Central Vascular Tissue Research Articles

Advances in the development and function of plant root endodermis

<p indent="0mm">Root, an important underground vegetative organ of plants, plays diversified roles in plant lifecycle, including fixation and support, absorption and transportation, storage and reproduction. In meristem zone of root tip, the apical meristem is continuously divided to produce new cells, promoting the growth of root tip. Mature tissues formed through primary growth constituted the primary structure of plant roots. Water and mineral ions entered the epidermis of root, passed through the outer cortex, cortex, and endodermis in a radial direction, and reached the middle column, and were transported upward in xylem through transpiration, so as to meet the needs of aboveground part. The endodermis lies the innermost cortex surrounding the central vascular tissue system, as is constructed by only monolayer cells Endodermis is a cylindrical boundary separating the central vascular tissue from the outer cortex, and is an extracellular barrier for plant selective nutrient absorption. There were mainly two stages in differentiation of endodermis, of which in differentiation stage I, lignin was continuously deposited on lateral and radial walls of endodermis cells near the root tip, forming Casparian strips. The nature of Casparian strips was a local modification on the primary wall of endodermal layer, which was formed by the lignin polymer “impregnate” the primary wall. The complete Casparian strips show a continuous belt thickening structure, which blocked the apoplast transport pathway, prevented the diffusion of solutes between cell walls, and forced solutes to transport through the plasma membrane. In differentiation stage II, and based on the Casparian strips, hydrophobic suberin was deposited on six cell walls of endodermal cells, forming suberin lamellae structure, which inhibited the transmembrane transport of substances and was a protective barrier structure. Development of Casparian strips and suberin lamellae structure are regulated by their own growth and development mechanism and closely regulated by external environmental factors. The structure of endodermis could respond to the stress environment, and the lack of necessary mineral elements, including potassium, iron or sulfur, may cause changes in composition and structure of the Casparian strips and suberin lamellae. The above change was called plant phenotypic plasticity, whose most significant function was to support plants to adapt to the environment, integrate environmental information, optimize nutrient absorption, and maintain ion homeostasis in plants, so as to improve their survival under stressful environment. A complete suberin lamellae barrier was capable of preventing calcium ions from flowing in and potassium ions from seeping out, maintaining the balance of calcium/potassium ions in plants, supporting to regulate calcium signaling system, promoting plants photosynthesis, and improving stress resistance of crops. The paper comprehensively introduced the general development mode of plant endodermis, and the structure, concluded the composition and formation regulation mechanism of Casparian strips and suberin lamellae, and discussed the changing law of endodermis plasticity responding to abiotic stress and its importance for maintaining plant ion homeostasis, as well as prospected the future research direction, providing a new idea for cultivating new stress-resistant species.

Gut microbe‐modulated metabolites are longitudinally associated with higher neurodegeneration biomarkers in cerebrospinal fluid (CSF)

AbstractBackgroundGut bacterial metabolites in circulation and postmortem brain tissue differ in abundance between cognitively unimpaired (CU) and Alzheimer’s disease (AD) individuals. Furthermore, in mice chronic injection of bacterial components exacerbates AD pathology. In CSF, we found the microbially‐produced trimethylamine‐N‐oxide was linked to AD and neurodegeneration biomarkers. However, the broader extent to which microbe‐modulated metabolites relate to AD and neurodegeneration is unknown. We tested the hypothesis that in CSF of non‐demented adults, elevated metabolites would longitudinally associate with higher biomarkers.MethodCSF was collected biannually in the Wisconsin Alzheimer’s Disease Research Center and Wisconsin Registry for Alzheimer’s Prevention studies. Biomarkers of pTau181, Aβ42, tTau, axonal (NfL) and synaptic (neurogranin, α‐synuclein) degeneration were quantified using Elecsys assays (NeuroToolKit robust prototype, Roche Diagnostics). Metabolite relative abundances were quantified via untargeted metabolomics analysis (ultrahigh performance liquid chromatography‐tandem mass spectrometry, Metabolon). A priori literature and database review identified 76 detected microbe‐modulated metabolites. 200 CU participants with two or more analyzed CSF samples were included in statistical analysis (Table 1). For each metabolite‐biomarker pair, multilevel linear growth modeling tested person‐level mean metabolite effects on biomarker slope and intercept, and time‐varying metabolite effects on biomarker intercept (Figure 1). Covariates included age, sex, and APOE risk score; Benjamini‐Hochberg corrections adjusted for Type I error inflation.ResultSeven metabolites were positively associated with biomarker slope or intercept (Table 2). Mean gluconate, glycerate, and oxalate levels were correlated with increased tTau slope, and N‐acetylaspartate, N‐acetylglutamate and cholesterol were correlated with higher a‐synuclein or neurogranin intercepts. Phenol sulfate was linked to tTau and α‐synuclein trajectories. To our knowledge, this is the first study detecting phenol sulfate in human CSF. No metabolites significantly associated with NfL or pTau181/Aβ42.ConclusionIn CSF, participants with higher mean levels of bacteria‐utilized metabolites exhibited accelerated neurodegeneration (tTau). Gluconate is metabolized by Bifidobacterium and Lactobacillus, two genera reduced in AD. Furthermore, the microbially‐produced phenol sulfate predicted greater tTau development and synaptic degeneration. Uremic toxins related to phenol sulfate dysregulate endothelial barriers in the central nervous system and vascular tissues. Additional studies should evaluate microbial metabolites as mediators of gut microbiome‐neurodegeneration relationships, and should determine if they represent modifiable pathways.

WITHDRAWN: Expression of the Arabidopsis SWEET genes during rice false smut infection in the transgenic Arabidopsis thaliana containing increased levels of ATP and sucrose

Ustilaginoidea virens is a causal agent of rice false smut disease which hampers the global rice production. This pathogen is able to continually reach into the central vascular tissues and is able to hijack nutrients from the host plants. It is known that during the later stage of U. virens colonization, the floral region particularly the basal area which is having conducive micro-environment in terms of nutrients and moisture content witnessed the presence of a dense mass of mycelia. Nevertheless, the mechanism which underlies U. virens infection in relation to sugar transport is not well established using a molecular approach. Here, we analyzed contents of sugar that are present in leaves pre and post U. virens infection using a transgenic Arabidopsis thaliana leaves containing increased levels of ATP and sucrose. Glucose and sucrose contents in the leaves of the transgenic Arabidopsis lines were significantly increased compared to WT after U. virens infection, suggesting that U. virens growth and translocation of sugar are directly linked. From the four A. thaliana SWEET genes tested, two AtSWEETs which belong to Clade III were up-regulated significantly upon U. virens infection particularly in the OE7 transgenic line. Our studies suggest that active sugar translocation between the sugar producing tissues probably was triggered by U. virens infection, and the identified SWEET family genes are directly involved in facilitating this process.

SCARECROW, SCR‐LIKE 23 and SHORT‐ROOT control bundle sheath cell fate and function in Arabidopsis thaliana

Bundle sheath (BS) cells form a single cell layer surrounding the vascular tissue in leaves. In C3 plants, photosynthesis occurs in both the BS and mesophyll cells, but the BS cells are the major sites of photosynthesis in C4 plants, whereas the mesophyll cells are only involved in CO2 fixation. Because C4 plants are more efficient photosynthetically, introduction of the C4 mechanism into C3 plants is considered a key strategy to improve crop yield. One prerequisite for such C3-to-C4 engineering is the ability to manipulate the number and physiology of the BS cells, but the molecular basis of BS cell-fate specification remains unclear. Here we report that mutations in three GRAS family transcription factors, SHORT-ROOT (SHR), SCARECROW (SCR) and SCARECROW-LIKE 23 (SCL23), affect BS cell fate in Arabidopsis thaliana. SCR and SCL23 are expressed specifically in the BS cells and act redundantly in BS cell-fate specification, but their expression pattern and function diverge at later stages of leaf development. Using ChIP-chip experiments and sugar assays, we show that SCR is primarily involved in sugar transport whereas SCL23 functions in mineral transport. SHR is also essential for BS cell-fate specification, but it is expressed in the central vascular tissue. However, the SHR protein moves into the BS cells, where it directly regulates SCR and SCL23 expression. SHR, SCR and SCL23 homologs are present in many plant species, suggesting that this developmental pathway for BS cell-fate specification is likely to be evolutionarily conserved.

Impact of IAA, GA 3 , and ABA on Negative Root Phototropism and Root Growth of Rice

The objective of this research was to investigate the relationship between rice roots negative phototropism and endogenous plant hormone levels, as well as the impact of exogenous hormones on rice root growth. The adventitious roots and primary roots of conventional cultivars Yangdao 6(indica), Nipponbare(japonica), Zhonghua 11(japonica) and its OsPIN1 a over expressed transgenic seedlings were used to observe the process of root negative phototropism, measure contents of endogenous auxin(IAA), gibberellin(GA3) and abscisic acid(ABA) in the bending part of root tips, and investigate the impact of exogenous hormones to rice root morphology and anatomy. Results indicated that seedling roots of both Yangdao 6 and Nipponbare had negative phototropism, and the lateral roots and adventitious roots of Yangdao 6 showed stronger negative phototropism. Plant hormones and negative phototropism of rice root and root development are closely linked. Under unilateral illumination, endogenous IAA, GA3 and ABA levels declined. The content of these phytohormones in the irradiated side was lower than that in the shaded side. The treatment of unilateral light and exogenous IAA induced a large number of root hairs in Yangdao 6. Compared with conventional rice, the angle of negative phototropism was lager in OsPIN1 a transgenic plant. There was no direct relationship between exogenous GA3 and ABA treatments and root negative phototropism of rice. Exogenous ABA inhibited root growth obviously. The total absorption surface area, root number and root length per plant were significantly reduced by ABA treatment. The treatment of 10 μmol L–1ABA not only caused a dysplasia of both meristematic and elongation zone but also promoted the development of the central vascular tissue in root tips.

1-MCP partially alleviates dehydration-induced abscission in cut leaves of the fern Nephrolepis cordifolia

Fern leaves, also called fronds, are often used in bouquets. Leaves of the sword fern ( Nephrolepis cordifolia) consist of a central vascular tissue, with numerous leaflets (pinnae) at each side. Leaves that have been cut and immediately placed in water show abscission of the pinnae, starting from about day 4 of vase life, with 50% pinnae abscission on day 13. The onset of pinnae abscission was hastened by a period of dehydration (3, 6, 9 or 12 h at 25 °C). The time to 50% pinnae abscission was between 7.0 and 4.7 days after 3 h and 12 h of dehydration, respectively. Dehydration treatments might induce air emboli in the xylem, but in these experiments did not inhibit water uptake. Dehydration did increase the rate of ethylene production of the cut leaves throughout vase life. A 3 h treatment with 1-MCP at concentrations of 200 or 300 nL L −1 prior to the period of dehydration reduced the rate of ethylene production and reduced the rate of abscission. 1-MCP treatments were also effective if given after the period of dehydration. The data show that pinnae abscission limits the vase life of cut leaves of the sword fern, and that a short period of water stress drastically increases the rate of abscission. The increase in pinnae abscission was correlated with an increase in ethylene production. As 1-MCP alleviated the effect of dehydration on pinnae abscission, the dehydration effect involved ethylene perception. The data suggest that a small water stress induced an autocatalytic rise in ethylene production which was the direct cause of the increase in pinnae abscission.

CRESCIMENTO DO BONITO LISTRADO (Katsuwonus pelamis) E UM MÉTODO PARA A CORREÇÃO DE VÍCIOS DECORRENTES DA VASCULARIZAÇÃO CENTRAL DOS ESPINHOS DAS NADADEIRAS DORSAIS

The dorsal spines are often used in studies on the age and growth of tuna. However, there are vascular tissues in the central area of the spines that make difficult the estimation of the number of growth increments. Some growth increments are missed as the organism and as the central vascular tissue grows. Therefore the age estimations can be biased (i.e. underestimation). In this work we propose methods to minimize the biases in the age estimation of tunas, and we show an application for the skipjack tuna (Katsuwonus pelamis), which is a very important fishery resource. The growth estimations using the best corrective method (L8 =97,9 cm; k = 0,14 por ano; to = -0,071 anos; r2 = 0,71) were very different from that gathered by not trying to minimize the biases (L8 = 80,6 cm; k = 0,25 por ano; to = -1,94 anos; r2 = 0,27). If the results from the corrective method are more reliable than the others, the scientific opinion on the skipjack life history should change. The skipjack would has a longer life history, reaches larger sizes, and grows slower than one could deduce relying on the traditional estimation procedure. The multiple regressions are good tools to correct aging estimates, when it is difficult to identity growth increments in a part of the aposition structure (e.g. spine) being analyzed.

Getting to the root of plant biology: impact of the Arabidopsis genome sequence on root research

Prior to the availability of the genome sequence, the root of Arabidopsis had attracted a small but ardent group of researchers drawn to its accessibility and developmental simplicity. Roots are easily observed when grown on the surface of nutrient agar media, facilitating analysis of responses to stimuli such as gravity and touch. Developmental biologists were attracted to the simple radial organization of primary root tissues, which form a series of concentric cylinders around the central vascular tissue. Equally attractive was the mode of propagation, with stem cells at the tip giving rise to progeny that were confined to cell files. These properties of root development reduced the normal four-dimensional problem of development (three spatial dimensions and time) to a two-dimensional problem, with cell type on the radial axis and developmental time along the longitudinal axis. The availability of the complete Arabidopsis genome sequence has dramatically accelerated traditional genetic research on root biology, and has also enabled entirely new experimental strategies to be applied. Here we review examples of the ways in which availability of the Arabidopsis genome sequence has enhanced progress in understanding root biology.

Inheritance of the cracking characteristics of abscission layer and its relation with the degree of grain shedding

Seed shattering is an undesirable characteristic of rice cultivars affecting field loss of grain yield before and during harvesting. This research was designed to clarify the inheritance of the cracking characteristics of abscission layers formed between the rice grains and its pedicel using F2 populations derived from the crossing of Milyang 23 and Ashahi. The results indicated that the cracking characteristic of Milyang 23 was dominant to absence of cracking in Ashahi, and controlled by a single dominant gene. The degrees of grain shedding were somewhat related with the cracking characteristics of the abscission layer. The breaking tensile strengths closely correlated with the diameter of the supporting zone, the thickness of sclerenchyma tissue between the inner side of abscission layer and the central vascular tissues in the lemma side, and in the palea side, respectively. Some negative correlations were observed between the breaking tensile strengths and some agronomic traits such as plant height, culm length, panicle length, number of matured grains, and the ratio of fertility.

Anatomy of ethylene-induced floral-organ abscission in Chamelaucium uncinatum (Myrtaceae)

Postharvest abscission of Geraldton waxflower (Chamelaucium uncinatum Schauer) flower buds and flowers is ethylene-mediated. Exposure of floral organs to exogenous ethylene (1 µL L–1) for 6 h at 20°C induced separation at a morphologically and anatomically distinct abscission zone between the pedicel and floral tube. Flower buds with opening petals and flowers with a nectiferous hypanthium were generally more responsive to exogenous ethylene than were flower buds enclosed in shiny bracteoles and aged (senescing) flowers. The anatomy of abscission-zone cells did not change at sequential stages of floral development from immature buds to aged flowers. The zone comprised a layer of small, laterally elongated-to-rounded, closely packed and highly protoplasmic parenchyma cells. Abscission occurred at a two- to four-cell-wide separation layer within the abscission zone. The process involved degradation of the middle lamella between separation layer cells. Following abscission, cells on both the proximal and distal faces of the separation layer became spherical, loosely packed and contained degenerating protoplasm. Central vascular tissues within the surrounding band of separation layer cells became torn and fractured. For flower buds, bracteoles that enclose the immature floral tube also separated at an abscission zone. However, this secondary abscission zone appeared less sensitive to ethylene than the primary (central) floral-tube abscission zone as bracteoles generally only completely abscised when exposed to 10 µL L–1 ethylene for the longer period of 24 h at 20°C. The smooth surfaces of abscised separation-layer cells suggest that hydrolase enzymes degrade the middle lamella between adjacent cell walls.

Anatomia comparada do pulvino, pecíolo e raque de Pterodon pubescens Benth. (Fabaceae - Faboideae)

Em Pterodon pubescens, os pulvinos primário e secundário, órgãos responsáveis pelos movimentos foliares, apresentam peculiaridades estruturais em relação ao pecíolo e raque. Estas peculiaridades incluem cutícula mais espessa, córtex mais desenvolvido formado por células parenquimáticas de formas variáveis, sistema vascular em posição central, floema circundado por fibras septadas não lignificadas e medula parenquimática reduzida ou ausente. Pecíolo e raque apresentam córtex reduzido constituído por células parenquimáticas isodiamétricas, sistema vascular periférico, floema envolvido por calota de fibras gelatinosas e medula ampla. Compostos fenólicos foram detectados no vacúolo das células corticais dos pulvinos, enquanto que no córtex e medula do pecíolo e da raque foram observados amiloplastos em abundância e ausência de compostos fenólicos. Em geral, as características estruturais do pulvino de P. pubescens são comuns aos pulvinos das demais espécies de Fabaceae e têm sido relacionadas com a intensificação de trocas laterais de íons e água entre células do córtex e sistema vascular, além de proporcionar maior flexibilidade e capacidade de movimento desta região. Já as características do pecíolo e da raque conferem maior rigidez a estas estruturas foliares.

Methods to Study Oxygen Sensing Sodium Channels

This chapter discusses methods that can be used to study oxygen-sensing sodium channels. Various K + and Ca 2+ channels in central neurons and vascular tissue respond to low O 2 levels, and several hypotheses of how this O 2 sensing occurs have been put forward. For example, the channel itself, or an associated subunit, may sense changes in the O 2 levels, there may be a nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase associated with the channel in the plasma membrane, or there may be other nonhaem-containing structures closely associated with the channel. Ion channel O 2 sensing involves a redox reaction and/or a haem-containing structure. The chapter describes the dissociation of ventricular myocytes, dissociation of young rat hippocampal neurons, hippocampal cell cultures, and expression of recombinant channels in L9292/HEK293 cell lines. Patch clamping persistent Na + channels, bath and pipette solutions, and the role of INaP in hypoxic cell damage and arrhythmias are discussed as well.

Expression patterns of a putative homolog of AGAMOUS, STAG1, from strawberry

MADS box genes function to regulate vegetative, floral, and fruit development in plants. Here we characterize the expression pattern of a MADS box gene from strawberry ( Fragaria× ananassa), designated STAG1. Sequence analysis revealed that STAG1 shared 68–91% amino acid sequence identity to AGAMOUS homologs from a variety of plant species. STAG1 transcripts were detected in stamens, carpels, and developing fruit. In situ hybridization revealed that STAG1 mRNA expression was restricted to the endothelium and the vascular bundles connecting the achenes to the inner part of the receptacle and was not evident in the receptacle of the fruit. Analysis of the expression of a GUS marker gene driven by the STAG1 promoter showed that during floral development, STAG1 was active in stamens, the base of the receptacle and the petals, and in the central pith and vascular tissue. During the ripening stage of fruit development, STAG1 activity was detected in achenes, pith cells, and cortical cells. Sequence analysis and expression patterns indicate that STAG1 is an AGAMOUS homolog of strawberry.

Immunopathogenesis and prevention of uveitis with the Behçet's disease-specific peptide linked to cholera toxin B.

Behcet’s disease (BD) is a multisystemic disease affecting mucocutaneous, ocular, central nervous system, joint and vascular tissues. The aetiology of BD has been associated with a variety of microorganisms, especially Herpes simplex virus and some strains of Streptococcus sanguis. A common microbial agent may be involved, such as heat shock protein (HSP) which has significant homology with human cellular HSP. The evidence in favour of HSP as an aetiological factor in BD is based on T cell proliferative responses and B cell antibodies to the defined HSP60 epitope p336-351 in patients with BD in Britain, Japan and Turkey. The proportion of γδ T cells is significantly increased in BD and HSP65 or HSP70 upregulates γδ T cells. The peptide determinant defined in patients with BD elicits uveitis in Lewis rats when administered by the oral mucosal or systemic route. It is noteworthy that the MICA gene is a cell stress response gene and shares nucleotide sequences with the human HSP70 promoter. The cytokine and chemokine networks stimulated by HSP induce polarisation towards the TH1 cytokines during active disease. The evidence from these studies has converged towards the concept that the multi-system immunopathology of BD is generated by an over-reaction to microbial stress proteins. We now demonstrate that p336-351 induced uveitis in rats can be prevented by oral tolerization with the peptide linked to recombinant cholera toxin B subunit. Preliminary evidence in an open clinical trial suggests that this novel tolerizing strategy may be effective in preventing relapse of uveitis in BD.

Ocular fundus abnormalities in children born before 29 weeks of gestation: a population-based study.

Preterm birth has been found to be associated with increased morbidity of the central nervous and vascular tissues. To investigate the influence of preterm birth on the optic disc and retinal vessels, we examined the ocular fundus in school-aged children born very preterm. A prospective, population-based study was performed in 50 very preterm children (median age 7 years, range 5-9 years) with a median gestational age at birth of 27 weeks (range 24-28 weeks) and a median birth weight of 1055 g (range 450-1520 g). The ocular fundus was examined by ophthalmoscopy in 50 children, and the optic nerve and retinal vessel morphology was evaluated by digital image analysis of ocular fundus photographs in 45 of these children. The median optic disc area was significantly smaller (p = 0.0002) in the preterm children compared with a reference group. There was no difference in cup area and, consequently, the rim area was significantly smaller (p = 0.0002) in the preterm children. Children with early signs of brain lesions commonly had a rim area below the median of the reference group. Preterm children also commonly had an abnormal retinal vascular pattern that was independent of a previous history of retinopathy of prematurity. Very preterm birth was associated with subnormal optic disc and rim areas and an abnormal vascular pattern. The findings clearly demonstrate the effect of preterm birth on the development of these structures. The long term clinical prognosis of these findings has yet to be determined.

Study of the nutrient distribution in root tips of the tropical forage Brachiaria

A Brachiaria breeding project is being conducted at CIAT, Colombia to combine acid soil adaptation of B. decumbens with other favourable traits of Brachiaria species that are less adapted to acid soils. Micro-PIXE was applied to investigate nutrient uptake and distribution in root tips of different species grown in hydroponic culture under control and simulated acid soil stress conditions. Different sample measurement approaches were evaluated, including: (1) linear scans with single point measurements along the root axis; (2) mapping of whole root tips; and (3) mapping of root cross sections. Different tissue types could be distinguished on the base of differences in nutrient concentrations and/or Al stress. Al, if supplied under nutrient stress conditions, increased P accumulation in the central vascular tissue of the meristematic and elongation zone of B. decumbens and B. brizantha. Furthermore, a negative correlation was found between Al and Cl accumulation in the root cap of B. decumbens.

Composite Bundles, the Host/Parasite Interface in the Holoparasitic Angiosperms Langsdorffia and Balanophora (Balanophoraceae)

Composite bundles are not simply a type of vascular bundles, but an integrated host/parasite interface. We investigated their structure in tubers of Langsdorffia and Balanophora. Composite bundles in both genera have similar components: 1) a central mass of host vascular tissues among which are located large parasite transfer cells; 2) a sheath of parasite parenchyma surrounding the central host vascular tissues; 3) specialized conducting tissues in the sheath; and 4) apical meristems composed of both host and parasite meristematic cells. Sheath parenchyma is recognizable from parasite tuber matrix by having thinner cell walls, and, especially in Langsdorffia, by the presence of collapsed matrix cells between the bundle sheath and tuber matrix. Sheath-conducting tissues consist of densely cytoplasmic transfer cells and small sieve tube members; in Langsdorffia, tracheary elements are also present. These sheath bundles connect with vascular bundles of the tuber matrix. Direct host/parasite contact only occurs by means of parasite transfer cells in the composite bundles. There is no xylem-xylem contact at the host/parasite interface. Abundance of parasite transfer cells suggests that they play an important role in nutrient absorption and translocation.

Composite bundles, the host/parasite interface in the holoparasitic angiosperms Langsdorffia and Balanophora (Balanophoraceae)

Composite bundles are not simply a type of vascular bundles, but an integrated host/parasite interface. We investigated their structure in tubers of Langsdorffia and Balanophora. Composite bundles in both genera have similar components: 1) a central mass of host vascular tissues among which are located large parasite transfer cells; 2) a sheath of parasite parenchyma surrounding the central host vascular tissues; 3) specialized conducting tissues in the sheath; and 4) apical meristems composed of both host and parasite meristematic cells. Sheath parenchyma is recognizable from parasite tuber matrix by having thinner cell walls, and, especially in Langsdorffia, by the presence of collapsed matrix cells between the bundle sheath and tuber matrix. Sheath‐conducting tissues consist of densely cytoplasmic transfer cells and small sieve tube members; in Langsdorffia, tracheary elements are also present. These sheath bundles connect with vascular bundles of the tuber matrix. Direct host/parasite contact only occurs by means of parasite transfer cells in the composite bundles. There is no xylem‐xylem contact at the host/parasite interface. Abundance of parasite transfer cells suggests that they play an important role in nutrient absorption and translocation.

Stems of Allium cepa L. contain starch.

Stems of Allium cepa L. were prepared for histochemical analysis and stained by periodic acid-Schiff reaction and Lugol's iodine for detecting starch grains (amyloplasts). Starch was found in the root cap of the root initials, around the central cylinder vascular tissues of the stem and in the primary thickening meristem during sprouting but not during dormancy.

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.