Impact of <Emphasis Type="Italic">Heterobasidion</Emphasis> root-rot on fine root morphology and associated fungi in <Emphasis Type="Italic">Picea abies</Emphasis> stands on peat soils

We examined differences in fine root morphology, mycorrhizal colonisation and root-inhabiting fungal communities between Picea abies individuals infected by Heterobasidion root-rot compared with healthy individuals in four stands on peat soils in Latvia. We hypothesised that decreased tree vitality and alteration in supply of photosynthates belowground due to root-rot infection might lead to changes in fungal communities of tree roots. Plots were established in places where trees were infected and in places where they were healthy. Within each stand, five replicate soil cores with roots were taken to 20 cm depth in each root-rot infected and uninfected plot. Root morphological parameters, mycorrhizal colonisation and associated fungal communities, and soil chemical properties were analysed. In three stands root morphological parameters and in all stands root mycorrhizal colonisation were similar between root-rot infected and uninfected plots. In one stand, there were significant differences in root morphological parameters between root-rot infected versus uninfected plots, but these were likely due to significant differences in soil chemical properties between the plots. Sequencing of the internal transcribed spacer of fungal nuclear rDNA from ectomycorrhizal (ECM) root morphotypes of P. abies revealed the presence of 42 fungal species, among which ECM basidiomycetes Tylospora asterophora (24.6 % of fine roots examined), Amphinema byssoides (14.5 %) and Russula sapinea (9.7 %) were most common. Within each stand, the richness of fungal species and the composition of fungal communities in root-rot infected versus uninfected plots were similar. In conclusion, Heterobasidion root-rot had little or no effect on fine root morphology, mycorrhizal colonisation and composition of fungal communities in fine roots of P. abies growing on peat soils.     

Root system development of Larix gmelinii trees affected by micro-scale conditions of permafrost soils in central Siberia

Spatial distributions of root systems of Larix gmelinii (Rupr.) Rupr. trees were examined in two stands in central Siberia: an even-aged stand (ca. 100 yrs-old) and a mature, uneven-aged (240–280 yrs-old) stand. Five larch trees of different sizes were sampled by excavating coarse roots (diameter > 5 mm) in each stand. Dimensions and ages of all first-order lateral roots were measured. Micro-scale conditions of soil temperature and soil water suction (each 10 cm deep) were also examined in relation to earth hummock topography (mound vs. trough) and/or ground floor vegetation types (moss vs. lichens). All larch trees developed superficial root systems, consisting of the aborted short tap root (10–40 cm in soil depth) and some well-spread lateral roots (n= 4-13). The root network of each tree was asymmetric, and its rooting area reached about four times the crown projection area. Lateral roots generally expanded into the upper soil layers of the mounds where summer soil temperature was 1–6°C higher than inside nearby troughs. Chronological analysis indicated that lateral root expansion started successively from lower to upper parts of each aborted tap root, and some lateral roots occurred simultaneously at several decades after tree establishment. The process of root system development was likely to be primarily linked with post-fire dynamics of rhizosphere environment of the permafrost soils.     

Fine root distribution in a young loblolly pine (Pinus taeda L.) stand: Effects of preplant phosphorus fertilization

The effects of preplant phosphorus fertilization on fine root (2 mm) distribution were examined in an 11-year-old stand of loblolly pine (Pinus taeda L.) located on the lower Coastal Plain of North Carolina. Root auger cores were collected from the planting bed and interbed areas from two depths (0–10 and 10–20 cm) from fertilized (45 kg P ha^–1 at time of planting) and nonfertilized plots. Root samples were collected and aboveground growth measured during the 11th year after fertilization. Despite significant increases in aboveground volume and biomass due to fertilization, fine root biomass was unaffected. No differences in rooting density (root length per volume of soil) due to phosphorus additions were detected. However, the ratio of fine root biomass to aboveground (shoot) biomass (R:S) was significantly smaller on plots receiving phosphorus fertilization.operated by Martin Marietta Energy Systems, Inc., under Contract No. DE-AC05-840 R21400 with the U.S. Department of Energy     

Environmental Regulation of Lateral Root Initiation in Arabidopsis

Plant morphology is dramatically influenced by environmental signals. The growth and development of the root system is an excellent example of this developmental plasticity. Both the number and placement of lateral roots are highly responsive to nutritional cues. This indicates that there must be a signal transduction pathway that interprets complex environmental conditions and makes the "decision" to form a lateral root at a particular time and place. Lateral roots originate from differentiated cells in adult tissues. These cells must reenter the cell cycle, proliferate, and redifferentiate to produce all of the cell types that make up a new organ. Almost nothing is known about how lateral root initiation is regulated or coordinated with growth conditions. Here, we report a novel growth assay that allows this regulatory mechanism to be dissected in Arabidopsis. When Arabidopsis seedlings are grown on nutrient media with a high sucrose to nitrogen ratio, lateral root initiation is dramatically repressed. Auxin localization appears to be a key factor in this nutrient-mediated repression of lateral root initiation. We have isolated a mutant, lateral root initiation 1 (lin1), that overcomes the repressive conditions. This mutant produces a highly branched root system on media with high sucrose to nitrogen ratios. The lin1 phenotype is specific to these growth conditions, suggesting that the lin1 gene is involved in coordinating lateral root initiation with nutritional cues. Therefore, these studies provide novel insights into the mechanisms that regulate the earliest steps in lateral root initiation and that coordinate plant development with the environment.     

Developmental changes in peanut root structure during root growth and root-structure modification by nodulation

Background and Aims

Basic information about the root and root nodule structure of leguminous crop plants is incomplete, with many aspects remaining unresolved. Peanut (Arachis hypogaea) forms root nodules in a unique process. Structures of various peanut root types were studied with emphasis on insufficiently characterized lateral roots, changes in roots during their ontogenesis and root modification by nodule formation.

Methods

Peanut plants were grown in the field, in vermiculite or in filter paper. The taproot, first-order and second-order lateral roots and root nodules were analysed using bright-field and fluorescence microscopy with hand sections and resin sections.

Key Results

Three root categories were recognized. The primary seminal root was thick, exhibiting early and intensive secondary thickening mainly on its base. It was tetrarch and contained broad pith. First-order lateral roots were long and thin, with limited secondary thickening; they contained no pith. Particularly different were second- and higher-order lateral roots, which were anatomically simple and thin, with little or no secondary growth. Unusual wall ingrowths were visible in the cells of the central part of the cortex in the first-order and second-order lateral roots. The nodule body was formed at the junction of the primary and lateral roots by the activity of proliferating cells derived originally from the pericycle.

Conclusions

Two morphologically and anatomically distinct types of lateral roots were recognized: long, first-order lateral roots, forming the skeleton of the root system, and thin and short second- and higher-order lateral roots, with an incomplete second state of endodermal development, which might be classified as peanut ‘feeder roots’. Formation of root nodules at the base of the lateral roots was the result of proliferating cell divisions derived originally from the pericycle.Key words: Endodermis, lateral root structure, nodule structure, peanut, Arachis hypogaea, primary root structure     

The inhibition of infection thread development in the cultivar-specific interaction ofRhizobium and subterranean clover is not caused by a hypersensitive response

Summary The cultivar specific interaction ofTrifolium subterranean cv. Woogenellup andRhizobium leguminosarum bv.trifolii strain ANU 794 was examined to establish the basis for nodulation failure on this cultivar. Infections were initiated by strain ANU 794 on cv. Woogenellup. Root hair curling, the initiation of infection threads, and cortical cell divisions were evident on the tap root and appeared normal after microscopic observation. However, in most cases, the infection threads stayed confined to the root hairs. No evidence was found for a hypersensitive response by the plant. The progress of infections on the tap roots was different from that on the lateral roots. This was confirmed by the differential tap and lateral root nodulation patterns of the mutants derived from strain ANU 794, which show enhanced nodulation on cv. Woogenellup. On the lateral roots, cortical cell divisions progressed further than those on the tap root and formed macroscopically visible swellings, which could be divided into two morphological classes. In some cases infection threads developed into these primordia but successful nodules were not established. The inhibition of infection appeared to be manifested at two levels: first, on the tap roots in the root hairs, where many of the infection threads are contained and secondly, in the primordia induced on the lateral roots, where the infection threads sometimes penetrate further than the root hair cell but stop in the primordial cells. It appears that an essential factor or trigger in the communication between plant and bacteria is missing or altered, resulting in an array of primordia-structures, which cease to develop.Abbreviations bv biovar - cv cultivar - Fix⁺ nitrogen fixing - GUS -glucuronidase - Nod⁺ nodulating - HR hypersensitive response - Km kanamycin - LOSs lipo-oligosaccharides - Sm streptomycin - Sp spectinomycin - X-Gluc 5-bromo-4-chloro-3-indonyl--glucuronic acid     

Immunohistological localization and quantification of IAA in studies of root growth regulation

The content and distribution of auxins were studied in gravistimulated roots of maize (Zea mays L.) and primary roots of 7-day-old wheat (Triticum durum Desf.) seedlings, which branching was enhanced by excision of adventitious roots. IAA localization was observed immunohistochemically, using specific anti-IAA antibody in combination with second (anti-species) antibody labeled with colloidal gold. Differences in the IAA content (staining intensity) were found between upper and lower parts of gravistimulated maize roots. We also observed IAA accumulation in the primary wheat root after adventitious root excision; the cells of lateral root primordia were characterized by more intense IAA staining. The role of auxin redistribution in plants for lateral root initiation and development is discussed.     

Lateral root initiation by asymmetrical transverse divisions of pericycle cells in four plant species:Raphanus sativus,Helianthus annuus,Zea mays,andDaucus carota

Summary In roots ofRaphanus sativus, Helianthus annuus, Zea mays, andDaucus carota, lateral root initiation occurs when a pair of neighbouring elongated and highly vacuolated pericycle cells in the same column almost simultaneously undergo asymmetrical transversal division. This produces a pair of very short pericycle cells lying end-to-end, flanked above and below by two longer cells. This occurs because both mitoses occur close to the ends of the neighbouring pericycle cells. The two longer daughter pericycle cells divide again later. In roots of radish, sunflower, and carrot these cells divide transversely and asymmetrically, producing more short cells adjacent to the previous ones. In corn roots, they undergo oblique divisions. Much later, the first pair of short pericycle cells undergoes periclinal divisions. Although such periclinal divisions of pericycle cells are generally thought to mark lateral root initiation, our results show that the first pair of short neighbouring pericycle cells in the same column offers another morphological criterion which permits identification of the site of lateral root initiation, both earlier and nearer to the apex than previously documented.     

A comparative study between two citrus rootstocks: Effect of nitrate on the root morpho-topology and net nitrate uptake

Root morpho-topology and net nitrate uptake of two citrus seedlings, Volkamer Lemon and Carrizo Citrange, grown at two nitrogen supplies (NO₃-N 5 M and 1000 M, respectively) were studied. Root morphological and topological parameters were gauged by an image-specific analysis system (WinRHIZO). Net nitrate uptake was estimated using the nitrate depletion method. The main findings showed that Carrizo seedlings had a dichotomous branching root system characterized by high root tip numbers and long 2^nd order lateral roots. Conversely, Volkamer root systems had a herringbone structure with a long tap root and 1^st order lateral root. Nitrate treatment did not seem to affect the pattern of the two genotypes, except for the 2^nd order lateral roots (Carrizo more than Volkamer) and root/shoot ratio and root mass ratio (Volkamer more than Carrizo) that were significantly different at low nitrate supply. Nitrate treatments induced a diverse net nitrate uptake regulation between citrus rootstocks. Indeed, at low nitrate supply, Carrizo showed a more efficient nitrate acquisition process in terms of: 1) higher net nitrate uptake maximum of the inducible high affinity transport system or full induction (A), (2) higher cumulative nitrate uptake (A_t) and (3) lower t₁ parameter defined as the half time of the net nitrate uptake rate of the inducible transport system during the induction phase, compared to Volkamer. Conversely, at the high nitrate level, only the genotypical difference of the t₁ parameter was maintained. The results suggested that, at the low nitrate level, the morphological root traits such as higher 2^nd order lateral roots and greater root tip numbers of the Carrizo compared with Volkamer seedlings, enhance the capacity to absorb nitrate from nutrient solution.     

Does liquid fertilization affect fine root dynamics and lifespan of mycorrhizal short roots?

We studied effects of nitrogen, other nutrients and water (liquid fertilization; LF) on fine root dynamics (production, mortality) and life span of mycorrhizal short roots in a Norway spruce stand, using minirhizotrons. Data were collected and analyzed during a two-year period at depths of 0–20 cm, 21–40 cm and 41–85 cm, six years after the start of treatment. Relative to control (C), root production was lower in LF plots at depth 0–20 cm. Root production increased significantly at depth 41–85 cm. Fine root mortality in LF plots was higher at all depths. Life span of mycorrhizal short roots in LF plots was significantly lower than C plots and at the end of the study no mycorrhizal short roots were alive. It is suggested that the water and nitrogen input lower longevity of mycorrhizal short roots and promote fine root production at deeper soil layers.     

Long range lateral root activity by neo-tropical savanna trees

The extent of water uptake by lateral roots of savanna trees in the Brazilian highlands was measured by irrigating two 2 by 2 m plots with deuterium-enriched water and assaying for the abundance of deuterium in stem water from trees inside and at several distances from the irrigation plots. Stem water of trees inside the irrigation plots was highly enriched compared to that of control trees, whereas stem water of trees just outside the plot was only slightly enriched compared with that from control trees. Therefore, bulk water uptake in the savanna trees studied occurred in a horizontally restricted area, indicating that their rooting structure was characterized by a dense cluster of short roots associated with the main trunk and a few meandering long range lateral roots. This root architecture was confirmed by extensive excavations of several species. The same deuterium labeling pattern was observed in an Amazonian tropical forest. The savanna ecosystem, however, differed from the tropical forest ecosystem by having a greater proportion of trees outside the irrigation plots having stem water with deuterium levels significantly above background. This leads us to the conclusion that savanna trees have more or longer lateral roots compared to tropical forest trees. The greater lateral root development in savanna trees may be an adaptation for more efficient nutrient absorption.     

Lateral root development in the maize (Zea mays) lateral rootless1 mutant

Background and Aims

The maize lrt1 (lateral rootless1) mutant is impaired in its development of lateral roots during early post-embryonic development. The aim of this study was to characterize, in detail, the influences that the mutation exerts on lateral root initiation and the subsequent developments, as well as to describe the behaviour of the entire plant under variable environmental conditions.

Methods

Mutant lrt1 plants were cultivated under different conditions of hydroponics, and in between sheets of moist paper. Cleared whole mounts and anatomical sections were used in combination with both selected staining procedures and histochemical tests to follow root development. Root surface permeability tests and the biochemical quantification of lignin were performed to complement the structural data.

Key Results

The data presented suggest a redefinition of lrt1 function in lateral roots as a promoter of later development; however, neither the complete absence of lateral roots nor the frequency of their initiation is linked to lrt1 function. The developmental effects of lrt1 are under strong environmental influences. Mutant primordia are affected in structure, growth and emergence; and the majority of primordia terminate their growth during this last step, or shortly thereafter. The lateral roots are impaired in the maintenance of the root apical meristem. The primary root shows disturbances in the organization of both epidermal and subepidermal layers. The lrt1-related cell-wall modifications include: lignification in peripheral layers, the deposition of polyphenolic substances and a higher activity of peroxidase.

Conclusions

The present study provides novel insights into the function of the lrt1 gene in root system development. The lrt1 gene participates in the spatial distribution of initiation, but not in its frequency. Later, the development of lateral roots is strongly affected. The effect of the lrt1 mutation is not as obvious in the primary root, with no influences observed on the root apical meristem structure and maintenance; however, development of the epidermis and cortex are impaired.     

The lateral root initiation index: an integrative measure of primordium formation

Background and Aims

Lateral root initiation is an essential and continuous process in the formation of root systems; therefore, its quantitative analysis is indispensable. In this study a new measure of lateral root initiation is proposed and analysed, namely the lateral root initiation index (I_LRI), which defines how many lateral roots and/or primordia are formed along a parent-root portion corresponding to 100 cortical cells in a file.

Methods

For data collection, a commonly used root clearing procedure was employed, and a new simple root clearing procedure is also proposed. The I_LRI was determined as 100dl, where d is the density of lateral root initiation events (number mm⁻¹) and l is the average fully elongated cortical cell length (mm).

Key Results

Analyses of different Arabidopsis thaliana genotypes and of a crop plant, tomato (Solanum lycopersicum), showed that I_LRI is a more precise parameter than others commonly used as it normalizes root growth for variations in cell length. Lateral root primordium density varied in the A. thaliana accessions Col, Ler, Ws, and C24; however, in all accessions except Ws, I_LRI was similar under the same growth conditions. The nitrogen/carbon ratio in the growth medium did not change the lateral root primordium density but did affect I_LRI. The I_LRI was also modified in a number of auxin-related mutants, revealing new root branching phenotypes in some of these mutants. The rate of lateral root initiation increased with Arabidopsis seedling age; however, I_LRI was not changed in plants between 8 and 14 d post-germination.

Conclusions

The I_LRI allows for a more precise comparison of lateral root initiation under different growth conditions, treatments, genotypes and plant species than other comparable methods.Key words: Arabidopsis thaliana, auxin, lateral root density, lateral root initiation index, mutant phenotype, pericycle, root architecture, root branching, root primordium, Solanum lycopersicum     

Lateral root initiation by asymmetrical transverse divisions of pericycle cells in adventitious roots ofAllium cepa

Summary In onion adventitious roots cellular events have been identified that indicate that lateral root initiation occurs earlier and nearer the apex than previously documented. Lateral roots are not initiated when a pericycle cell divides periclinally but earlier, when a pair of neighbouring pericycle cells in the same column divide transversely and asymmetrically, with both mitoses close to the end towards the neighbouring pericycle cell. Each cell therefore produces two cells of unequal length. The shorter cells produced by the mother pericycle cells are adjacent, while the longer cells are located above and below the shorter cells. This objective morphological criterion allows clear identification of the site of lateral root initiation. Subsequent to these asymmetric divisions, both the longer pericycle cells again divide transversely and asymmetrically producing more short cells adjacent to the previous ones. The first periclinal division occurs in one of these short pericycle cells.     

Fine root dynamics and soil carbon accretion under thinned and un-thinned Cupressus lusitanica stands in, Southern Ethiopia

Background and aims

Forest management activities influences stand nutrient budgets, belowground carbon allocation and storage in the soil. A field experiment was carried out in Southern Ethiopia to investigate the effect of thinning on fine root dynamics and associated soil carbon accretion of 6-year old C. lusitanica stands.

Methods

Fine roots (≤2 mm in diameter) were sampled seasonally to a depth of 40 cm using sequential root coring method. Fine root biomass and necromass, vertical distribution, seasonal dynamics, annual turnover and soil carbon accretion were quantified.

Results

Fine root biomass and necromass showed vertical and temporal variations. More than 70 % of the fine root mass was concentrated in the top 20 cm soil depth. Fine root biomass showed significant seasonal variation with peaks at the end of the major rainy season and short rainy season. Thinning significantly increased fine root necromass, annual fine root production and turnover. Mean annual soil carbon accretion, through fine root necromass, in the thinned stand was 63 % higher than that in the un-thinned stand.

Conclusions

The temporal dynamics in fine roots is driven by the seasonality in precipitation. Thinning of C. lusitanica plantation would increase soil C accretion considerably through increased fine root necromass and turnover.     

The effect of drought on mycorrhizal production and very fine root system development of Norway spruce under natural and experimental conditions

Mycorrhizal growth rates were measured monthly, using a new method, in two neighouring plots of a natural spruce stand. One of the plots was irrigated while the other suffered from drought during the late summer and autumn months. Drought did not completely stop mycorrhizal growth. It caused a higher rate of root dormancy and a reduced elongation rate of the parent roots but an increased development of new mucorrhizal last order laterals. Thus, the branching density of the very fine root system was increased, even though fewer growing mycorrhizae were found in the non-irrigated plot during the dry period. Similar results were observed in a water-stress experiment with pot-cultures. After rewetting, elongation rate was stimulated and the number of growing mycorrhizae increased rapidly on the non-irrigated plot. Possible relationships between dry weight, distribution and branching density of growing fine root systems are presented.     

Functional characterization of NtCDPK1 in tobacco

We previously showed that NtCDPK1, a tobacco cal-cium-dependent protein kinase, interacts with and phosphorylates the Rpn3 regulatory subunit of the 26S proteasome, and that both NtCDPK1 and Rpn3 are mainly expressed in rapidly proliferating tissues, in-cluding shoot and root meristem. In this study, we ex-amined NtCDPK1 expression in roots using GUS ex-pression in transgenic Arabidopsis plants, and investi-gated its function in root development by generating transgenic tobacco plants carrying a sense NtCDPK1 transgene. GUS activity was first detected in roots two days after sowing. In later stages, strong GUS expres-sion was detected in the root meristem and elongation zone, as well as the initiation sites and branch points of lateral roots. Transgenic tobacco plants in which NtCDPK1 expression was suppressed were smaller, and their root development was abnormal, with reduced lateral root formation and less elongation. These re-sults suggest that NtCDPK1 plays a role in a signaling pathway regulating root development in tobacco.     

Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils

Fertilizer-induced reductions in CO(2) flux from soil ((F)CO(2)) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to (F)CO(2) reductions in fertilized forest soils. Fertilization reduced (F)CO(2) by 16-19% in 65-yr-old plantations of northern red oak (Quercus rubra) and sugar maple (Acer saccharum), and in a natural 85-yr-old yellow birch (Betula allegheniensis) stand. In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to (F)CO(2).     

MORPHOLOGICAL STUDIES OF THE ROOT OF RED PINE,PINUS RESINOSA I. GROWTH CHARACTERISTICS AND PATTERNS OF BRANCHING

Observations were made of the seasonal root growth behavior under natural conditions and under controlled conditions in plant observation boxes. Under natural conditions root growth conformed to the commonly reported pattern of a surge of growth in the spring, a mid-summer low, and a renewed burst in the fall. Growth of individual roots was cyclic. Growth patterns ordinarily varied according to root diameter and branching and in the plantations were modified by soil moisture conditions. Observations of roots during periods of constant elongation showed that the distance from the root apex to the first lateral root primordium varied directly with growth rate. Laterals did not arise in strict acropetal succession, and lateral root abortion was common, particularly in large-diameter, fast-growing roots. Observations of root initiation in relation to seasonal growth increments and to dormancy structures showed an increase in numbers of laterals on both the proximal and distal portions of a seasonal increment.