Contributions of the surface of the root tip to the growth of Zea roots in soil

The development of the epidermal layer of roots of Zea is traced from the quiescent centre to the zone where root hairs develop. In the zone of cell division a three layered coat forms on the outside of the epidermal cells consisting of the outer epidermal walls, overlaid by a two-layered pellicle composed of a thick fibrillar inner layer of polysaccharide, and a thin fibrillar outer layer of protein. The epidermal cells divide several times in the same longitudinal file but rarely across a radius to give a new longitudinal file. Thus, the radial walls become much thicker than all but the original transverse walls, and packets of up to 32 daughter cells derived from a single initial may be distinguished. The pellicle develops during these divisions as a continuum over the outer walls of the daughter cells. It is proposed that the pellicle provides a stiffening to the forward end of the root which permits it to penetrate soil without bending. Support for this hypothesis is shown by the Zea mays mutant Ageotropic in which the pellicle is absent, the epidermal surface is disorganized, and which grows crookedly through soil. In the zone of extension growth of normal roots of two Zea species the pellicle thins and disappears. Circumferential strips of the pellicle were peeled off the young epidermal cells and could be stretched to twice their length. This deformation is partly the result of the pellicle stretching and breaking above the attachments of the radial walls. After normal thinning of the pellicle, detachment of the radial walls at their outer ends produces a corrugated surface in the proximal zone of the root tips. In dicotyledons (e.g., soybean), there is no similar pellicle, but a stiff root tip is produced by a long multi-layered root cap, the proximal portion of which covers the elongating epidermal surface.     

The root epidermis of<Emphasis Type="Italic"> Echium plantagineum</Emphasis> L.: a novel type of pattern based on the distribution of short and long root hairs

The great majority of angiosperm species form a group in which either every cell in the root epidermis produces a root hair, or the cells that produce these hairs are randomly distributed. We describe, for the first time, pattern in the root epidermal cells of a species within this group. The seedling root of Echium plantagineum L. (Boraginaceae) has an epidermis in which almost every cell produces a root hair, but these are of two types, short hairs (up to 200 micro m) and long hairs (>200 micro m), which are in separate cell files, with the cells bearing long hairs usually separated by one or two files of cells bearing short hairs; the epidermal cells with the long root hairs are longer than the epidermal cells with the short root hairs. The long root hairs are initiated and develop earlier than the short root hairs. Transverse sections of the region of the root which contains only developing long root hairs show that the hair cells are located above anticlinal walls between underlying cortical cells. We regard the distribution of root epidermal cells in E. plantagineum as a sub-type of this group. We discuss the possible evolution, from this sub-type, of another group that is characterised by hair cells and non-hair cells occurring in separate files.     

Role of the Differentiation of Root Epidermal Cells in Nod Factor (from Rhizobium meliloti)-Induced Root-Hair Depolarization of Medicago sativa

The stage of differentiation of epidermal cells and the development of root hairs was found to be important for the induction of depolarization in root hairs of Medicago sativa by Nod factor [NodRm-IV(S)] isolated from the bacterium Rhizobium meliloti. The electrical membrane response was concentration dependent, having its major effect (amplitude of the depolarization and number of root hairs that responded) at 10-8 and 10-7 M Nod factor. This response was correlated with a morphological effect of Nod factor in the root-hair-deformation bioassay at similar concentrations. The effect of Nod factor on depolarization and root-hair deformation showed specificity with respect to the structure, since unsulfated Nod molecules were inactive, as was the synthetic N,N',N",N"'- tetraacetylchitotetraose. The Nod factor that is O-acetylated at the nonreducing sugar was as efficient in root-hair deformation and membrane depolarization as the sulfated Nod factor.     

EVIDENCE FOR DIVERSE CELL TYPES IN THE APICAL REGION OF THE ROOT EPIDERMIS OF PANICUM VIRGATUM

All epidermal cells in root tips of panicoid grasses have been considered to be capable of hair formation. Observations made in this investigation suggested that cells of two maturation potentials may be present in the root-tip epidermis of Panicum virgatum. Protein bodies which swell and fuse in the region of elongation were revealed in the meristem of this grass by different staining procedures. In many roots not all cells seemed to receive the same amount of these bodies or of the protein-positive material which appeared to arise from them. Only deeply stained cells with large nucleoli were seen to form hairs. Epidermal cells of very hairy roots contained uniform nucleoli and exhibited similar distributions of protein material. The protein positive inclusions were never found in the cortex, a region of cells with one maturation potential. Following chloramphenicol treatment, root tips were found to contain epidermal cells with nucleoli of similar size, a reduced amount of protein bodies, and a reduction in the number of root hairs. RNase treatment did not appear to affect the integrity of the inclusions. The significance of such protein bodies is discussed in relation to differentiation of epidermal cells in P. virgatum.     

星星草营养器官适应盐胁迫的结构特征

采用0．6％Na2CO3胁迫处理星星草[Puccinellia tenuiflora（Turcz．）Scribn．et Merr．]幼苗,光镜和电镜观察其根和叶的显微和超微结构。结果表明,星星草根的表皮向外突出形成密集的根毛;外皮层由1～2层细胞组成,排列较紧密;中皮层薄壁细胞排列疏松,形成发达的通气组织;内皮层呈典型的五面加厚;中柱鞘排列紧密,其壁加厚;初生木质部与初生韧皮部相间排列,初生木质部为5～7原型,中央为后生木质部导管,无髓存在。叶的表皮有表皮毛和丰富的蜡质层;叶上表皮泡状细胞数目较少,且深陷;气孔下陷,其下有较大的气室;叶脉有大、中、小3种维管束,大、中型维管束为C3型,小型维管束为C4型。星星草可能是介于C3和C4植物之间的类型,具有耐盐碱及耐干旱特征。     

Scanning Electron Microscopy of Plant Roots

A glycol methacrylate infiltration and polymerization techniquewas used to prepare clover roots inoculated with Rhizobium forscanning reflection electron microscopy. Root hairs and epidermalcells were coated with many bacteria; some bacteria seemed tobe embedded in the wall surface. Root hair tips were often smoothbut some older root hair surfaces showed a fibrillar meshworkpattern. Small granules c. 0.18 µm diameter were presenton the root hair and epidermal cell walls. The root cap, someroot hairs, and some epidermal cells were covered by an amorphousfilm thought to be the mucigel.     

The effect of the Agrobacterium tumefaciens attR mutation on attachment and root colonization differs between legumes and other dicots

Infections of wound sites on dicot plants by Agrobacterium tumefaciens result in the formation of crown gall tumors. An early step in tumor formation is bacterial attachment to the plant cells. AttR mutants failed to attach to wound sites of both legumes and nonlegumes and were avirulent on both groups of plants. AttR mutants also failed to attach to the root epidermis and root hairs of nonlegumes and had a markedly reduced ability to colonize the roots of these plants. However, AttR mutants were able to attach to the root epidermis and root hairs of alfalfa, garden bean, and pea. The mutant showed little reduction in its ability to colonize these roots. Thus, A. tumefaciens appears to possess two systems for binding to plant cells. One system is AttR dependent and is required for virulence on all of the plants tested and for colonization of the roots of all of the plants tested except legumes. Attachment to root hairs through this system can be blocked by the acetylated capsular polysaccharide. The second system is AttR independent, is not inhibited by the acetylated capsular polysaccharide, and allows the bacteria to bind to the roots of legumes.     

The Effect of the Agrobacterium tumefaciens attR Mutation on Attachment and Root Colonization Differs between Legumes and Other Dicots

Infections of wound sites on dicot plants by Agrobacterium tumefaciens result in the formation of crown gall tumors. An early step in tumor formation is bacterial attachment to the plant cells. AttR mutants failed to attach to wound sites of both legumes and nonlegumes and were avirulent on both groups of plants. AttR mutants also failed to attach to the root epidermis and root hairs of nonlegumes and had a markedly reduced ability to colonize the roots of these plants. However, AttR mutants were able to attach to the root epidermis and root hairs of alfalfa, garden bean, and pea. The mutant showed little reduction in its ability to colonize these roots. Thus, A. tumefaciens appears to possess two systems for binding to plant cells. One system is AttR dependent and is required for virulence on all of the plants tested and for colonization of the roots of all of the plants tested except legumes. Attachment to root hairs through this system can be blocked by the acetylated capsular polysaccharide. The second system is AttR independent, is not inhibited by the acetylated capsular polysaccharide, and allows the bacteria to bind to the roots of legumes.     

Observations on the Responses of Lentil Root Cells to Hyphae of Fusarium oxysporum

The infection of lentil roots by Fusarium oxysporum Schlecht and the responses of the host cells to invading hyphae were examined by light microscopy. Hyphae from inoculum placed on the zone of cell elongation entered the roots at the juncture of epidermal cells within 8 h after inoculation. Although swollen hyphal apices were observed on the epidermal cells, root penetration occurred without formation of these structures or appressoria. The sheath of material found on the surface of uninoculated roots was absent from inoculated roots penetrated by hyphae. Prior to penetration, the epidermal cells became irregular in shape and their cytoplasm appeared to be plasmolysed or granular. Hyphae were observed in the cortex 10—12 h after inoculation and non–penetrated cortical cells were distinctly lobate. Often these lobed cells had a broad, peripheral band of diffuse cytoplasm. When hyphae were first observed in the cortical cells, the walls were ruptured and only slightly stained or unstained by toluidine blue. The inability of such walls to bind the stain may have been the result of the removal of wall components by fungal enzymes. Although extensive proliferation of hyphae was evident throughout the cortex after 24 h of incubation, the endodermis and vascular cylinder were free of hyphae for at least 72 h. Hyphae from inoculum placed on the root hairs or the root apex failed to penetrate the roots during the first 24 h of incubation. The cytological results herein are discussed in relation to the infection of field plantings by this pathogen.     

Correlation between infection by Rhizobium leguminosarum and lectin on the surface of Pisum sativum L. roots

The lectin on the surface of 4- and 5-dold pea roots was located by the use of indirect immunofluorescence. Specific antibodies raised in rabbits against pea seed isolectin 2, which crossreact with root lectins, were used as primary immunoglobulins and were visualized with fluorescein- or tetramethylrhodamine-isothiocyanate-labeled goat antirabbit immunoglobulin G. Lectin was observed on the tips of newly formed, growing root hairs and on epidermal cells located just below the young hairs. On both types of cells, lectin was concentrated in dense small patches rather than uniformly distributed. Lectin-positive young hairs were grouped opposite the (proto)xylematic poles. Older but still-elongating root hairs presented only traces of lectin or none at all. A similar pattern of distribution was found in different pea cultivars, as well as in a supernodulating and a non-nodulating pea mutant. Growth in a nitrate concentration which inhibits nodulation did not affect lectin distribution on the surface of pea roots of this age. We tested whether or not the root zones where lectin was observed were susceptible to infection by Rhizobium leguminosarum. When low inoculum doses (consisting of less than 10⁶ bacteria·ml^-1) were placed next to lectin-positive epidermal cells and on newly formed root hairs, nodules on the primary roots were formed in 73% and 90% of the plants, respectively. Only a few plants showed primary root nodulation when the inoculum was placed on the root zone where lectin was scarce or absent. These results show that lectin is present at those sites on the pea root that are susceptible to infection by the bacterial symbiont.Abbreviations FITC fluorescein isothiocyanate - TRIC tetramethylrhodamine isothiocyanate     

冬小麦生育中后期次生根特殊根毛的发生与形态结构

大田试验条件下,研究了冬小麦次生根特殊根毛的发生规律和形态结构特征.结果表明,拔节后次生根近植株基部根体上特殊根毛普遍发生,其在次生根根体上的分布可区分为集中区、适中区和稀少区.特殊根毛集中区根毛的长度、直径、密度分别均大于适中区和稀少区,其中,长度和密度的差异达极显著水平(P<0.01).随着生育期推进, 特殊根毛长度、直径和密度均呈下降趋势.大多数特殊根毛呈现出不同程度的扭曲、变形, 根毛细胞突起和分枝现象较为普遍,特殊根毛细胞次生壁出现加厚现象.     

Early cytological events in the infection of the root hairs ofTrifolium repens byRhizobium leguminosarum biovartrifolii observed by glass slide culture in living seedlings

This report describes the early cytological events in the infection byRhizobium leguminosarum biovartrifolii of the root hairs ofTrifolium repens seedlings kept alive on agar medium in glass slide culture experiment. The infection threads bearing rhizobia were formed as soon as the epidermal cells began to emerge as root hairs. On the top of some of these infected emerging root hairs, there were smoky, cell-debris-like bodies, which appeared to be derived from the cell wall dug by rhizobia. Similar bodies were also observed in longer root hairs. None of the root hair cells along the length of the roots which contained infection threads were curled or distorted. A substantial number of pink-colored nodules were later formed on the roots with non-curled infected root hairs.     

Infection and Nodule Development in Aotus ericoides (Vent.) G. Don, A Woody Native Australian Legume

Infection and nodule development were studied by light and electronmicroscopy in Aotus ericoides, a woody native Australian legume,inoculated with a slow-growing field isolate of Rhizobium. Rhizobiabound to straight, but not deformed, root hairs, as detectedby immunofluorescence. Neither markedly curled root hairs norroot hairs with infection threads were seen. Nodules were indeterminate(astragaloid), with a peripheral meristematic layer, few vasculartraces and both infected and uninfected cells in the centralinfected zone. Infection threads containing contorted bacteriawere present throughout the nodule. Swollen, rod-shaped bacteriain infected cells were in groups in vesicles bounded by plasmalemma-derivedperibacteroid membranes. Senescence in infected cells was associatedwith accumulation of a fibrillar matrix inside peribacteroidmembranes, distortion of bacteria and destruction of most cytoplasmiccontents of the bacteria and host cells; however, most bacterialand plant membranes and plant cell walls remained intact. Ineffectivenesswas associated with relatively little, short-lived infectedtissue. Events in infection and nodule development were similarto those in most herbaceous legumes but showed characters ofboth determinate and indeterminate nodules. Key words: Bacteroids, Legume, Nitrogen-fixing, Nodule, Rhizobium     

Invasion of Lotus japonicus root hairless 1 by Mesorhizobium loti involves the nodulation factor-dependent induction of root hairs

In many legumes, including Lotus japonicus and Medicago truncatula, susceptible root hairs are the primary sites for the initial signal perception and physical contact between the host plant and the compatible nitrogen-fixing bacteria that leads to the initiation of root invasion and nodule organogenesis. However, diverse mechanisms of nodulation have been described in a variety of legume species that do not rely on root hairs. To clarify the significance of root hairs during the L. japonicus-Mesorhizobium loti symbiosis, we have isolated and performed a detailed analysis of four independent L. japonicus root hair developmental mutants. We show that although important for the efficient colonization of roots, the presence of wild-type root hairs is not required for the initiation of nodule primordia (NP) organogenesis and the colonization of the nodule structures. In the genetic background of the L. japonicus root hairless 1 mutant, the nodulation factor-dependent formation of NP provides the structural basis for alternative modes of invasion by M. loti. Surprisingly, one mode of root colonization involves nodulation factor-dependent induction of NP-associated cortical root hairs and epidermal root hairs, which, in turn, support bacterial invasion. In addition, entry of M. loti through cracks at the cortical surface of the NP is described. These novel mechanisms of nodule colonization by M. loti explain the fully functional, albeit significantly delayed, nodulation phenotype of the L. japonicus ROOT HAIRLESS mutant.     

Water-relation parameters of epidermal and cortical cells in the primary root ofTriticum aestivum L.

Water-relation parameters of root hair cells, hairless epidermal cells, and cortical cells in the primary root of wheat have been measured using the pressure-probe technique. Under well-watered conditions the mean cell turgor of cortical cells was 6.8±1.9 (30) bar (mean±SD; the number of observations in brackets). In hairless epidermal and root hair cells the mean cell turgor was 5.5±1.9 (22) and 4.4±1.5 (15) bar, respectively. Despite the large variability, turgor pressure was significantly lower (confidence interval=0.95) in epidermal cells relative to cortical cells. This may be a consequence of the ultrafiltration of ions by the external cell wall and-or plasmalemma of epidermal cells. The volumetric elastic modulus of the cells ranged from 10 to 150 bar. This parameter was dependent on cell volume, but within experimental accuracy, was independent of cell type. No pressure dependence of the volumetric elastic modulus was observed in these cells. The half-times for water exchange ranged from 1.8 to 48.8 s. The mean value increased in the order root hair < hairless epidermal < cortical cells and was directly related to volume to surface area ratio. Thus the hydraulic conductivities of the three cell types were similar and averaged 1.2±0.9·10^-6 (170) cm s^-1 bar^-1. No polarity was observed between inwardly and outwardly directed water flow. The similarity of the hydraulic conductivities of root hairs to those of other cells indicates that the membranes of root hairs are not particularly specialized for water transport. The overall hydraulic conductivity for radial water flow across the root was estimated from the pressure-probe data using a simple model and was compared with that measured directly on whole roots using an osmotic backflow technique. It was tentatively concluded that upon sudden osmotic perturbation, the major pathway for water transfer across the root may be through the symplasm and involve net flow from vacuole to vacuole.     

Involvement of the mitogen-activated protein kinase SIMK in regulation of root hair tip growth

Mitogen-activated protein kinases (MAPKs) are involved in stress signaling to the actin cytoskeleton in yeast and animals. We have analyzed the function of the stress-activated alfalfa MAP kinase SIMK in root hairs. In epidermal cells, SIMK is predominantly nuclear. During root hair formation, SIMK was activated and redistributed from the nucleus into growing tips of root hairs possessing dense F-actin meshworks. Actin depolymerization by latrunculin B resulted in SIMK relocation to the nucleus. Conversely, upon actin stabilization with jasplakinolide, SIMK co-localized with thick actin cables in the cytoplasm. Importantly, latrunculin B and jasplakinolide were both found to activate SIMK in a root-derived cell culture. Loss of tip-focused SIMK and actin was induced by the MAPK kinase inhibitor UO 126 and resulted in aberrant root hairs. UO 126 inhibited targeted vesicle trafficking and polarized growth of root hairs. In contrast, overexpression of gain-of-function SIMK induced rapid tip growth of root hairs and could bypass growth inhibition by UO 126. These data indicate that SIMK plays a crucial role in root hair tip growth.     

Floral nectary ultrastructure of Prunus persica (L.) Batch cv. Forastero (Newcomer), an Argentine peach

 Flowers of Prunus persica (L.) Batch. cv. Forastero have an orange toral nectary. The nectariferous tissue was formed by densely packed parenchyma cells (secretory cells) and an epidermis with hairs and modified stomata. The epidermal cells were highly vacuolated with a striated cuticle. The ultrastructure of these cells contained a cytoplasm with endoplasmic reticulum, plastids, mitochondria and dictyosomes. Sub-epidermal cells were barely vacuolated and their ultrastructure was similar to that of the epidermal cells. Differences were observed only in the endoplasmic reticulum, which is organized in a parallel configuration. Plasmodesmata were found between adjacent secretory cells and between secretory and epidermal cells. An electron dense secretion occurred in the intercellular spaces and between the external tangential wall and the cuticle of the epidermal cells. According to the ultrastructural observations, the sugar solution could be passed through the symplast or the apoplast. The nectar could be exuded from the stomata and the micro-channels of the cuticle covering the epidermal cells. Received July 7, 2002; accepted September 24, 2002 Published online: June 2, 2003