The Initiation of the Secondary Haustorium in Alectra vogelii Benth

Anatomical studies were carried out on initiation of the secondaryhaustorium in Alectra vogelii, a root parasite of leguminouscrops in Nigeria. In both the normal and self-haustorium, theformation of the haustorial initial on the parasite root soonafter initial contact between the host and parasite roots isfollowed by the penetration of the host root by the haustorium.Specialized penetrating cells (intrusive cells) at the haustorialfront prise apart and loosen the host root cortical cells, whichlater become digested. Through the same processes, a few ofthese columnar intrusive cells at the haustorial front piercethe endodermis to make contact with the xylem of the host root.Thereafter, a true conductive bridge consisting of short, isodiametric,reticulate vessel elements is established between the parasiteand host roots through the secondary haustorium. No pholem tissuewas observed in the connection. There is a close similaritybetween the mode of initiation of the secondary haustorium ofAlectra vogelii and that previously described for its primaryhaustorium. Alectra vogelii Benth, haustorium, self-haustorium, root parasite, hemiparasitism, Vigna unguiculata, Arachis hypogaea     

The haustorium of the root parasite Triphysaria (Scrophulariaceae), with special reference to xylem bridge ultrastructure

Haustoria of Triphysaria pusilla and T. versicolor subsp. faucibarbata from a natural habitat were analyzed by light and electron microscopy. Secretory trichomes (root hairs) participate in securing the haustorium to the surface of the host root. The keel-shaped intrusive part of the secondary haustorium penetrates to the depth of the vascular tissue of the host. Some of the epidermal interface cells differentiate into xylem elements. A significant number of haustoria do not differentiate further, but in most haustoria one to five of the epidermal xylem elements terminate a similar number of xylem strands. The strands mostly consist of vessel members and they connect host xylem or occasionally host parenchyma to the plate xylem adjacent to the stele of the parasite root. Each strand of this xylem bridge is accompanied by highly protoplasmic parenchyma cells with supposed transfer cell function. Increased surface area of the plasmalemma occurs in these cells as it does in interface parenchyma cells. Graniferous tracheary elements are restricted to the haustorium and occur most frequently in the plate xylem. The plate xylem is also accompanied by highly protoplasmic parenchyma cells. Hyphae of mycorrhizal fungi of the host root occasionally penetrate into the distal part of the xylem bridge. We combine structural observations and physiological facts into a hypothesis for translocation of water and nutrients between host and parasite. Some evolutionary aspects related to endogeny/exogeny of haustoria are discussed, and it is argued that the Triphysaria haustorium represents a greatly advanced and/or reduced condition within Scrophulariaceae.     

Morphology and Anatomy of the Haustorium of the Root Hemiparasite Olax phyllanthi (Olacaceae), with Special Reference to the Haustorial Interface

Gross morphology and internal structure of haustoria of Olaxphyllanthi are described in parasitism with a range of hosts,including roots of woody and herbaceous dicotyledons and certainmonocotyledons, and occasional instances of autoparasitism andhaustorial formation on monocotyledon rhizomes. Successful penetrationto xylem occurs on virtually all hosts across broad diameters,ages and anatomies of host root, but anatomical impedimentsto haustorial establishment and penetration are recorded forcertain host taxa. Each haustorium is a comparatively simpleand ephemeral structure. Its developing sucker (endophytic regionof the haustorium) spreads laterally around the surface of thehost xylem, yet never completely encircles the host stele. Damageto hosts is minimal and secondary thickening (of hosts) continueson the side of a host root opposite to a haustorium. The haustorialsucker lacks phloem and its interface with host xylem is comprisedalmost entirely (more than 98.7%) of parenchyma. The few terminatingtracheids at an interface lie in very close proximity to oroccasionally directly against exposed xylem vessels, but lumento lumen continuity between tracheary elements of the partnersis not achieved. Three dimensional reconstructions based onserial transverse sectioning indicate that well defined filesof tracheids connect back from an interface to the core of graniferoustracheary elements in the external body of the haustorium, andthence to the xylem of the parent parasite root. The findingsare discussed in relation to existing studies on haustorialanatomy. Root parasite, Olacaceae, haustorial anatomy, host specificity     

Development and Structure of the Haustorium of the Parasite Rhamphicarpa fistulosa (Scrophulariaceae)

Rhamphicarpa fistulosa (Hochst.) Benth. (Scrophulariaceae), a parasite of African cereals, develops secondary haustoria which penetrate the roots of the host plant. Light and electron microscopy have been used to study the structure and development of haustoria in this species, which, until now, have not been well characterized. Haustoria are initiated in the hypodermis of the parasite roots. A meristematic strand is developed between the parasite root stele and the host-parasite interface. From this strand, cells differentiate into xylem elements after penetration of the host root. Xylem differentiation follows an acropetal pattern. Mature haustoria are characterized by a continuous xylem bridge between water conducting elements of parasite and host. A detailed study of the hostparasite interface revealed the presence of collapsed and compressed host cells at the lateral interface (between parasite cells and host cortex), whereas the central interface between parasite cells and the host stele is almost devoid of host cell remnants. Implications of these observations for the penetration mechanisms are discussed.     

The structure and development of the haustorium in parasitic Scrophulariaceae*

The structure and development of roots and haustoria in 37 species of parasitic Scrophulariaceae was studied using light microscopy. The mature haustorium consists of two regions: the swollen “body” and the parent root, which resembles non-haustorial roots in structure. The body arises from the parent root and is composed of an epidermis, cortex, central region of xylem (the vascular core), a region of parenchyma (the central parenchymatous core), and the portion of the haustorium contained in the host tissue (the endophyte). The xylem of the vascular core is composed predominately of vessel elements. The central parenchymatous core is composed of parenchyma and col-lenchyma. Vessels extend from the vascular core through the central parenchymatous core to the endophyte. The endophyte is composed of parenchyma cells and vessel elements. No phloem is present in the body of the haustorium. Early stages in the development of the haustorium are exogenous. Initial periclinal divisions in the epidermis or outer cortex are followed by hypertrophy of cortical parenchyma. These events are followed by development of the vascular core from the pericycle, attachment of haustorium to the host by a specialized layer of cementing cells or root hairs, and penetration of the host by dissolution of host cells.     

In Vitro Haustorium Development in Roots and Root Cultures of the Hemiparasitic Plant Triphysaria Versicolor

Parasitic plants in the Orobanchaceae invade host plant roots through root organs called haustoria. Parasite roots initiate haustorium development when exposed to specific secondary metabolites that are released into the rhizosphere by host plant roots. While molecular approaches are increasingly being taken to understand the genetic mechanism underlying these events, a limitation has been the lack of a transformation system for parasitic plants. Since the haustorium development occurs in roots of Orobanchaceae, root cultures may be suitable material for transient or stable transformation experiments. To this end, root cultures were obtained from explants, and subsequently calluses, from the hemiparasitic plant Triphysaria versicolor. The cultured roots retained their competence to form haustoria when exposed to host roots, host root exudates, or purified haustorium-inducing factors. The root culture haustoria invaded host roots and initiated a vascular continuity between the parasite and host roots. The ontogeny of haustoria development on root cultures was indistinguishable from that on seedlings roots. Root cultures should provide useful material for molecular studies of haustorium development.     

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.     

Ontogenesis of the collapsed layer during haustorium development in the root hemi‐parasite Santalum album Linn

The structure and development of collapsed layers of the haustorium were studied in Santalum album Linn. Through light and transmission electron microscopy, it was shown that the collapsed layers originated from starch‐containing cells when the haustorium developed an internal gland, thickened gradually and ultimately developed into the mantle, which, combined with the sucker, buckled the host root. We report on the presence of inter‐collapsed layers for the first time. These layers develop after penetration into the host and are located between the intrusive tissues and the vascular meristematic region, gradually linking the collapsed layers and remains around the sucker. The proliferation of cells in the meristematic region and the ‘host tropism’ of cortical layers contribute to pressure within the haustorium and result in development of the collapsed layers. Besides, starch‐containing cells that turn into collapsed layers are vulnerable to pressure as they lack a large vacuole, have uneven cell wall thickness and a loose cell arrangement. We proposed that the functions of collapsed layers are to efficiently assure that cell inclusion and energy concentrate at the inner meristematic region and are recycled to affect penetration, reinforce the physical connection between the sandalwood haustorium and host root, and supply space for haustorial development.     

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.     

An anatomical study of the haustoria of Rhinanthus minor attached to roots of different hosts

Roots of a range of potential hosts responded differently when Rhinanthus minor attempted to form haustoria. Roots of Fabaceae show the weakest reaction as apart from slight lignification, no reaction was observed at the interface between the endophyte and the cortical tissue of the host root. Grass roots react with strong lignification of all cells within the stele with the exception of a small number of phloem cells whilst the endodermis fully enters the tertiary stage. In the case of Phleum bertolonii the cortical cells also become lignified. The lignification is even observed in the host root tissue in a distance of about 1 mm from the haustorium (both apically and basipetally). In the case of Leucanthemum vulgare, strong suberisation can be observed in the cell walls of the interface between endophyte (tip of the sucker) and host. Plantago lanceolata exhibits the strongest reactions against the haustorial tissues. Cells of the interface between the endophyte and the host cortex are completely destroyed, as well as a few cell layers outside the central xylem cylinder, even in some distance from the haustorium. Thus, host xylem is completely isolated from the haustorium in this case. Extraction of sap from xylem vessels is likely to be drastically impaired in such a situation.     

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.     

Epidermal derivatives as xylem elements and transfer cells: a study of the host-parasite interface in two species of Triphysaria (Scrophulariaceae)

Summary Haustoria ofTriphysaria pusilla andT. versicolor subsp.faucibarbata from a natural habitat were analysed by light and electron microscopy. The keel-shaped edge of the secondary haustorium generally splits the epidermis and cortex of the host root parallel to the root axis, and penetrates to the host vascular tissue. Anticlinally elongated epidermal cells of the haustorium constitute most of the host/parasite interface. Some of these epidermal cells are divided by oblique cell walls. Some of their oblique daughter cells as well as some undivided epidermal cells differentiate into xylem elements. Single epidermal cells occasionally intrude into the vascular tissue of the host and individual host cells can be invaded. The surface area of the plasmalemma in parasitic parenchymatous interface cells is increased by the differentiation of wall labyrinths characteristic of transfer cells and by the development of membrane-lined cytoplasmic tubules or flattened sacs which become embedded in the partly lignified interface cell-wall. Mycorrhizal fungal hyphae enter the xylem bridge in some haustoria. Implications of these observations for the function of the haustorium are discussed.     

Structure and Development of the Mature Secondary Haustorium in Alectra vogelii Benth

Anatomical investigations were carried out on the structureand development of the mature secondary haustorium in Alectravogelii growing on Arachis hypogaea or Vigna unguiculata. Followingthe formation of the young secondary haustorium, both the cambiumand pericycle of the host root directly opposite the young secondaryhaustorium are stimulated to divide and form new tissues andorgans including haustorial roots. Further proliferations ofthe host root pericycle and the haustorial cortex give riseto a large, tuberous and complex mature secondary haustoriumwithin which the tissues of the host and parasite remain inintimate contact forming a perfect graft union with a wide zoneof contact. Apart from the haustorial axial xylcm strand whichnormally connects the xylem of the parasite secondary root withthat of the host, direct xylary connections are also establishedbetween the axial xylem of the haustorium and the xylem of thehaustorial roots. The entire surface of the mature secondaryhaustorium of Alectrais covered with these haustorial rootsas was previously observed in its mature primary haustorium. Alectra vogelii Benth, secondary haustorium, haustorium, haustorial roots, root parasite, hemiparasitism, Arachis hypogaea, Vigna unguiculata     

檀香吸器维管组织的个体发育

为了解檀香吸器维管组织的发育过程,采用激光共聚焦显微镜、光学显微镜和透射电镜观察檀香吸器维管组织的个体发育。结果表明,檀香维管组织的分化分为两个时期:入侵前和入侵后。吸器维管组织发育始于盘状吸器时期,起源于吸器基部具有分生能力的细胞,后分为两束。侵入前无向顶的分化,处于吸器基部。侵入后随吸管深入寄主根与寄主根维管束连通,形成具有吸收功能的维管组织。成熟吸器维管组织呈倒烧瓶结构,仅处于吸器烧瓶核心两边,由木质部组成而无韧皮部。檀香的吸器维管组织发育有两个因素诱导,一个是遗传因素,另一个为寄主。这些为檀香半寄生性特性研究提供了形态解剖学基础。     

Ultrastructure of the haustorial interface and apoplastic continuum between host and the root hemiparasiteOlax phyllanthi (Labill.) R. Br. (Olacaceae)

Summary Structural features of haustorial interface parenchyma of the root hemiparasiteOlax phyllanthi are described. Walls contacting host xylem are thickened non-uniformly with polysaccharides, not lignin, and show only a thin protective wall layer when abutting pits in walls of host xylem vessels or tracheids. Lateral walls of interface parenchyma exhibit an expanded middle layer of open fibrillar appearance, sometimes with, but mostly lacking adjoining layers of dense wall material. Free ribosomes and rough endoplasmic reticulum are prominent and occasional wall ingrowths present. Experiments involving transpirational feeding of the apoplast tracers lanthanum nitrate or uranyl acetate to host roots cut below haustorial connections, indicate effective apoplastic transfer from host to parasite root via the haustorium. Deposits of the tracers suggest a major pathway for water flow through host xylem pits, across the thin protective wall layer, and thence into the haustorium via the electronopaque regions of the terminal and lateral walls of the contact parenchyma. Graniferous tracheary elements and walls of parenchyma cells of the body of the haustorium appear to participate in tracer flow as do walls of cortical cells, stele parenchyma and xylem conducting elements of the parasite root, suggesting that both vascular and non-vascular routes are involved in extracytoplasmic transfer of xylem sap from host to parasite. The Casparian strip of the endodermis and the suberin lamella of the exodermis of theOlax root act as barriers to flow within the system.     

蔗糖对列当科两种根部半寄生植物吸器发生的促进作用

吸器是寄生植物的特征器官,研究影响其发生的因素,有助于了解寄生关系的建立和调控过程。该研究以两种列当科(Orobanchaceae)根部半寄生植物甘肃马先蒿(Pedicularis kansuensis)和松蒿(Phtheirospermum japonicum)为材料,通过皿内培养试验,分析了蔗糖、DMBQ(2,6-二甲氧基-对-苯醌,一种高效的列当科根部半寄生植物吸器诱导化合物)和寄主植物诱导下两种根部半寄生植物吸器发生情况。结果表明：(1)蔗糖显著促进两种根部半寄生植物吸器发生,无寄主存在时,2%蔗糖处理使甘肃马先蒿和松蒿吸器发生率分别提高39.9%和20.2%。(2)蔗糖明显提升寄主植物对两种根部半寄生植物的吸器诱导水平,添加蔗糖后,寄主诱导的甘肃马先蒿单株吸器数和具木质桥的吸器比例分别增加5.7个/株和17.9%,松蒿吸器发生率和具木质桥的吸器比例分别提升76.7%和16.2%。(3)蔗糖对松蒿吸器发生的促进作用与已知吸器诱导化合物DMBQ相当,均能诱导50%以上的植株产生吸器。(4)培养基中添加4%蔗糖对两种根部半寄生植物的吸器诱导效果最好,其中甘肃马先蒿吸器发生率为56%...     

Light and Electron Microscopy of the Compatible Interaction Between Arabidopsis and the Downy Mildew Pathogen Peronospora parasitica

In this study, we focused on compatible interactions between Peronospora parasitica isolate Emoy‐2 and wild‐type (Oy‐0) and mutant (Ws‐eds1) Arabidopsis thaliana accessions by using light and transmission electron microscopy (TEM). Light microscopy of compatible interactions revealed that conidia germinated and penetrated through the anticlinal cell walls of two epidermal cells. Rapid spreading of the hyphal growth with formation of numerous haustoria within the mesophyll cells was subsequently followed by profuse sporulation in the absence of host cell necrosis on both wild‐type and mutant accessions. TEM observations revealed that coenocytic intercellular hyphae ramified and spread intercellularly throughout the host tissue forming several haustoria in host mesophyll cells. Intracellular haustoria were lobed with the diameter of 6–7 μm. Each haustorium was connected to intercellular hyphae in the absence of apparent haustorial neck. The cytoplasm of the haustorium included the organelles characteristic of the pathogen. Callose‐like deposits were frequently observed at sites of penetration around the proximal region of the haustorial neck. Apart from a few callose ensheatments, no obvious response was observed in host cells following formation of haustoria. Most of mesophyll cells contained normal haustoria and the host cytoplasm displayed a high degree of structural integrity. Absence of host cell wall alteration and cell death in penetrated host cell of both accessions suggest that the pathogen exerts considerable control over basic cellular processes and in this respect, response to this biotroph oomycete differs considerably from responses to other pathogens such as necrotrophs.     

Interface between haustoria of parasitic members of the Scrophulariaceae and their hosts: A histochemical and immunocytochemical approach

Summary The haustorial structure of three African parasitic members of the family Scrophulariaceae (Buchnera hispida, Rhamphicarpa fistulosa, andStriga hermonthica) has been studied with regard to the interface between haustoria and the invaded host roots. Immunocytochemical observations at the light and electron microscopical level were carried out with monoclonal antibodies against pectin. JIM5, JIM7, and hydroxyproline-rich glycoprotein (HRGP), LM1. Lignins have been visualized by phloroglucinolhydrochloric acid staining. At the margin of the lateral interface (contact area of host root cortex and parasite cells), JIM5- and JIM7-labelled substances accumulate between parasite papillae and the host root surface indicating that pectins are implicated in sealing the parasite to the attacked host organ. The lateral interface is characterized by the presence of compressed, necrotic host cells, whereas the central interface (contact area between host stele and parasite cells) is generally devoid of host cell remnants. Phenolic substances and/or lignins can be found at the site of penetration of the haustorium into the host root. These observations and the fact that HRGPs accumulate at the host side of the interface support the view of, at least, a partial defense reaction in the invaded host root tissues. Within haustoria, HRGPs were restricted to differentiating xylem elements, implying a spatio-temporal regulation of HRGPs in developmental processes.Abbreviations BSA bovine serum albumin - FITC fluorescein isothiocyanate - HRGP hydroxyproline-rich glycoprotein - LM light microscopy - MAb monoclonal antibody - TBSB Tris-buffered saline with bovine serum albumin - TBSB-T Tris-buffered saline with bovine serum albumin and Tween 20 - TEM transmission electron microscopy     

Haustorial Structure and Functioning of the Root Hemiparastic Tree Nuytsia floribunda(Labill.) R.Br. and Water Relationships with its Hosts

Observations on the origin and mature structure of the haustoriumof the Western Australian Christmas tree (Nuytsia floribunda)corroborate and extend the findings of earlier workers. We showthat the previously described sclerenchymatous ‘horn’or ‘prong’ formed within the haustorium acts asa sickle-like cutting device which transversely severs the hostroot and then becomes lodged in haustorial collar tissue directlyopposite to that where it originated. The cutting process isdeduced to be rapid and the gland-like fluid filled structurein the haustorium is suggested to generate a hydrostatic forcedriving the device through the host root. The haustorial parenchymacells at the tight junction between the endophytic part of thehaustorium and the cut face of the host root develop balloon-likeoutgrowths which intrude into the lumina of severed xylem vesselsof the host. Experiments feeding 0.05% (w/v) basic fuchsin tofreshly cut ends of host root segments distal to terminally-attachedmature haustoria demonstrate an apoplastic pathway from hostxylem elements fractured at the interface into haustorial parenchyma,and thence through vascular tissue to the haustorium into thetranspiring plant of Nuytsia. Application of labelled water(D₂O) to uncut basal roots of potted plants ofAcacia acuminataparasitized by Nuytsia results in labelling of leafy shootsof parasite and host, indicative of haustorial uptake of waterby Nuytsia from host root xylem in the intact association. Measurementsof xylem water potentials of pot-cultured seedling Nuytsia associatedwith a range of hosts, or of mature trees of Nuytsia and partnerwoody hosts in the native habitat, demonstrate consistentlymore negative potentials in the parasite than host, suggestingthat the parasite may regularly obtain xylem water through itshaustorial apparatus. Copyright 2000 Annals of Botany Company Root hemiparasite, Nuytsia floribunda, Loranthaceae, haustorial structure, host–parasite water relations