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.     

EXPERIMENTAL INITIATION OF HAUSTORIA IN AGALINIS PURPUREA (SCROPHULARIACEAE)

The initiation and early developmental stages of the haustorium were studied in Agalinis purpurea (Scrophulariaceae). Plants were grown in a 0.9% agar inorganic medium with a 0.5% sucrose supplement. Root exudate collected from Lespedeza sericea induced the initiation of haustoria, with earliest stages evident in 6-12 hr. A 30-min exposure to exudate produced an increased frequency of haustoria and a 24-hr exposure yielded haustorial frequencies equal to the number that were initiated on control plants continuously exposed to root exudate for the 5-day growth period. The early cytological features of haustorial development are described and the possible significance of haustorial initiation in host recognition is discussed.     

Signaling Organogenesis in Parasitic Angiosperms: Xenognosin Generation, Perception, and Response

Abstract Parasitic strategies within the angiosperms generally succeed by tightly coupling developmental transitions with host recognition signals in a process referred to as xenognosis. Within the Scrophulariaceae, Striga asiatica is among the most studied and best understood parasitic member with respect to the processes of host recognition. Specific xenognosins regulate seed germination, the development of the host attachment organ, the haustorium, and several later stages of host-parasite integration. Here we discuss the signals regulating the development of the haustorium, the critical vegetative/parasitic transition in the life cycle of this obligate parasite. We provide evidence for the localized production of H₂O₂ at the Striga root tip and suggest how this oxidant is used to exploit host peroxidases and cell wall pectins to generate a simple benzoquinone signal. This benzoquinone xenognosin proves to be both necessary and sufficient for haustorial induction in cultured seedlings. Furthermore, evidence is provided that benzoquinone binding to a redox active site completes a ``redox circuit' to mediate signal perception. This redox reaction regulates the time-dependent expression of specific marker genes critical for the development of the mature host attachment organ. These studies extend the emerging series of events necessary for the molecular regulation of organogenesis within the parasitic plants and suggest novel signaling features and molecular mechanisms that may be common across higher plants.     

Analysis of In Vivo Protein Synthesis and Histological Studies of Haustorial Formation in Root Cultures of Witch weed (Striga asiatica L Kuntze)

We recently described an in vitro approach that uses root culturesto study haustorial formation in Striga asiatica. Previous studieshave used haustoria formed on intact radicles of Striga seedlings.In vitro cultured roots formed haustoria that appeared morphologicallysimilar to those formed by Striga radicles, but were 5–10-foldlarger. In this study, we provide biochemical and histologicalevidence to support further the similarity of root culture haustoriato haustoria formed on radicles of seedlings. We examined invivo protein synthesis during haustorial development on rootcultures and radicles by 2-D PAGE. Four proteins increased inabundance in both root cultures and radicles after 6 h of haustorialinduction. All four proteins appeared transiently in root culturesand radicles, being more abundant at 6 h, and less abundantafter 24 h of haustorial induction. Only three of the four haustorial-specificproteins were more abundant in root cultures after 2 h of haustorialinduction; all four had decreased in abundance after 12 h ofhaustorial induction. Using light microscopic analysis we comparedthe ontogeny of root culture haustoria to that of haustoriaon radicles. These studies revealed that root culture haustoriaundergo developmental changes similar to those reported forradicle haustoria such as early expansion of cortical cells,the emergence of haustorial hairs from epidermal cells, andthe development of densely staining cells at the haustorialapex. In addition, these changes occurred within a similar time-frameand sequence in root culture and radicle haustoria. Finally,root culture haustoria were found to be capable of attachingto sorghum host roots. Key words: Striga asiatica L., Kuntze, haustoria, root cultures, proteins, histology, 2D-PAGE     

Vegetative/Parasitic transition: control and plasticity in striga development

Striga asiatica (Scrophulariaceae), an obligate parasite of grasses including many of the world's major grain crops, switches from vegetative to parasitic development by the differentiation of the root meristem into the host attachment organ, the haustorium. This change was induced in culture by the exposure to a single, low molecular weight signal molecule, 2,6-dimethoxy-p-benzo-quinone. A concentration of 10⁻⁶ molar quinone and an exposure time of ≥6 hours were required before the developmental process could be completed. With shorter exposure times, haustorial development was prematurely aborted and meristematic elongation was reestablished. The new meristem was capable of developing a second haustorium if reexposed to the signal molecule. These results are discussed in terms of the transition to the parasitic phase and the general control of plant cellular development.     

An haustoria inducer for the root parasite Agalinis purpurea

Host recognition by the angiosperm root parasite Agalinis purpurea occurs through the development of haustoria, organs specialized for the attachment and penetration of host roots. Axenic cultures of Agalinis do not develop haustoria, but low molecular weight compounds produced by host plants induce haustoria development. Structure-activity studies have shown that highest levels of haustoria induction are afforded by flavonoids bearing specifically substituted methoxyphenol functionality. This paper examines Lespedeza sericea to determine the extent of this structure-activity relationship among hosts of Agalinis. A pentacyclic triterpene has been isolated from Lespedeza roots as a haustoria inducer, and extensive NMR and mass spectral experiments led to its characterization as soyasapogenol B (3β,22β,24-trihydroxy-olean-12-ene). The activity of soyasapogenol B is lower than the phenolic inducers, and modification of hydroxyl substitutents on soyasapogenol B abolishes activity. This haustoria inducer appears to be preferentially secreted from the roots of Lespedeza, but not in quantities sufficient to account for all haustoria inducing activity exhibited by the crude root exudate.     

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.     

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     

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     

A time-course study of early establishment stages of parasitic angiosperm Striga asiatica on susceptible sorghum roots

A time-course study of the early establishment stages of Striga asiatica was carried out on a susceptible sorghum hybrid, CSH 1, using polyethylene bags and whole-root clearing and staining techniques. Preconditioned Striga seeds were applied to different aged segments of primary root but the results did not differ for these different aged segments. Most of the Striga seeds (63%) germinated within 24 h of inoculation on the host roots. The attachment of Striga radicles to host root was rapid and it occurred between 36 and 48 h after inoculation. Only 9% of the germinated Striga seeds attached to the host root but 65% of these attachments successfully penetrated through the epidermis and entered the host cortex within 72 h. Penetration through the cortical cells was difficult; only 17% of attachments were able to reach the endodermis. Penetration took from 12 to 43 h after the first appearance of haustorial cells in the cortex; a total of 84 to 120 h after inoculation on the host root. Penetration through the endodermis and establishment on the host stele was relatively easier, as most of the haustoria reaching the endodermis were able to establish on the host stele. But this is a slow process taking a minimum of 24 h, and a maximum of 60 h after first contact of haustorial cells with the endodermis. The minimum time taken from inoculation of ungerminated Striga seed on the host root to establishment is about 108 h. The results are discussed in relation to published reports on other parasitic species such as Agalinis purpurea.     

Progress in parasitic plant biology: host selection and nutrient transfer

Host range varies widely among species of parasitic plants. Parasitic plants realize host selection through induction by chemical molecular signals, including germination stimulants and haustoria-inducing factors (HIFs). Research on parasitic plant biology has provided information on germination, haustorium induction, invasion, and haustorial structures and functions. To date, some molecular mechanisms have been suggested to explain how germination stimulants work, involving a chemical change caused by addition of a nucleophilic protein receptor, and direct or indirect stimulation of ethylene generation. Haustorium initiation is induced by HIFs that are generated by HIF-releasing enzymes from the parasite or triggered by redox cycling between electrochemical states of the inducers. Haustorium attachment is non-specific, however, the attachment to a host is facilitated by mucilaginous substances produced by haustorial hairs. Following the attachment, the intrusive cells of parasites penetrate host cells or push their way through the host epidermis and cortex between host cells, and some types of cell wall-degrading enzymes may assist in the penetration process. After the establishment of host-parasite associations, parasitic plants develop special morphological structures (haustoria) and physiological characteristics, such as high transpiration rates, high leaf conductance, and low water potentials in hemiparasites, for nutrient transfer and resource acquisition from their hosts. Therefore, they negatively affect the growth and development and even cause death of their hosts.     

High rates of nucleotide substitution in nuclear small-subunit (18S) rDNA from holoparasitic flowering plants

Relative rate tests, using Gnetum as a reference taxon, were conducted on nuclear 18S rRNA sequences from 10 angiosperms including autotrophic nonparasites (Arabidopsis, Asarum, Glycine, Malpighia, and Zea), a chlorophyllous hemiparasite (Arceuthobium—Viscaceae), and achlorophyllous holoparasites (Balanophora—Balanophoraceae, Prosopanche—Hydnoraceae, and Rafflesia and Rhizanthes—Rafflesiaceae). Compared with Glycine, the mean number of substitutions per site (K) for five autotrophic angiosperms is 0.036 whereas for the holoparasites K = 0.126, i.e., 3.5 times higher. Comparisons of autotrophic species with short and long generation times showed no differences in K; hence, divergent rRNA sequences in the holoparasites are likely attributable to other mechanisms. These might include genetic bottlenecks, effective population size, and/or molecular drive. High substitution rates appear to be associated only with those parasitic angiosperms that have developed a highly modified haustorial system and extreme nutritional dependence upon the host. At present, high substitution rates in these parasites confound attempts to determine their phylogenetic position relative to other angiosperms. Correspondence to: D.L. Nickrent     

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.     

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.     

The Regulation of the Water Potential Gradient in the Host and Parasite Relationship betweenSorghum bicolorandStriga hermonthica

A glasshouse experiment was carried out to investigate the factorscontrolling the abstraction of xylem fluid from its host bythe parasiteStriga hermonthica(Scrophulariaceae).Strigahad amean daily transpiration rate far exceeding that of its hostsorghum (Sorghum bicolor), with infestation byStrigaalso shownto lower the transpiration rate of the host. Stopping the host'stranspiration was shown to decrease the transpiration rate ofthe parasite. Stopping the parasite's transpiration only gavean initial increase in the host's transpiration rate which wasnot sustained. The parasite had a lower water potential thanits host, values being -0.42 MPa and -0.23 MPa, respectively,and an accompanying higher osmotic pressure of 0.68 MPa against0.51 MPa for sorghum. Modifying the water potential gradientby bagging both partners together showed that the differentialin osmotic pressure and water potential was largely maintainedby the parasite's higher rate of transpiration. A favourablewater potential gradient towards the parasite still existedfollowing the cessation of transpiration, this being generatedby the haustorial resistance to hydraulic conductivity whichwas found to be some 1.5–4.5 times greater than that offeredby the parasite shoot. Both the high rate of transpiration andthe increased resistance across the haustoria would appear tobe necessary means to facilitate the diversion of host resourcesto the parasite.Copyright 1997 Annals of Botany Company Striga hermonthica; sorghum; water relations; haustorium; root parasite     

Ultrastructure of Angiosperm Haustoria--A Review

It has been clear for many years that the evolution of parasitismin the angiosperms has taken place independently in a numberof unrelated groups (Kuijt, 1969). In other words, the parasiticorgans (haustoria) which both structurally and physiologicallyjoin host and parasite are remarkable instances of convergentevolution in these different groups. The main thread which canbe followed throughout the many anatomical studies which inthe past have been carried out on haustoria is the search forcommon denominators between various groups; that is, as to howmuch the haustoria of unrelated taxa have come to resemble eachother. When Solms-Laubach (1867–1868) more than a century agoreviewed the structure of angiosperm haustoria it required wellover one hundred pages to do so. Needless to say the bulk ofour present knowledge has been obtained since that time, andlight microscopy continues to unearth significant new information.In a recent review of haustorial anatomy (Kuijt, 1969), severalstructural parallelisms are noted. The advent of electron microscopyhas, since the latter review, resulted in a great deal of additionalinformation which has enabled certain comparisons between groupsto be made. Ultrastructural studies have quite naturally concentratedon the most readily available which, of course, are also thosewhich have been most i ntensively studied with the light microscopein the past. The haustoria of several families, such as Rafflesiaceae,Balanophoraceae, Hydnoraceae, and Lennoaceae, have scarcelybeen studied even with the light microscope. The purpose of the present paper is to give a general reviewof present knowledge of the ultrastructure of haustoria. Todo so we must of necessity ignore most recent purely anatomicalwork, except where relevant to the topics under discussion.Ultrastructural observations have been made on mistletoes (Arceuthobiumand Phthirusa, Loranthaceae), Santalaceae (several genera),Scrophulariaceae(Castilleja), Orobanchaceae (Orobanche) andConvolvulaccae (Cuscuta), in which order the discussion willproceed.     

Cell Wall Degrading Enzymes in Cuscuta reflexa and Its Hosts

Pectin degrading enzymes, hemicellulose degrading enzyme andcellulose degrading enzymes were studied in Cuscuta reflexaRoxb., its susceptible hosts, Brassica campestris L., Cocciniaindica W. & A. Datura innoxia Mill, Helianthus annuus L.,Holoptelea indica Planch, Lantana camara L., Medicago sativaL., Manihot utilissima Pohl, Petunia hybrida X Hort exvilm,Pisum sativum L., Phaseolus vulgaris L. and Solanum nigrum L.and non-susceptible plants Ipomoea batata Lam. and Solanum tuberosumL. Pectin esterase and polygalacturonase were present in higheramounts in Cuscuta parasitic on P. vulgaris and S. nigrum, whichneeded more time for haustorial establishment. Exo-l, 4-ß-D-glucosidaseactivity was found in Cuscuta but could not be detected in itshosts. Xylanase and cellulase activity of host plants increasedwhile cellobiase activity decreased as a result of infectionby the parasite. Higher pectin esterase, polygalacturonase,xylanase and exo-l, 4-ß-D-glucosidase activities inthe haustorial region of the parasite is likely to bring aboutthe lysis of the cell wall of the host plant and thus facilitatethe penetration of the parasite haustoria into the host sieveelement, which is necessary for the transport of nutrients betweenthe host and the parasite. Key words: Cell wall degrading enzymes, Cuscuta reflexa     

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     

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.