Rhizosphere Microbial Community Structure in Relation to Root Location and Plant Iron Nutritional Status

Root exudate composition and quantity vary in relation to plant nutritional status, but the impact of the differences on rhizosphere microbial communities is not known. To examine this question, we performed an experiment with barley (Hordeum vulgare) plants under iron-limiting and iron-sufficient growth conditions. Plants were grown in an iron-limiting soil in root box microcosms. One-half of the plants were treated with foliar iron every day to inhibit phytosiderophore production and to alter root exudate composition. After 30 days, the bacterial communities associated with different root zones, including the primary root tips, nonelongating secondary root tips, sites of lateral root emergence, and older roots distal from the tip, were characterized by using 16S ribosomal DNA (rDNA) fingerprints generated by PCR-denaturing gradient gel electrophoresis (DGGE). Our results showed that the microbial communities associated with the different root locations produced many common 16S rDNA bands but that the communities could be distinguished by using correspondence analysis. Approximately 40% of the variation between communities could be attributed to plant iron nutritional status. A sequence analysis of clones generated from a single 16S rDNA band obtained at all of the root locations revealed that there were taxonomically different species in the same band, suggesting that the resolving power of DGGE for characterization of community structure at the species level is limited. Our results suggest that the bacterial communities in the rhizosphere are substantially different in different root zones and that a rhizosphere community may be altered by changes in root exudate composition caused by changes in plant iron nutritional status.     

Development of the Fifth Leaf is Indicative for Whole Plant Performance at Low Temperature in Tomato

In the search for early-detectable selection criteria for growthat low temperature conditions in tomato, first the initiationand growth of individual leaves was analysed. Scanning electronmicroscopy revealed that the first four primordia had alreadydeveloped during the germination period at 25°C. The primordiumof the fifth leaf, however, was initiated after the transferof seedlings to the experimental conditions. The increase inlength of the first three leaves, and to a lesser extent ofthe fourth leaf, was considerably smaller in comparison withthat of later formed leaves. Moreover, the morphology of thefirst three to four leaves was deviant, whereas the others showedthe normal compound leaf architecture. All these results indicatedthat the fifth leaf was the earliest formed leaf with growthcharacteristics that might reflect the growth potential of thewhole plant. Development of the fifth leaf was tested as a marker for wholeplant growth. At three temperature, 18, 15 and 12°C, growthresponses of the fifth leaf were similar to that of whole plantsin four tomato genotypes: Line A, Line B, Premier and MXXIV-13.Significant differences in relative growth rate of dry weightof whole plants and fifth leaves (RGR_W)and of leaf area of thefifth leaves (RGR_LA between two fast growing and two slow growinggenotypes were found. No genotype by temperature interactionfor RGR_W and RGR_LA was found, indicating that the effect oftemperature decrease was similar for the four genotypes. The structure of the mature fifth leaf of one fast and one slowgrowing genotype, Line A and MXXIV-13, was analysed. For bothgenotypes, leaves were small and thick at low temperature, 12°C.The total number of epidermis and palisade parenchyma cellsper leaf was smaller but the size of the cells developed at12°C was larger than at 18°C. Consequently, the slowgrowth at 12°C was due to a low rate of cell division. Atboth temperatures, the fifth leaf to MXXIV-13 was smaller comparedto that of line A. Since the size of the cells were similar,the smaller leaf size was due to lower number of leaf cells. The results confirm the suitability of the growth, especiallyexpressed as RGR_LA , of the fifth leaf as a nondestructive marketfor vegetative development of tomato at low temperature. Growthdifferences between genotypes were mainly reflected by differencesin cell number of leaves, which might be correlated with geneticallydetermined differences in cell number of leaf primordia.Copyright1993, 1999 Academic Press Lycopersicon esculentum Mill. genotypes, plant growth, selection criteria, low temperature, leaf initiation, leaf development, RGR, leaf structure, cell expansion     

Contrasted Responses to Root Hypoxia in Tomato Fruit at Two Stages of Development

The biochemical consequences of root hypoxia have been documented in many sink organs, but not extensively in fruit. Therefore, in the present study, the response to root hypoxia in tomato fruit (Solanum lycopersicum L.) was investigated at two developmental stages, during the cell division and the cell expansion phases. Our results showed that in dividing fruit, root hypoxia caused an exhaustion of carbon reserves and proteins. However, ammonium and major amino acids (glutamine, asparagine and γ–aminobutyric acid (GABA)) significantly accumulated. In expanding fruit, root hypoxia had no effect on soluble sugar, protein and glutamine contents, whereas starch content was significantly decreased, and asparagine and GABA contents slightly increased. Metabolite contents were well correlated with activities of the corresponding metabolising enzymes. Contrary to nitrogen metabolising enzymes (glutamine synthetase, asparagine synthetase and glutamate decraboxylase), the activities of enzymes involved in sugar metabolism (invertase, sucrose synthase, sucrose phosphate synthase and ADP glucose pyrophosphorylase) were significantly reduced by root hypoxia, in diving fruit. In expanding fruit, only a slight decrease in ADP glucose pyrophosphorylase and an increase in asparagine synthetase and glutamate decarboxylase activities were observed. Taken together, the present data revealed that the effects of root hypoxia are more pronounced in the youngest fruits as it is probably controlled by the relative sink strength of the fruit and by the global disturbance in plant functioning.     

Role of Pseudomonas putida Indoleacetic Acid in Development of the Host Plant Root System

Many plant-associated bacteria synthesize the phytohormone indoleacetic acid (IAA). While IAA produced by phytopathogenic bacteria, mainly by the indoleacetamide pathway, has been implicated in the induction of plant tumors, it is not clear whether IAA synthesized by beneficial bacteria, usually via the indolepyruvic acid pathway, is involved in plant growth promotion. To determine whether bacterial IAA enhances root development in host plants, the ipdc gene that encodes indolepyruvate decarboxylase, a key enzyme in the indolepyruvic acid pathway, was isolated from the plant growth-promoting bacterium Pseudomonas putida GR12-2 and an IAA-deficient mutant constructed by insertional mutagenesis. The canola seedling primary roots from seeds treated with wild-type P. putida GR12-2 were on average 35 to 50% longer than the roots from seeds treated with the IAA-deficient mutant and the roots from uninoculated seeds. In addition, exposing mung bean cuttings to high levels of IAA by soaking them in a suspension of the wild-type strain stimulated the formation of many, very small, adventitious roots. Formation of fewer roots was stimulated by treatment with the IAA-deficient mutant. These results suggest that bacterial IAA plays a major role in the development of the host plant root system.     

Reduction of the Cytosolic Phosphoglucomutase in Arabidopsis Reveals Impact on Plant Growth,Seed and Root Development,and Carbohydrate Partitioning

Phosphoglucomutase (PGM) catalyses the interconversion of glucose 1-phosphate (G1P) and glucose 6-phosphate (G6P) and exists as plastidial (pPGM) and cytosolic (cPGM) isoforms. The plastidial isoform is essential for transitory starch synthesis in chloroplasts of leaves, whereas the cytosolic counterpart is essential for glucose phosphate partitioning and, therefore, for syntheses of sucrose and cell wall components. In Arabidopsis two cytosolic isoforms (PGM2 and PGM3) exist. Both PGM2 and PGM3 are redundant in function as single mutants reveal only small or no alterations compared to wild type with respect to plant primary metabolism. So far, there are no reports of Arabidopsis plants lacking the entire cPGM or total PGM activity, respectively. Therefore, amiRNA transgenic plants were generated and used for analyses of various parameters such as growth, development, and starch metabolism. The lack of the entire cPGM activity resulted in a strongly reduced growth revealed by decreased rosette fresh weight, shorter roots, and reduced seed production compared to wild type. By contrast content of starch, sucrose, maltose and cell wall components were significantly increased. The lack of both cPGM and pPGM activities in Arabidopsis resulted in dwarf growth, prematurely die off, and inability to develop a functional inflorescence. The combined results are discussed in comparison to potato, the only described mutant with lack of total PGM activity.     

植物侧根发育的研究进展

侧根是植物根系的重要组成部分,其发生和发育受到内源植物激素和外界环境因素的共同影响。生长素在侧根发生起始、侧根原基的发育和侧根突破母体表皮等阶段均发挥关键作用。研究侧根的发育和形态解剖结构以及信号调控途径等,都具有重要的理论和实践意义。本文结合近年来的研究进展,综述了拟南芥和水稻侧根发育的详细过程和影响因素,重点关注生长素在侧根原基发生和发育过程中的作用。     

The Reaction of the Medium in Relation to Root Formation in Coleus

Background: As seed dispersal can vary among years and individuals, studies that focus on a single year or on a few individuals may lead to erroneous conclusions.

Aims: To study temporal and spatial intraspecific variation of seed dispersal in Scrophularia canina, a widespread species with capsule-type fruit.

Methods: Primary seed dispersal was quantified by placing traps in each cardinal direction around 10 individuals during two consecutive years. We correlated several seed shadow parameters (modal dispersal distance, kurtosis, skewness, percentiles, slope, and seed percentage beneath the plant canopy) with three plant features (maximum height, lateral spread and seed production).

Results: Scrophularia canina dispersed their seeds by boleochory, giving rise to a typical leptokurtic curve, but behaving as a barochorous species, because about 90% of seeds landed beneath the plant canopy. Temporal dispersal in S. canina included several seed waves associated with maximum wind speeds. Plant lateral spread was significantly positively correlated with seed percentiles and percentage of seeds beneath the plant canopy regardless of year. A seed production effect was only evident when both years were considered together.

Conclusions: Although time-consuming, investigation of the dispersal process for more than 1 year provides more realistic information on seed dispersal. Lateral spread is the main plant feature determining seed shadow.     

The Relative Toxicity of Manganese and Cobalt to the Tomato Plant

1. At equivalent concentrations, cobalt was more effective thanmanganese in inducing chlorosis and necrosis in tomato plants. 2. Chlorotic plants suffering from either cobalt or manganesetoxicity alone, or from the two conditions together, containedas much or more iron than normal plants, and chlorotic leavesresponded positively to painting with a solution of ferroussulphate. 3. When cultures were supplied with 5 p.p.m. Mn, distinct injuryof the lower leaves occurred regardless of the cobalt supply,but the addition of cobalt up to 0·01 p.p.m. broughtabout a gradual decrease in the chlorosis of the top leavesand an increase in dry weight. 4. With cobalt at the 0·1 p.p.m. level, severe chlorosisand necrosis occurred and growth was stunted, but by raisingthe manganese level to 5·0 p.p.m. a substantial antidotingeffect on the toxicity was obtained. 5. Cobalt and manganese had an antidoting effect on each other,whereby at high manganese supplies small quantities of cobaltalleviated manganese toxicity, while toxic concentrations ofcobalt were antidoted by high levels of manganese.     

A Mechanistic Model of Carbohydrate Dynamics During Adventitious Root Development in Leafy Cuttings

The rooting of vegetatively propagated leafy cuttings involvesthe complex interaction of many processes. For this reason,the influence of carbohydrate status, nutrition, water and hormonalfactors on root formation is poorly understood at a mechanisticlevel. We present a mechanistic model of the growth of pre-formedroot initials on a cutting consisting of leaf, internode androots. The processes represented are leaf photosynthesis, starchmobilization, sugar transport, and sugar utilization for rootgrowth. The model provides a quantitative scheme for understandinghow root development depends on properties of cuttings suchas leaf area, internode length and initial carbohydrate content.The potential of the model to interpret rooting experimentson a whole-cutting basis is illustrated using published datafor cuttings of Triplochiton scleroxylon (a West African hardwood)with different leaf areas. Observed rooting times are reproducedin the model by varying the leaf photosynthetic rate per unitarea. The simulated starch and sugar dynamics during root growthare in qualitative agreement with observations. A sensitivityanalysis is performed to examine the effect of key parameterson the timing or success of rooting. The model provides a frameworkfor examining the role of other factors known to affect rooting,such as nutrient and water status, but requires further parameterisationbefore it can be used as a predictive tool in vegetative propagation. Model, carbohydrates, rooting, vegetative propagation     

Tomato (Lycopersicon esculentum Mill., cv. 'Better Bush') Plant Response to Root Restriction

Observations presented here describe changes in the growth ofLycopersicon esculentum Mill., cv. ‘Better Bush’,at the onset of root-restriction stress. Plants were grown ineither 1500 cm³ (control) or 25 cm³ (treated) containers ina flow-through hydroponic culture system (FTS). Seeds were imbibed,germinated, held for 13 d in sterile germination bags, thenplaced into the FTS and designated day zero plants. Plants grownin the FTS were harvested, every 7 d, from day 11 to day 46.Plant height, root and shoot weights, leaf areas, leaf lengths,branching and flowering were measured to illustrate morphologicalchanges that occurred in response to root-restriction stress.Plants restricted to a 25 cm³ root volume experienced a markedstress after 18 d in the FTS. A reduction in leaf elongationwas evident and preceded the time (prior to day 25) when maximumdry weights were attained by restricted root systems. Controlplants had greater leaf areas and plant dry weights than thoseof root-restricted plants after day 18. Treated plants showedno significant increase in dry weights from day 32 to the endof the experiment. Between day 18 and 25 the root: shoot ratiofor the treated plants dropped below that of the controls. Adventitiousrooting was evident in treated plants by day 25. Roots of thetreated plants showed an apparent waterlogging indicated bybrowning of roots by day 32. Root turnover followed as the primaryroot system was replaced by adventitious roots. The number ofbranches or flowers did not differ between control and treatedplants. The timing of the events described here can be usedto further the study of specific physiological responses oftomato plants to root-restriction stress.     

The Carbohydrate Nutrition of Tomato Roots: V. The Promotion and Inhibition of Excised Root Growth by Various Sugars and Sugar Alcohols

Iron is only consistently present in an available form in White'sroot culture medium if the inorganic salts are autoclaved withthe sugar. The substitution of ferric ethylenediamine-tetra-acetatefor the inorganic ferric salt of White's medium ensures ironavailability when the carbon source of the medium is renderedsterile by ether treatment and subsequently added to the remainderof the constituents which have been sterilized by autoclaving. The biological activity of sugars, and particularly of dextroseand laevulose, is altered by autoclaving them in presence ofthe inorganic salt solution of White's medium. The only sugar which supports a considerable growth of excisedtomato roots is sucrose. The activity of this sugar is not affectedby alcohol-precipitation nor is it dependent upon the simultaneouspresence of traces of its constituent mono-saccharides. Dextrose or laevulose or a mixture of the two sugars supporta low but sustained level of excised-root growth. All othersugars and sugar alcohols tested were inactive as carbon sources. The addition of sucrose at low concentration (0–2 percent.) to a medium containing dextrose as the main carbon compounddoes not make possible a level of growth comparable with thatobtained with an adequate sucrose supply. It has not been possibleto enhance the growth-rate of excised roots supplied with dextroseby previous presentation of this sugar under conditions permittingactive growth. Using media containing 'etherized' sucrose anddextrose, no evidence was obtained of any competitive inhibitionof sucrose utilization by dextrose. Certain sugars when added to a medium, containing the optimumconcentration of sucrose₁, markedly inhibited excised root growth.Thus l-sorbose, l- and d-arabinose, and d-xylose caused notless than 80 per cent, inhibition at a concentration of 0-5per cent. d-mannose and d-galactose completely inhibited growthat o-1 per cent. The oligosaccharides, dextrose, laevulose,and the sugar alcohols tested had, by contrast, very low inhibitoryactivity.     

Carbohydrate Production in Relation to Microphytobenthic Biofilm Development: An Integrated Approach in a Tidal Mesocosm

Experiments were performed to evaluate short-term changes in sediment extracellular carbohydrates for a multispecific assemblage of benthic diatoms in relation to physiological status, endogenous migratory rhythms, and environmental conditions. For this purpose, a mesocosm was used, which simulated both tidal and dark: light alternating cycles under controlled conditions. Scanning electronic microscopy in combination with picture analyses indicated that natural diatom migration patterns were reproduced in the mesocosm. Two EPS fractions were operationally separated in colloidal carbohydrate measurements: alcohol-soluble EPS (termed “soluble EPS”) and alcohol-insoluble EPS (termed “bound EPS”). Microphytobenthic biomass followed a logistic-type curve and converged toward a maximal value termed the “biotic capacity of the local environment.” Both EPS fractions showed oscillations with production during photosynthetic periods and sharp decreases during night immersion periods. Productions of both EPS fractions increased with Chl a production during light periods suggesting a light dependence in relation to migratory patterns. The decreases in both EPS fractions, which occurred during night immersion periods suggest that carbohydrate hydrolysis and/or washaway affected both EPS fractions similarly in benthic environments. Our results confirm the theory according to which the two distinct fractions are under different metabolic controls. No change in soluble EPS release was obtained during the transition from logarithmic to stationary phase. On the other hand, a metabolism modification of microalgae, probably related to ammonium depletion, occurred when cells entered the stationary phase, since there was a high enhancement in bound EPS production. Mesocosm results can serve as a system of reference useful to characterize biofilm development in field investigations and to revisit the effective implication of each EPS fraction in sediment stability.     

The Influence of Steep Slopes on Root System Development

Mechanical forces active on steep slopes tend to overturn plants, which respond by developing a specific asymmetrical architecture in the root system. This asymmetric architecture is the consequence of preferential lateral root emergence and elongation in the up-slope and down-slope directions. Root systems show a normal symmetrical architecture when the same species is grown on plane soil. The asymmetrical root architecture on steep slopes seems to increase the plant's stability by modifying the distribution of mechanical forces into the soil. This hypothesis is supported by the observation that lateral roots developing in the up-slope or down-slope directions present considerable anatomical modifications in shape and tissue-organization compared with lateral roots from plants growing on plane soil.     

The Influence of Steep Slopes on Root System Development

Mechanical forces active on steep slopes tend to overturn plants, which respond by developing a specific asymmetrical architecture in the root system. This asymmetric architecture is the consequence of preferential lateral root emergence and elongation in the up-slope and down-slope directions. Root systems show a normal symmetrical architecture when the same species is grown on plane soil. The asymmetrical root architecture on steep slopes seems to increase the plant's stability by modifying the distribution of mechanical forces into the soil. This hypothesis is supported by the observation that lateral roots developing in the up-slope or down-slope directions present considerable anatomical modifications in shape and tissue-organization compared with lateral roots from plants growing on plane soil.