THE HOST-PARASITE INTERFACE OF ALBUGO CANDIDA ON RAPHANUS SATIVUS

The fine structure of the intercellular hyphae of the obligate parasite Albugo candida infecting radish does not differ markedly from that described previously for cells of Peronospora manshurica. The stalked, capitate haustoria do not contain nuclei and are packed with mitochondria and lomasomes. The fungal plasma membrane and cell wall are continuous from the intercellular hypha throughout the haustorium except that there is no evidence of fungal cell wall around a portion of the haustorial stalk proximal to the haustorial head. Within the vacuolate host mesophyll cell, the haustorium is always surrounded by host plasma membrane and with at least a thin layer of host cytoplasm. The host cell wall invaginates at the point of haustorial penetration to form a short sheath around the region of penetration, but normally there is no host cell wall around the balance of the haustorium. About 1% of the haustoria observed were necrotic, and these were invariably walled-off completely from host cytoplasm by host cell wall. An amorphous, moderately electron-dense encapsulation lies between the haustorium proper and the host plasma membrane and extends into the penetration region between the sheath and the fungal cell wall. Invaded host cells contain more ribosomal-rich ground cytoplasm than uninfected cells. Glandular-like systems of tubules and connecting vesicles are often numerous in host cytoplasm in the vicinity of haustorial heads. These tubules open into the encapsulation, their limiting unit membranes being continuous with the host plasma membrane. We suggest that these represent a secretory mechanism of the host specifically induced by the parasite.     

Tremelloid haustorial cells with haustorial filaments and potential host range of Tremella mesenterica

Zugmaier, W. & Oberwinkler, F. 1995. Tremelloid haustorial cells with haustorial and potential host ranges of Tremella mesenterica . - Nord. J. Bot. 15: 207–213. Copenhagen. ISSN 0107–055X.
In vitro and in vivo haustorial cells of Tremella mesenterica , consisting of a subglobose basal part with one or more thread-like haustorial filaments, were examined using light and electron microscopy. The haustorial cells developed from clamp connections of hyphal septa or intercalarily. Mostly such haustorial cells were monokaryotic, but in a few cases also dikaryotic haustorial cells were observed. In vitro, the host range of Tremella mesenterica was tested with the corticiaceous homobasidiomycetes Peniophora erikssonii, P. quercina and Phanerochaete eremea . The in vitro interaction of Tremella mesenterica with all these fungi was principally the same as in vivo with Peniophora laeta . A single micropore connected the cytoplasm of the haustorial filament with that of the respective host cell. The pore domain at the host side was delimited by curved ER cisternae only in Peniophora quercina , but not in Peniophora erikssonii and Phanerochaete cremea .     

EXPERIMENTAL STUDIES OF HAUSTORIUM INITIATION AND EARLY DEVELOPMENT IN AGALINIS PURPUREA (L.) RAF. (SCROPHULARIACEAE)

In parasitic angiosperms the haustorium, an organ specialized for attachment and penetration of host tissue, functions in the transport of water and nutrients from the host to the parasite. In Agalinis purpurea (L.) Raf. (Scrophulariaceae) these organs are initiated laterally along its roots, opposite a primary xylem pole. Analyses of haustoria distribution and cellular root profiles show that the portion of the root which is most sensitive to haustorial elicitor molecules is the area distal to the zone of elongation and near the root meristem. Sectioned material supports this finding and, further, indicates that the cells which are the first to respond to haustorial elicitors are located in the inner cortex. Haustoria develop rapidly in response to a host root or to isolated chemical elicitors (xenognosins) normally contained in host root exudate. By 6 hr, vacuolation and radial cellular enlargement are observed in the cortex, and a lateral swelling along the root is visible. By 12 hr, cells of the epidermis divide anticlinally to establish a group of densely cytoplasmic cells at the apex of the haustorial swelling. Accompanying these divisions is the differentiation of specialized hair cells which elongate from epidermal cells flanking the presumptive haustorial apex. Next, the internal, radially enlarged cortical cells divide periclinally. Periclinal divisions are subsequently initiated in the pericycle as early as 18 hr post-induction. Cellular division and enlargement continue so that by 24–36 hr a mature pre-contact haustorium is formed. There is a reduction in root elongation concomitant with haustorial initiation. Depending upon the number of haustoria produced, elongation typically returns to the preinduction level within 2 or 3 days.     

Anatomical and Histochemical Studies of Haustrial Development of Cassytha filiformis L.

This paper deals with the haustorium development of parasitic plant (Cassytha filiformis L.) parasitized on purple willow (Salix purpurea L.).The polarity occurred in the portion near the host; and the cushion-shaped haustorial plate formed, and then the haustorial primordium initiated in the cortex. Finally, the haustoria reached the cortex of pith of the host, penetrating through its own cortex and epidermis. Tracheary elements were differentiated from the base of the haustoria and the sieve elements were not observed in the haustoria. Histochemical studies revealed that there were starch grains in normal stem cortex. The starch grains were increased in the portion near the axis after twisting on host. After the haustorial plate was formed, the starch grains were richly accumulated in the central group of cells, which were followed by the haustorial development; The starch disappeared in the meristem, in which protein stained deeply; The dynamic change of protein turned oK to be contrary to the tendency of starch accumulation. The structure and parasitic mechanism and the dynamic change of starch and protein are discussed.     

Scanning electron microscope observations on the pathological changes of Malpighian tubules in the worker honeybee,Apis mellifera,infected by Malpighamoeba mellificae

A mixed population of cysts, immature cysts, and trophozoites was observed in the lumen of Malpighian tubules of Apis mellifera infected with Malpighamoeba mellificae. The most distinct pathological change in the infected tubules was swelling of the brush border of epithelial cells. In some areas the brush border was broken down into pieces, and in some cases it was completely engulfed by the parasites. The microvilli of the brush border were long and slender in noninfected tubules. Nuclei were not found in the infected tubules, but were frequently observed in the non-infected tubules. Numerous secretion globules were observed in the cytoplasm of epithelial cells of noninfected Malpighian tubules, but not in the cells of infected tubules. Epithelial cells in the infected tubules were destroyed and consumed by the trophozoites of M. mellificae. Trophozoites possessed protrusions and pits on their surfaces; many also had pseudopodia and apparently consumed the host cell by a mechanism of phagocytosis or endocytosis.     

Structure and Development of the Primary Haustorium in Alectra vogelii Benth. (Scrophulariaceae)

Anatomical observations were made on the structure and developmentof the primary haustorium of Alectra vogelii. Its developmentinvolves a mutual aggressive growth of both the host and parasitetissues resulting in the formation of a very large and complextuberous organ. One of the host tissues whose growth is stimulatedby parasite infection is the pericycle whose cells divide repeatedlyand grow around and within the parasite haustorial cortex. Fromvarious points of the proliferating host pericycle, roots becomeinitiated and eventually the entire surface of the haustoriumbecomes covered with these roots. We have referred to them as‘haustorial roots’, a term which we have re-examinedand redefined. True xylary connections are established not onlybetween the parasite and the host root but also between theparasite and these ‘haustorial roots’. The uniquedevelopment of primary haustorium and ‘haustorial roots’in A. vogelii is discussed in relation to the development ofprimary haustoria in other root parasites.     

Anatomy of the endophyte of Viscum album L. (Loranthaceae)

Anatomy of the endophyte of Viscum album L. (Loranthaceae). An anatomical investigation into the nature of the host-parasite interaction of V. album and several of its phanerogamic hosts using SEM and light microscopy was conducted. Three kinds of parasite cell (haustorial parenchyma cells, cells resembling transfer cells and haustorial tracheids) were identified at the host-parasite interface. The terms haustorial parenchyma and haustorial tracheid are defined. Haustorial tracheids were seen to have penetrated the walls of host vessel elements and it is suggested that V. album is able to establish on a wide range of hosts because of the anatomically plastic nature of its haustorium. The development of the haustorium depends to a large extent on the nature of the surrounding host tissues. Parasite-induced host abnormalities including hypertrophy, distorted xylem elements, vessel-wall penetration and tylosis-occluded vessels were observed. The macroanatomical features observed are discussed and interpreted by-proposing a new theory for the ontogenesis of the V. album haustorium. Cortical strands with 'chisel' and 'pencil' shaped apices were both found to be present at the same time on one plant and thus were not seasonally separated.     

Ultrastructural specialization at the host-pathogen interface in rust-infected flax

Summary Ultrastructure of the association between the rust fungus, Melampsora lini, and a compatible variety of flax, Linum usitatissimum, was studied to clarify the structural relationships and interactions at the site of host penetration and at the host-parasite interface. Results of freeze-etching as well as a special section-staining procedure consisting of periodate-chromate-phosphotungstate (PACP) are shown with a host-parasite combination for the first time. The host plasma membrane is invaginated by the fungus and forms a continuous boundary around the fungal haustoria which penetrate the host cells. No morphological continuities are observed linking the protoplasts of host and fungus. With both freeze-etching and the PACP stain, the invaginated portion of the host plasma membrane at the host-parasite interface shows distinctive features that are not characteristic of the non-invaginated portion of the membrane. This localized specialization of host plasma membrane in response to the fungus appears as a significant and consistent feature of the host-parasite interaction. The host plasma membrane is separated from the haustorial wall by an amorphous layer of sheath material which covers the body but not the neck of the haustorium. This sheath provides the environment in which the haustorium exists and functions during the course of the host-parasite association. Occasionally, a collar of wall-like material derived from the host cell forms around the haustorial neck. The collar is continuous with the host wall and is distinct and discontinuous from the haustorial sheath. In fewer than 5% of the infected cells this wall material encases entire haustoria. The fungal wall is structurally specialized around the site of host penetration, and it becomes intimately associated with the host wall where the fungus penetrates into the lumen of the host cell. During penetration, the host and fungal walls appear to be fused so that the interface between them is not clearly delineated. The haustorial wall is continuous, via the haustorial neck, with the wall of the haustorial mother cell which lies outside the host cell. Different staining properties reveal this wall continuum to consist of several well-defined regions having different structure or composition. A ring of fungal wall material midway along the haustorial neck stains densely with lead citrate, but is preferentially etched away by periodic acid. The neck ring denotes a transition in the staining reaction of the fungal wall, from that present in the region of host penetration to that of the wall surrounding the haustorium. The findings demonstrate specialization of the fungal wall in the area of host penetration as well as specialization of the host plasma membrane at the host-parasite interface to a degree not previously realized from ultrastructural information.     

Physiology of the interaction of angiosperm parasites and their higher plant hosts

Abstract. Interactions between parasitic angiosperms and their hosts occur at the level of seed germination, haustorial development and resource transfer. Chemicals released from the host function as cues for host recognition, and trigger germination as well as haustorial initiation. Transpiration is a key process regulating solute transfer from host to parasite, and some parasitie plants have unusual stomatal characteristics. Although solute transfer is apoplastic, the haustorium appears to play a role in regulating solute composition. Host responses to infection are reviewed, and it is concluded that competition for water and solutes are unlikely to play a major role in determining reductions in host productivity: metabolic incompatability is suggested to be the major cause of this.     

Subtilase activity in intrusive cells mediates haustorium maturation in parasitic plants

Parasitic plants that infect crops are devastating to agriculture throughout the world. These parasites develop a unique inducible organ called the haustorium that connects the vascular systems of the parasite and host to establish a flow of water and nutrients. Upon contact with the host, the haustorial epidermal cells at the interface with the host differentiate into specific cells called intrusive cells that grow endophytically toward the host vasculature. Following this, some of the intrusive cells re-differentiate to form a xylem bridge (XB) that connects the vasculatures of the parasite and host. Despite the prominent role of intrusive cells in host infection, the molecular mechanisms mediating parasitism in the intrusive cells remain poorly understood. In this study, we investigated differential gene expression in the intrusive cells of the facultative parasite Phtheirospermum japonicum in the family Orobanchaceae by RNA-sequencing of laser-microdissected haustoria. We then used promoter analyses to identify genes that are specifically induced in intrusive cells, and promoter fusions with genes encoding fluorescent proteins to develop intrusive cell-specific markers. Four of the identified intrusive cell-specific genes encode subtilisin-like serine proteases (SBTs), whose biological functions in parasitic plants are unknown. Expression of SBT inhibitors in intrusive cells inhibited both intrusive cell and XB development and reduced auxin response levels adjacent to the area of XB development. Therefore, we propose that subtilase activity plays an important role in haustorium development in P. japonicum.

Subtilases specifically expressed in intrusive cells regulate auxin-mediated host–parasite connections in the parasitic plant Phtheirospermum japonicum.     

Initiation, Development and Structure of the Primary Haustorim in Striga gesnerioides (Scrophulariaceae)

A light-microscopic study is reported on the initiation, establishmentand structure of the primary haustorium of Striga gesnerioideson the host, cowpea (Vigna unguiculata). The radicular apexof the germinated parasite seed dissolves its way through thehost root cortex to the stele. Thus, it is converted into aprimary haustorium. Some of the haustorial front-line cellsin contact with the host endodermis penetrate into the steleand make contact with the xylem vessels. Differentiation ofthese haustorial cells into xylem vessels occurs and extendsbackwards through the median axial region of the haustorialtract in the host cortex to connect with the conductive xylemof the radicle outside the host root. Subsequently the parasite'splumule develops into a leafy shoot. On penetrating the steleof the host, the haustorium stimulates cell division in thehost pericycle whose triggered proliferation together with expansionof the parasite haustorial tissues result in the formation ofa large, tuberous primary haustorium. At various points of thehost-parasite interface, differentiation of xylem elements occurs,presumably maximizing nutrient transfer from host to parasite.In spite of this, many proliferated host cells at the interfaceremain apparently meristematic showing densely-stained cytoplasmand prominent nuclei.     

PARASITISM IN PHOLISMA (LENNOACEAE). I. EXTERNAL MORPHOLOGY OF SUBTERRANEAN ORGANS

The external features of the subterranean organs of coastal Californian Pholisma depressum Greene are described. A dimorphic root system exists. Long, stout roots (pilot roots) serve for vegetative propagation in reaching other host roots. Short, unbranching, more slender roots originating from the pilot roots are haustorial in function. Shoots seem to arise only on pilot roots. Host attachment is achieved through the direct transformation of the apical meristem of haustorial roots into the haustorial organ. The basal part of the shoot is also able to produce roots when near host roots, and may form lateral branch shoots either directly or after new haustorial connections are formed. Both types of roots arise endogenously. The invaded host root undergoes considerable hypertrophy. The intrusive organ itself forms a massive but irregular body. More than one haustorial root may enter even a small host root. Self parasitism may occur but is not common.     

ELECTRON MICROSCOPY OF THE HOST-PARASITE RELATIONSHIPS IN STEM RUST OF WHEAT

Ehrlich , H. G., and Mary A. Ehrlich . (Duquesne U., Pittsburgh, Pa.) Electron microscopy of the host-parasite relationships in stem rust of wheat.—Amer. Jour. Bot. 50(2): 123–130. Illus. 1963.—A series of micrographs showing intercellular dikaryotic mycelium, haustorial mother cells, stages in haustorial formation, and haustoria within host cells are presented in the present report. Of special interest and potential significance in a study of obligate parasitism is an encapsulation ranging from 800 to 3400 A in thickness which surrounds the haustorium, but which is not present around the intercellular hyphae. The encapsulation completely encases the haustorium proper; it is bounded on the inside by the cell wall of the haustorium, and its thin membranous outer margin abuts directly on the protoplast of the host cell. The nature of the material composing the encapsulation is uncertain, but it appears to originate from the haustorial protoplast, and at least a portion of it may be fungal cytoplasm. This newly described structure represents the actual interface between the host and pathogen. Small vesicles which seem to originate from the outer margin of the encapsulation are sometimes found in the host cytoplasm surrounding apparently vigorous haustoria. The vesicles are bounded by a membrane and contain particulate material.     

RELATION OF TOBACCO MOSAIC VIRUS TO THE HOST CELLS

The relation of tobacco mosaic virus (TMV) to host cells was studied in leaves of Nicotiana tabacum L. systemically infected with the virus. The typical TMV inclusions, striate or crystalline material and ameboid or X-bodies, which are discernible with the light microscope, and/or particles of virus, which are identifiable with the electron microscope, were observed in epidermal cells, mesophyll cells, parenchyma cells of the vascular bundles, differentiating and mature tracheary elements, and immature and mature sieve elements. Virus particles were observed in the nuclei and the chloroplasts of parenchyma cells as well as in the ground cytoplasm, the vacuole, and between the plasma membrane and the cell wall. The nature of the conformations of the particle aggregates in the chloroplasts was compatible with the concept that some virus particles may be assembled in these organelles. The virus particles in the nuclei appeared to be complete particles. Under the electron microscope the X-body constitutes a membraneless assemblage of endoplasmic reticulum, ribosomes, virus particles, and of virus-related material in the form of wide filaments indistinctly resolvable as bundles of tubules. Some parenchyma cells contained aggregates of discrete tubules in parallel arrangement. These groups of tubules were relatively free from components of host protoplasts.     

Endogenous hormone levels and anatomical characters of haustoria in Santalum album L. seedlings before and after attachment to the host

The physiological and anatomical attributes of haustoria tissues in hemi-parasitic Santalum album L. seedlings, growing on the potential host, Kuhnia rosmarnifolia Vent., were investigated before and after attachment to the host. Quantization of endogenous levels of indole-3-acetic acid (IAA), zeatin (Z), zeatin riboside (ZR), GA-like substances (GAs) and abscisic acid (ABA) was performed by HPLC. Histological preparations were used to characterize structural differences between pre- and post-attachment haustoria. The contents of GAs and ABA were higher in attached haustoria, with 3.61 and 3.50μgg(-1) fresh weight, respectively, and three times higher than in non-attached haustoria. Cytokinins, Z, ZR and IAA levels were also high, and their contents in attached haustoria increased 2.04-, 2.17-, and 2.82-fold more, respectively, than in non-attached haustoria. A high auxin-to-cytokinin ratio contributed to haustorial development of S. album. A numerous amount of starch in parenchyma cells around the meristematic region above the haustorial gland and the endophyte tissue of the post-attachment haustoria were reported in a Santalaceae member for the first time. Many lysosomes were present and large-scale digestion of host cells occurred at the interface between the parasite and host. The haustorial penetration in S. album into the host stele was suggested to be a function of mechanical force and enzymatic activity. Analysis of the endogenous hormone levels and the structural characters in S. album haustoria indicated that the haustoria were able to synthesize phytohormones, which appeared to be necessary for cell division and differentiation during haustorial development. These results suggest that endogenous hormones are involved in the haustorial development of S. album and in water and nutrient transport in the host-parasite association.     

梨胶锈菌性孢子和锈孢子阶段吸器的超微结构研究

本文对梨胶锈菌性子期和锈子期菌丝吸器的形成方式、吸器及其与寄主细胞界面的超微结构进行了研究。观察结果表明：梨胶锈菌性子期和锈子期寄主胞间菌丝吸器的形成方式有两种：一种是由寄主胞间菌丝直接形成吸器;另一种是由寄主胞间菌丝先形成吸器母细胞,然后由吸器母细胞形成吸器。吸器在开始形成时只是一个乳头状的侵入楔,以后逐渐形成囊状、镰刀状、指状及其它不规则形状的吸器。多数吸器分化为颈和吸器主体两部分,在颈部及部分吸器主体外有一个由类似寄主细胞壁物质形成的领圈。吸器内部的超微结构与寄主胞间菌丝基本相同,但吸器壁比胞间菌丝或吸器母细胞的壁薄。吸器鞘的厚度随着吸器伸长膨大 而逐渐增厚。     

Internalization of Flax Rust Avirulence Proteins into Flax and Tobacco Cells Can Occur in the Absence of the Pathogen

Translocation of pathogen effector proteins into the host cell cytoplasm is a key determinant for the pathogenicity of many bacterial and oomycete plant pathogens. A number of secreted fungal avirulence (Avr) proteins are also inferred to be delivered into host cells, based on their intracellular recognition by host resistance proteins, including those of flax rust (Melampsora lini). Here, we show by immunolocalization that the flax rust AvrM protein is secreted from haustoria during infection and accumulates in the haustorial wall. Five days after inoculation, the AvrM protein was also detected within the cytoplasm of a proportion of plant cells containing haustoria, confirming its delivery into host cells during infection. Transient expression of secreted AvrL567 and AvrM proteins fused to cerulean fluorescent protein in tobacco (Nicotiana tabacum) and flax cells resulted in intracellular accumulation of the fusion proteins. The rust Avr protein signal peptides were functional in plants and efficiently directed fused cerulean into the secretory pathway. Thus, these secreted effectors are internalized into the plant cell cytosol in the absence of the pathogen, suggesting that they do not require a pathogen-encoded transport mechanism. Uptake of these proteins is dependent on signals in their N-terminal regions, but the primary sequence features of these uptake regions are not conserved between different rust effectors.     

First catalogue of the rust fungi of French Guiana, northern South America

An annotated list of the rust fungi (Uredinales or Pucciniales) of French Guiana is presented. It enumerates 68 species of which 57 are new reports for the department and 3, Aecidium plukenetiae, Puccinia kourouensis and P. parianicola, are new to science. Dicheirinia guianensis and Hapalophragmium angylocalycis are excluded from the French Guianan mycobiota. New host plants are reported for Batistopsora crucis-filii, B. pistila, Cerotelium ficicola, C. sabiceae, Crossopsora piperis, Desmella aneimiae, Endophyllum guttatum, Kweilingia divina, Puccinia lateritia, Uredo anthurii and Uromyces anguriae. Previously undescribed characters are presented for Achrotelium lucumae, Chaconia ingae, Cerotelium sabiceae, Prospodium amapaensis, Sphenospora smilacina and Uromyces wulffiae-stenoglossae. Chaconia ingae showed haustorial complexes comprising both intracellular hyphae and D-haustoria. In Cerotelium sabiceae, the haustorial mother cells retained the nuclei while D-haustoria were enucleate. The occurrence of these haustorial types in tropical rust fungi is discussed. Internal basidium formation is described for the first time in Sphenospora: teliospores of S. smilacina produced external or internal basidia. The species richness and composition of the French Guianan rust mycobiota are discussed in a neotropical context.     

Studies on new types of haustoria in some Central EuropeanRhinanthoideae (Scrophulariaceae)

Two new types of haustoria have been found in many representatives ofRhinanthoideae. Metahaustoria are stimulated by host roots but lack contact with them. They develop a massive haustorial core with a large- and a smalled-celled portion, but little vascular core. In very old metahaustoria the vascular core is much enlarged and numerous tracheal elements develop in the reduced haustorial core and cortex. Warthaustoria are very small, occur only in the upper portions of the root system, and have host contacts. Vascular and haustorial core, and the endophyte are lacking, the intrusive cells are usually not endogenous, but rather roothair-like. Wart-haustoria may represent reduced and partly inhibited normal haustoria.     

小麦条锈菌吸器母细胞超微结构的研究

本文利唱电镜技术研究了小麦条锈菌吸器母细胞的超微特征。观察发现,小麦条锈菌吸器母细胞大部位于胞间菌丝的顶部,但也可形成于两菌丝细胞间,同一菌丝细胞上可着生多个吸器母细胞,吸器母细胞的多核现象较为普遍。吸器母细胞壁由6层组成,而其隔膜可分辩为4层。细泡化学染色结果表明隔膜突中含有多糖类物质,吸器母细胞隔膜突的出现和消失与吸器的发育密切相关。本文结果表明小麦条锈菌吸器母细胞的一些超微特征明显不同于其它锈菌。