Haustorial Development Research Articles

Histological and ultrastructural comparisons of compatible, incompatible and [formula omitted] -β-amino- n -butyric acid-induced resistance responses of pepper stems to Phytophthora capsici

The compatible interaction of pepper stems with Phytophthora capsici showed more rapid and severe disease development than did the incompatible interaction, although pathogen penetration styles of host cells in compatible and incompatible interactions were similar to each other. Treatment with DL -β-amino- n -butyric acid (BABA) protected the pepper plants against P. capsici infection. Reduced hyphal growth and sporangial formation were found after P. capsici infection in BABA-induced resistant and incompatible reactions. One of the most noticeable ultrastructural features of the BABA-induced resistant reaction was the formation of electron-dense wall appositions. The thick and dense wall appositions that encased the haustoria restricted haustorial development, thus leading to limitation of further pathogen penetration into inner plant tissues. A main host response in the incompatible interaction was the occlusion of cortical cells with an amorphous material. Plugging of the intercellular spaces in the cortical cells with electron opaque material was frequently observed in the incompatible interaction, but not in the compatible interaction. Another common feature of the BABA-induced resistant and incompatible reactions was degeneration of mitochondrial structure within penetrating hyphal cytoplasm. The mitochondrial structure in the BABA-induced resistant or incompatible reactions had no distinct double membrane layer and well-shaped cristae.

Haustorial Development and Growth Benefit to Seedlings of the Root Hemiparasitic Tree Nuytsia floribunda(Labill.) R.Br. in Association with Various Hosts

Dry matter gains and haustorial production of pot-cultured seedlings of Nuytsia floribunda were assessed after a 12 month period of association singly with each of a range of potential woody host species. One species,Adenanthos cygnorum , of similar size to most parasitized hosts, served as measure of response of Nuytsia in a non-benefiting situation. Rated on this basis, all 23 parasitized hosts elicited greater mean dry weights of Nuytsia than when on Adenanthos, and seven of these instances were highly significant. Numbers and weights of penetrating and presumably functional haustoria formed on a host were broadly correlated with growth benefit to Nuytsia, but there were notable instances of unusually poor or great benefit from a host relative to the complement of haustoria involved. Experiments in which haustoria-bearing associations of Nuytsia partnered with nodulated Acacia hosts (Acacia acuminata and A. cyclops) were fed15N2showed significant transfer of15N to the parasite, but failed to determine whether the label had been acquired through haustoria or directly by Nuytsia roots following turnover of nodule and root residues of the host in the rooting medium. A parallel study using the unusual non-protein amino acid, djenkolic acid, as a marker of benefit from the djenkolic acid-containing host A. cyclops, showed appearance and progressive build-up of the compound in foliage of Nuytsia over a 6 month period after partnering the species in pot culture. Presence of the compound at final harvest in xylem sap of both partners but not in soil solution of the cultures strongly indicated xylem transfer via haustoria as the principal avenue for N benefit to the parasite. Results are discussed in relation to a recent evaluation of haustorial structure and functioning of N. floribunda.

Exploiting chemical ecology and species diversity: stem borer and striga control for maize and sorghum in Africa

Stem borers, comprising the larvae of a group of lepidopterous insects, and parasitic witchweeds, particularly Striga hermonthica and S asiatica, cause major yield losses in subsistence cereal production throughout sub-Saharan Africa. Studies are described that have led to the development of a ‘push-pull’ strategy for minimising stem borer damage to maize and sorghum. This involved the selection of plant species that could be employed as trap crops to attract colonisation away from the cereal plants, or as intercrops to repel the pests. The two most successful trap crop plants were Napier grass, Pennisetum purpureum, and Sudan grass, Sorghum sudanensis. The intercrop giving maximum repellent effect was molasses grass, Melinis minutiflora, but two legume species, silverleaf, Desmodium uncinatum, and greenleaf, D intortum, gave good results and had the added advantage of suppressing development of S hermonthica. In terms of stem borer control, the plant chemistry responsible involves release of attractant semiochemicals from the trap plants and repellent semiochemicals from the intercrops. With M minutiflora, parasitism of stem borers was also increased by certain chemicals repellent to ovipositing adults. The mechanism of striga control has not been fully elucidated, but allelopathic effects from the Desmodium species have been shown to involve stimulation of germination and interference with haustorial development. Significant beneficial effects have been obtained with the individual components of these push-pull strategies. However, the most robust crop-protection package is obtained when these components are combined. The trap crop and intercrop plants also provide valuable forage for cattle, often reared in association with subsistence cereal production. There has been considerable take-up of the system within the communities where farmer-managed trials have been carried out, particularly in the Trans Nzoia and Suba districts of Kenya, and the programme is set to expand throughout and beyond Kenya. © 2000 Society of Chemical Industry

Studien an Cuscuta reflexa Roxb.: VIII. Mechanische Haustorieninduktion an nichtwindenden Achsen des Parasiten

Summary The induction of haustoria formation in Cuscuta reflexa proved to be independent of the presence of a suitable potential host. The induction, caused by mechanical pressure, was also possible in non-twining stems of the isolated parasite without applying any exogenous hormones. Over a period of time it was possible to induce haustoria mechanically repeatedly on the isolated Cuscuta stems. Haustoria formation was restricted to a subapical region of the parasite’s stem, which is the area where the most intensive elongation of the Cuscuta stem takes place. During haustorial development, the growth rate of the parasite is retarded. An interaction of IAA and cytokinine, possibly involved in the regulation of these processes, is discussed.

Primary Haustorial Development of Striga asiatica on Host and Nonhost Species

ABSTRACT Initial interactions of Striga asiatica with a susceptible host and non-host plants were examined by histological methods. Haustorial development was initiated when radicles of S. asiatica were placed in contact with host or nonhost roots. Reorganization of the S. asiatica root apical meristem was rapid and involved the formation of a distal group of cells that penetrated the host or nonhost root. Penetration of the epidermis of the host (sorghum) roots and advance into the cortex occurred within 24 to 48 h of inoculation. Penetration of the endodermis by the developing endophyte was delayed for 72 to 96 h after initial contact. However, upon penetration vascular continuity was established between parasite and host. In contrast, interactions with nonhosts provided evidence of active resistance mechanisms. Penetration of lettuce, marigold, and cowpea roots by S. asiatica was most frequently arrested in the cortex, and endophytic cells were necrotic 72 h after inoculation. Some species-specific differences were observed in the reactions of nonhosts to penetration, although in their general nature the interactions with S. asiatica were similar.

Actin genes with unusual organization in the parasitic angiosperm Striga asiatica L. (Kuntze)

In order to better understand the regulation of cellular differentiation during haustorial development in parasitic angiosperms, we have begun to examine the structure and expression characteristics of genes encoding various components of the plant cytoskeleton in Striga asiatica L. (Kuntze). We describe here the cloning and characterization of three actin genes from Striga with significant similarity at the nucleotide level and encoding proteins having greater than 98% identity. However, the three genes (designated SAAc-1, SAAc-2 and SAAc-3) differ from each other in their organization and SAAc-2 contains an unusual exon-intron arrangement relative to genes encoding actins described in other higher plants. The significance of these observations concerning the evolutionary origins and potential roles of Striga actin genes is discussed.

A mechanism for inducing plant development: the genesis of a specific inhibitor.

Parasitic strategies are widely distributed in the plant kingdom and frequently involve coupling parasite organogenesis with cues from the host. In Striga asiatica, for example, the cues that initiate the development of the host attachment organ, the haustorium, originate in the host and trigger the transition from vegetative to parasitic mode in the root meristem. This system therefore offers a unique opportunity to study the signals and mechanisms that control plant cell morphogenesis. Here we establish that the biological activity of structural analogs of the natural inducer displays a marked dependence on redox potential and suggest the existence of a semiquinone intermediate. Building on chemistry that exploits the energetics of such an intermediate, cyclopropyl-p-benzoquinone (CPBQ) is shown to be a specific inhibitor of haustorial development. These data are consistent with a model where haustorial development is initiated by the completion of a redox circuit.

Changes in patterns of protein synthesis during haustorial development ofStriga hermonthica(Del.) Benth. seedlings

This study focuses upon the developmental transition of the parasitic plant Striga hermonthica from its freeliving state (germinated seedling) to its parasitic state after development of an infection organ: the haustorium. A new method has been developed that allows the production of gram quantities of germinated and haustorially-induced Striga seedlings, thereby facilitating biochemical and molecular analysis of haustorial induction. Water-soluble proteins have been extracted from germinated seeds (stage A) and seedlings treated with 2,6-dimethoxy-p-benzoquinone (2,6-DMBQ) to induce haustorium (stage B). Samples were analysed by two-dimension al polyacrylamid e gel electrophoresis and quantitative as well as qualitative differences could be observed. In particular a group of four highly abundant acidic proteins (molecular weight 39 kDa, pi 5.1, 5.3, 5.3, 5.6) and three other proteins (molecular weight 12 kDa, pi 6.9; 17 kDa, pi 4.4; 17 kDa, pi 4.45) were seen in stage A while at least four proteins (molecular weight 21.5 kDa, pi 6.4; 21.5 kDa, pi 6.3; 31 kDa, pi 5.1; 34 kDa, pi 6.2) were present in greater abundance in stage B. In order to compare watersoluble protein with newly synthesized protein patterns, mRNAs from the two stages of development were isolated and cell-free translation products analysed by 2-D PAGE. Two-D gels of cell-free translation products showed the appearance of six proteins in stage B (molecular weight ranging from 10 to 35 kDa) and the presence of three acidic proteins in stage A with one protein (molecular weight 40 kDa) very similar in size to the triplet of proteins in the water-soluble protein 2-D gels.

Resistance to Haustorial Development of Two Mistletoes, Amyema preissii (Miq.) Tieghem and Lysiana exocarpi (Behr.) Tieghem ssp. Exocarpi (Loranthaceae), on Host and Nonhost Species

Seeds of two mistletoe species, Amyema preissii and Lysiana exocarpi, were inoculated onto host and nonhost species, and haustorial development was examined. Resistance to haustorial penetration was observed in the bark and xylem. Bark resistance took one or all of three forms: (1) mechanical resistance; (2) production of wound periderm; and (3) abnormal changes in host tissues surrounding the mistletoe haustorium. Xylem resistance included physical constraints on haustorial growth and abnormal changes in host tissues surrounding the haustorium. Bark resistance to A. preissii occurred on all nonhost species, and mistletoe haustoria failed to penetrate the bark of all nonhost species except Geijera linearifolia. In the latter case, A. preissii haustoria reached host xylem, but further development was hindered by xylem resistance. No resistance was recorded on Acacia nyssophylla, the host species of A. preissii. Bark resistance to L. exocarpi was observed in haustoria growing on the two major host species, Heterodendrum oleifolium and Myoporum platycarpum, and haustorial development on these two species was delayed for up to 3 mo compared with other host species. While no bark resistance was observed in haustoria growing on the three nonhost species, Exocarpos aphyllus, Eucalyptus oleosa, and G. linearifolia, all three species showed resistance by the xylem to haustorial development. No xylem resistance, however, was recorded on the three host species, A. nyssophylla, H. oleifolium, and M. platycarpum.

Glycoproteins recognised by monoclonal antibodies UB7, UB8 and UB10 are expressed early in the development of pea powdery mildew haustoria

A new monoclonal antibody (MAb) raised against haustorial complexes isolated from pea leaves infected with the powdery mildew fungus Erysiphe pisi is described. The antibody (designated UB10) recognises a 45 kDa N-linked glycoprotein which is specifically located in the haustorial plasma membrane. This glycoprotein is clearly distinct from a previously characterized 62 kDa glycoprotein (identified with MAb UB8) which is also specifically located in the haustorial plasma membrane. These antibodies were used, along with MAb UB7 which binds to a major 62 kDa glycoprotein in the cell wall and plasma membrane of both haustoria and surface hyphae, to label haustoria within epidermal strips from infected pea leaves using indirect immunofluorescence. Different stages of haustorial development can be observed using this procedure and the results showed that all three glycoproteins recognised by the MAbs are expressed early haustorial development.

Molecular differentiation in the extrahaustorial membrane of pea powdery mildew haustoria at early and late stages of development

Two new monoclonal antibodies (MAbs) are described which recognise glycoprotein components of the extrahaustorial membrane ( ehm) which surrounds the haustorium formed during the infection of pea leaves by the powdery mildew fungus Erysiphe pisi. One of the MAbs, UB9, raised against enriched pea leaf plasma membranes, recognises a large (200 kDa) glycoprotein in this membrane within infected and uninfected epidermal cells. It does not label haustoria at early stages of development showing that at least one plasma membrane glycoprotein is excluded from the ehm. However, UB9 binds to the ehm of a subpopulation (approx 20%) of HCs late in the infection process and it is possible that the insertion of this glycoprotein into the ehm may represent a breakdown of biotrophy during senescence of haustoria or could be a late defence response by the plant. A second MAb, UB11, was raised against isolated haustorial complexes derived from the pea powdery mildew system. This antibody recognises a glycoprotein (250 kDa) which is present in the ehm of the haustorial complex and is expressed very early in haustorial development. UB11 does not bind to plant membranes of infected or uninfected plants and does not cross-react with other fungal components. This antibody therefore identifies a glycoprotein specifically located to the ehm of haustorial complexes and shows that there is molecular differentiation of this key interface between the biotrophic partners.

In vitro and transplantation experiments with germination of date embryos

In vitro experiments with date palm embryos were designed to determine (i) which carbon sources support growth and (ii) what culture conditions promote haustorial growth and development. Date palm embryos were also transplanted into Washington filifera seeds to deterine whether any developmental characteristics were determined by the seed or endosperm. Date embryos did not grow on media containing starch or galactomannan any better than they do on medium lacking a carbon source. Good growth occurred on medium containing mannose, glucose, or sucrose as the carbon source. Embryos imbibed in the seed for 6 days grew better and produced larger haustoria than those imbibed 2 days, imbibed in water, imbibed in 3% mannose, or unimbibed. Date embryos imbibed in Washington seeds grew as well as those imbibed in date seeds. Optimum seedling and haustorium weights occurred at pH 7. The transembryonic seedlings germinated and grew normally but the shape of the haustorium was altered. Haustoria produced by in vitro grown seedlings exhibited many normal anatomical features, but haustoria from transembryonic seedlings exhibited more normal histological features. Results support the hypotheses that (i) date embryos do not possess the enzymes necessary to digest mannans and (ii) haustorium development is controlled by at least two steps, an initial inductive step that occurs by 6 days of imbibition and a further step that occurs later. We hypothesize that diffusable substances from the endosperm are responsible for controlling haustorium initiation and development. Key words: germination, embryo culture, palms, palm tissue culture, Arecaceae, haustorium.

In vitro root culture: a novel approach to study the obligate parasite Striga asiatica (L.) Kuntze

Abstract We report the development of a novel approach to study haustorial formation in Striga asiatica based upon in vitro root culture. Previous studies have used intact radicles of Striga seedlings. The potential advantages of our root culture system over the use of radicles are that more plant material is available for biochemical and molecular genetic analysis and possible confounding effects of other contaminating organisms are eliminated. Striga roots cultured in a Gamborg B5 salts medium containing indole-3-acetic acid (22.8 μM) increased in size and proliferated lateral roots. We demonstrate that cultured lateral roots are capable of haustorial formation within 24 h of treatment with a haustorial initiation factor 2,6-dimethoxy-p-benzoquinone (2,6-DMBQ). Cultured root tips developed haustoria when in contact with a solid medium that contained 2,6-DMBQ. The solidity of the medium was observed to influence the extent of haustorial development. Haustoria formed on cultured roots appeared morphologically similar to those formed by Striga radicles within a similar time period and were 5–10 fold larger. We also show that root cultures can be used as an explant source for regeneration studies. Callus, derived from root culture explants, was induced to form either shoots or roots. The optimal concentrations of the growth regulators N6-(2-isopentenyl)adenine and indole-3-acetic acid for shoot formation were 49 μM and 2.85 μM, respectively, whereas the best root regeneration medium contained 5.4 μM 1-naphthaleneacetic acid and 8.8 μM 6-benzylaminopurine.

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.

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.

Failure of haustorial development as a factor in slow growth and development of Puccinia hordei in partially resistant barley seedlings

Observations on six barley lines differing for partial resistance, at 44 h and 6·5 days after inoculation with Puccinia hordei, indicated that a proportion of colonies fail to form haustoria, the proportion failing being related to the level of partial resistance. Following the formation of the first hauslorium, the rate of formation of further haustoria was reduced for some time in lines with high levels of partial resistance. Failed haustorium formation was quantified by determining the number of induced papillae. When colonies reach a certain size, the host-colony interaction appears to change abruptly towards high compatibility, since the hyphae formed from then on induce fewer papillae than before. Indications were found for differences in resistance levels between epidermal and mesophyll cells. Differences in size and phase of development of colonies at 6·5 days after inoculation both within and between barley lines were connected with differences in the degree of formation of wall appositions (papillae formation) around the point of entry into the leaf. The results are likely to have importance for understanding the mechanisms that determine basic compatibility.

The haustorium and the chemistry of host recognition in parasitic angiosperms

Two parasitic angiosperms,Agalinis purpurea (Scrophulariaceae) andStriga asiatica (Scrophulariaceae), are compared as to the chemical recognition events involved in host selection.Agalinis is a hemiparasite which can mature to seed-set without a host, whereasStriga is a holoparasite and survives for only a very limited time without a host. Both parasites, however, attach to a host through a specialized organ known as the haustorium and regulate the development of this organ through the recognition of chemical factors from host plants. We now describe the discovery of 2,6-dimethoxy-p-benzoquinone (2,6-DMBQ) as an haustoria-inducing principle fromSorghum root extracts. Our investigation of this compound has led us to suggest that one level of host recognition in these parasitic plants is mediated through their enzymatic digestion of the host root surface. Degradation of surface components liberates quinonoid compounds, such as 2,6-DMBQ, which in turn trigger haustorial development.

Haustorial development test to characterize metalaxyl resistance and genetic variability in Plasmopara viticola1

A new method is described for assessing haustorium formation in Plasmopara viticola on metalaxyl‐treated grape leaves. Using this method, three types of sensitivity of P. viticola to metalaxyl could be distinguished and the precise ratio of sensitive, ‘reduced sensitive’ and resistant zoospores of selected isolates could be determined.

Strigol Analogs Inhibit Seedling Growth and Haustorial Development in Sopubia delphinifolia G. Don. —A Hemi-root Parasite

The effects of synthetic strigol analogs, GR-7, GR-24, and GR-28, were studied on seed germination, seedling growth and haustorial development in a tropical hemi-root parasite, Sopubia delphinifolia. All the three GR-compounds significantly stimulated seed germination but inhibited seedling growth, particularly the root growth. They also inhibited gum tragacanth-induced haustorial development. It is discussed that GR-compounds may not only induce «suicidal germination» but also exert post-germination control, even in the presence of host plants.

An ultrastructural study of the late-blight fungus Phytophthora infestans and its interaction with the foliage of two potato cultivars possessing different levels of general (field) resistance

In the susceptible reaction of foliage of the cultivar Majestic there was extensive colonization of the host tissue prior to the onset of necrosis. The development of intracellular hyphae was confined to the initially parasitized palisade cells. Thereafter the fungus grew as intercellular hyphae, which penetrated host cells and formed haustoria. Haustorial morphology was highly variable, ranging in type from small spherical to much larger digitlike structures. Haustorial formation was preceded by the laying down of a moderately electron-dense penetration matrix bounded by the host plasmalemma. This material probably constituted the extrahaustorial matrix once haustorial development had taken place. The matrical material stained with silver proteinate reagent, and this reaction was blocked by dimedone, indicating that it was carbohydrate in nature. In the resistant reaction of foliage of the cultivar Shamrock, the epidermal cells rapidly became necrotic. Quantities of exceedingly electron-dense granules appeared in the necrotic host cells. Host organelles were no longer recognizable, but the fungal cytoplasm remained intact. In adjoining host cells, thin cell wall appositions were formed, which had a heterogeneous composition. On occasions when the fungus attempted to invade an underlying mesophyll cell, papillae usually formed in that cell at the site of incipient penetration.