indole-3-) butyric acid (TFIBA), a recently described root growth stimulator, on primary root growth of Lactuca sativa L. seedlings. TFIBA (100 micromoles) promoted elongation of primary roots by 40% in 72 h but inhibited hypocotyl growth by 35%. TFIBA induced root growth was independent of pH. TFIBA did not affect ethylene production, but reduced the inhibitory effect of ethylene on root elongation. TFIBA promoted root growth even in the presence of the ethylene biosynthesis inhibitor L-alpha-(2-aminoethoxyvinyl)glycine. TFIBA and the ethylene-binding inhibitor silver thiosulphate (STS) had a similar effect on root elongation. The results indicate that TFIBA-stimulated root elongation was neither pH-dependent nor related to inhibition of ethylene synthesis, but was possibly related to ethylene action.

In vitro propagation and iridoid analysis of the medicinal species Harpagophytum procumbens and H . zeyheri

Extracts of the tubers of Harpagophytum procumbens DC (Devil's Claw) are used widely for the relief of arthritis, lumbago and muscular pain. The anti-inflammatory activity has been attributed to their iridoid components. A two-step protocol was established for the in vitro propagation of plants of Harpagophytum sp. by the regeneration of new plantlets from nodal cuttings and their acclimatisation to ex vitro conditions. Single node cuttings were submitted to a root induction treatment with β-indoleacetic acid (5 days at 2 mg l–1) followed by a transfer to a phytohormone-free medium to promote root elongation and support plantlet development. The new plantlets were weaned under autotrophic conditions and subsequently acclimatised in a glasshouse where they grew into fertile flowering plants that produced the characteristic Devil's Claw fruits as well as tuberised roots. Analysis of the tuber tissue of the micropropagated plants showed the presence of the iridoids harpagoside and harpagide at concentrations comparable with those found in the wild plant material (1% dry weight). The leaves were also found to contain these iridoids, and therefore could potentially provide an alternative and more sustainable source of therapeutically active compounds. The application of in vitro methods for the propagation of Devil's Claw would contribute to the conservation of this species.

Comparative tolerances of riparian willows and cottonwoods to water-table decline

Cottonwoods (Populus sp.) and willows (Salix sp.) generally dominate riparian landscapes across western North America. To investigate their relative tolerances to water-table decline, rooted shoot cuttings (saplings) of two willows, Salix exigua and S. drummondiana, and two cottonwoods, Populus angustifolia, and P. balsamifera, were grown in rhizopods, controlled growth devices that allow water-table manipulation. Water-table-decline rates of 0 to 12 cm/d were applied and plant growth and survival were monitored. In a second study, seedlings of S. exigua, S. lutea, P. balsamifera, and P. deltoides were grown in rhizopods and exposed to water-table-decline rates of 0 to 8 cm/d. For saplings and seedlings of both genera, gradual declines of 1 or 2 cm/d promoted root elongation and often promoted shoot growth relative to the constant water-table treatment (0 cm/d). More abrupt declines (> 2 cm/d) reduced growth and survival, and there were some differences in sensitivity within as well as across the two genera. Thus, the willow and cottonwood saplings were similarly affected by abrupt water-table decline, but willow seedlings were slightly more vulnerable than cottonwood seedlings. It is recommended that gradual stream stage recession along regulated rivers of about 2.5 cm/d in the late spring would encourage the recruitment of cottonwood seedlings, but subsequently, more gradual recession of about 1 cm/d through the lower elevational streamside zones would enable recruitment of S. exigua and other willows.

Determination of 1-aminocycopropane-1-carboxylic acid (ACC) to assess the effects of ACC deaminase-containing bacteria on roots of canola seedlings.

Previously, it was proposed that plant growth-promoting bacteria that possess the enzyme, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, can reduce the amount of ethylene produced by a plant and thereby promote root elongation. To test this model, canola seeds were imbibed in the presence of the chemical ethylene inhibitor, 2-aminoethoxyvinyl glycine (AVG), various strains of plant growth-promoting bacteria, and a psychrophilic bacterium containing an ACC deaminase gene on a broad host range plasmid. The extent of root elongation and levels of ACC, the immediate precursor of ethylene, were measured in the canola seedling roots. A modification of the Waters AccQ.Tag Amino Acid Analysis Method was used to quantify ACC in the root extracts. It was found that, in the presence of the ethylene inhibitor, AVG, or any one of several ACC deaminase-containing strains of bacteria, the growth of canola seedling roots was enhanced and the ACC levels in these roots were lowered.

Effect of transferring 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase genes into Pseudomonas fluorescens strain CHA0 and its gacA derivative CHA96 on their growth-promoting and disease-suppressive capacities.

Pseudomonas fluorescens strain CHA0, a root colonizing bacterium, has a broad spectrum of biocontrol activity against plant diseases. However, strain CHA0 is unable to utilize 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of plant ethylene, as a sole source of nitrogen. This suggests that CHA0 does not contain the enzyme ACC deaminase, which cleaves ACC to ammonia and alpha-ketobutyrate, and was previously shown to promote root elongation of plant seedlings treated with bacteria containing this enzyme. An ACC deaminase gene, together with its regulatory region, was transferred into P. fluorescens strains CHA0 and CHA96, a global regulatory gacA mutant of CHA0. ACC deaminase activity was expressed in both CHA0 and CHA96. Transformed strains with ACC deaminase activity increased root length of canola plants under gnotobiotic conditions, whereas strains without this activity had no effect. Introduction of ACC deaminase genes into strain CHA0 improved its ability to protect cucumber against Pythium damping-off, and potato tubers against Erwinia soft rot in small hermetically sealed containers. In contrast, ACC deaminase activity had no significant effect on the ability of CHA0 to protect tomato against Fusarium crown and root rot, and potato tubers against soft rot in large hermetically sealed containers. These results suggest that (i) ACC deaminase activity may have lowered the level of plant ethylene thereby increasing root length; (ii) the role of stress-generated plant ethylene in susceptibility or resistance depends on the host-pathogen system, and on the experimental conditions used; and (iii) the constructed strains could be developed as biosensors for the role of ethylene in plant diseases.

Effect of Exogenously Applied L-Tryptophan on Allelochemical Activity of Plant-Growth-Promoting Rhizobacteria (PGPR)

The effect of L-tryptophan on root elongation of lettuce seedlings (Lactuca sativa), as well as on allelochemical activity of three species of root growth promoting rhizobacteria (PGPR), was studied under controlled axenic conditions. L-Tryptophan exhibited an hormonelike effect, promoting root elongation of lettuce seedlings at 10−5 and 10−3 mM by ca. 20%, but at 10 mM, root elongation was inhibited by ca. 52% compared to the control (L-tryptophan-free). Comamonas acidovorans 26, Agrobacterium sp., and Alcaligenes piechaudii promoted root elongation of lettuce seedlings by 15, 30, and 44%, respectively. When 10 mM L-tryptophan was applied, Agrobacterium sp. and A. piechaudii inhibited root elongation by 57.6 and 63.5%, respectively. However, at the same concentration of L-tryptophan, C. acidovorans 26 promoted root growth. It is suggested that L-tryptophan reverted root growth promotion through enhancing the secretion of indole-3-acetic acid by the rhizobacteria. L-Tryptophan may have both a direct effect on root growth and an indirect effect through affecting activity of root growth-mediating bacteria. Since the growth of C. acidovorans 26 was exclusively inhibited by L-tryptophan (10 mM), it is conjectured that reduced population of this bacterium could not generate IAA at a level sufficient to inhibit root elongation. Despite the mild root growth promoting effect of C. acidovorans 26, its consistent root growth promotion, perhaps justifies further experimentation.

Arbuscular mycorrhizal formation in undisturbed soil counteracts compacted soil stress for pigeon pea

Plant growth is sometimes restricted with soil compaction under no-tillage conditions, although undisturbed soils are favorable to arbuscular mycorrhizal (AM) fungi of a symbiont. We examined growth responses of the pigeon pea ( Cajanus cajan (L.) Millsp.) to soil disturbance and inoculation with an AM fungus ( Gigaspora margarita Becker & Hall) in a pot experiment. The AM fungal inoculum was added to the soil and wheat was grown. After 6 months the shoot of wheat was removed and the soil was either disturbed or remained undisturbed. Subsequently, pigeon pea was grown and harvested after 3 months. The colonization and spore density of Gigaspora were significantly greater in undisturbed soil than in disturbed soil. Undisturbed soil showed higher penetrometer resistance and resulted in impaired shoot growth of the pigeon pea with lower shoot-to-root ( S/ R) ratio than disturbed soil. However, inoculation with the AM fungus reduced the stress impact of undisturbed soil on the pigeon pea without affecting the soil resistance and S/ R ratio. A possible reason for reducing the stress impact was increase in specific root length, rather than P inflow with the AM formation. It is a novel finding that AM formation in undisturbed soil could promote root elongation despite the fact that soil was seriously compacted.

NO·–releasing substances that induce growth elongation in maize root segments

Root segments of maize were incubated in different solutions containing substances that non-enzymatically release nitric oxide, such as sodium nitrite (SN), sodium nitroprusside (SNP), nitrosoglutathione (NGLU) and nitrosocysteine (NCYS). We found that all of these substances induced root tip expansion in a dose-dependent manner. The decreasing order of potency for root-induced elongation was: 10 -7 M SN, pH 4.5; 10 -11 M NCYS, 10 -10 M SNP, 10 -9 M NGLU and 10 -7 M SN, pH 7.0. Nitric oxide scavenger such as methylene blue prevented the elongation induced by NO·–releasing substances, but had no effect on indole-3-acetic acid (IAA)-induced cell expansion. Our results suggest that nitric oxide is the putative elongation inducer and that IAA and NO·–releasing substances conceivably share common steps in the signal transduction pathway, since both elicited the same plant response. Vanadate, a plasmamembrane ATPase inhibitor, significantly reversed IAA-induced elongation when supplied at 10 μM concentration. IAA-induced elongation was strongly enhanced by 10 nM BAY K 8644, an agonist of voltage dependent Ca2+ channels. Promotion of root elongation in the absence of IAA occurred only at higher concentrations of BAY K. Vanadate and BAY K had no influence on the NCYS-induced elongation suggesting that the common steps in the signalling of IAA and NCYS are not at the level of the plasmamembrane.

Differential IAA dose response relations of the axr1 and axr2 mutants of Arabidopsis

In comparison to wild type Arabidopsis thaliana, the auxin resistant mutants axr1 and axr2 exhibit reduced inhibition of root elongation in response to auxins. Several auxin‐regulated physiological processes are also altered in the mutant plants. When wild‐type, axr1 and axr2 seedlings were grown in darkness on media containing indoleacetic acid (IAA), promotion of root growth was observed at low concentrations of IAA (10−11 to 10−7M) in 5‐day‐old axr2 seedlings, but not in axr1 or wild‐type seedlings. In axr1 there was little or no measurable root growth response over the same concentration range. In wild type, root growth was inhibited at concentrations greater than 10−10M and no detectable root growth response was observed at lower concentrations. In addition, production of lateral roots in response to IAA increased in axr2 seedlings and decreased in axr1 seedlings relative to wild type. Promotion of root elongation and initiation of lateral roots in axr2 seedlings in response to auxin indicate that axr2 seedlings are able to perceive and respond to IAA.

Low temperature growth, freezing survival, and production of antifreeze protein by the plant growth promoting rhizobacterium Pseudomonas putida GR12-2.

The plant growth promoting rhizobacterium Pseudomonas putida GR12-2 was originally isolated from the rhizosphere of plants growing in the Canadian High Arctic. Here we report that this bacterium was able to grow and promote root elongation of both spring and winter canola at 5 degrees C, a temperature at which only a relatively small number of bacteria are able to proliferate and function. In addition, the bacterium survived exposure to freezing temperatures, i.e., -20 and -50 degrees C. In an effort to determine the mechanistic basis for this behaviour, it was discovered that following growth at 5 degrees C, P. putida GR12-2 synthesized and secreted to the growth medium a protein with antifreeze activity. Analysis of the spent growth medium, following concentration by ultrafiltration, by SDS-polyacrylamide gel electrophoresis revealed the presence of one major protein with a molecular mass of approximately 32-34 kDa and a number of minor proteins. However, at this point it is not known which of these proteins contains the antifreeze activity.

Persistence in soil of the plant growth promoting rhizobacterium Pseudomonas putida GR12-2 and genetically manipulated derived strains

Transformation of the plant growth promoting rhizobacterium Pseudomonas putida GR12-2 with broad-host-range vectors can affect the growth of the bacterium, its ability to promote root elongation of canola seedlings under gnotobiotic conditions, and its persistence in soil. Plasmid transformants, and a transposon-mutagenized derivative of P. putida GR12-2, fell into two classes with respect to these three attributes: strains that were clearly diminished in these capabilities and strains that behaved like the nontransformed wild type. These differences can be accounted for by the imposition of a metabolic load that is created by some types of genetic modification that results in a physiological impairment of the modified bacterium and decreases its ability to function as a plant growth promoting rhizobacterium.Key words: plant growth promoting rhizobacteria, PGPR, bacterial fertilizer, soil bacteria, soil persistence, microcosm.

Comparative structural study ofQuercus serrata andQ. acutissima formed byPisolithus tinctorius andHebeloma cylindrosporum

Ectomycorrhizas were synthesized in pots and growth pouches betweenQuercus serrata, Q. acutissima, and two ectomycorrhizal fungi,Pisolithus tinctorius andHebeloma cylindrosporum. Root morphology and the structure of the mantle and Hartig net were compared using light, fluorescence, scanning and transmission electron microscopy.P. tinctorius initially colonized root cap cells, and eventually produced a highly branched lateral root system with a complete mantle, whereasH. cylindrosporum promoted root elongation with few hyphae on the root apex surface indicating that interaction between roots differs with fungal species. Hartig net structure and hyphal inclusions varied between all the combinations tested. There were structural differences between mycorrhizas ofH. cylindrosporum/Q. acutissima grown in soil and growth pouches, which indicate that the growth pouch environment can induce artefacts in roots. Fruit bodies ofH. cylindrosporum developed in pots withQ. acutissima. AlthoughP. tinctorius has been used to inoculate oak seedlings in the nursery, results of this study indicate thatH. cylindrosporum may also be an effective ectomycorrhizal fungus forQ. serrata andQ. acutissima.

Plant Regeneration from Root Tissue and Establishment of Root Culture of Japanese White Birch (Betula platyphylla var. japonica).

Protocols for in vitro regeneration from root explants and for rapid elongation of root cultures were established on Betula platyphylla var. japonica. Adventitious shoots were differentiated on IS medium supplemented with BAP and NAA from root explants. The highest differentiation rate (50%) was attained on the medium containing 0.8mg/l BAP and 0.03mg/l NAA. The adventitious shoots successfully regenerated perfect plantlets on the rooting medium. Root culture was established from root segments excised from intact plantlets grown in vitro in liquid IS, B5 and WP media containing 1-3% sucrose. B5 medium supported maximum root elongation among the tested media. The use of 100mg/l PVP in B5 medium successfully prevented browning of roots and promoted root elongation. There was no significant difference in the rate of adventitious shoot formation between root explant obtained from root culture and that obtained from intact root from plantlets.

Partial purification and characterization of 1-aminocyclopropane-1-carboxylate deaminase from the plant growth promoting rhizobacterium Pseudomonas putida GR12-2

The plant growth promoting rhizobacterium Pseudomonas putida GR12-2 can utilize 1-aminocyclopropane-1-carboxylate (ACC) as a sole nitrogen source because it possess the unusual enzyme ACC deaminase, which hydrolyzes ACC to ammonia and α-ketobutyrate. This enzyme, which is thought to be intimately involved in the mechanism that the bacterium uses to promote root elongation in developing canola seedlings, was partially purified and characterized. The native form of the enzyme is a trimer with a molecular mass of 105 kDa and a subunit molecular mass of 35 kDa. ACC deaminase activity is found in the cytoplasm of the bacterium, is induced by low levels (i.e., 100 nM) of ACC, and has a temperature optimum at approximately 30 °C and a pH optimum of 8.5. These properties are very similar to those reported for ACC deaminase from another soil bacterium, Pseudomonas sp. strain APC.Key words: 1-aminocyclopropane-1-carboxylate, ACC, plant growth promoting rhizobacteria, PGPR, ACC deaminase, bacterial fertilizer.

1-Aminocyclopropane-1-carboxylic acid deaminase mutants of the plant growth promoting rhizobacterium Pseudomonas putida GR12-2 do not stimulate canola root elongation

The plant growth promoting rhizobacterium Pseudomonas putida GR12-2 was mutagenized with nitrosoguanidine and three separate mutants that were unable to utilize 1-aminocyclopropane-1-carboxylic acid (ACC) as a sole nitrogen source were selected. These mutants are devoid of the ACC deaminase activity that is present in wild-type P. putida GR12-2 cells. Only wild-type cells, but not any of the ACC deaminase mutants, promoted root elongation of developing canola seedlings under gnotobiotic conditions. These results are interpreted in terms of a model in which P. putida GR12-2 promotes root elongation by binding to germinating seeds and sequesters and hydrolyzes some of the unbound ACC, thereby lowering the level of ACC and hence the endogenous ethylene concentration, allowing the roots to grow longer.Key words: 1-aminocyclopropane-1-carboxylate, ACC, plant growth promoting rhizobacteria, PGPR, ACC deaminase, bacterial fertilizer.