Haustorium Initiation Research Articles

LATERAL ORGAN BOUNDARIES DOMAIN 25 functions as a key regulator of haustorium development in dodders.

Parasitic plants reduce crop yield worldwide. Dodder (Cuscuta campestris) is a stem parasite that attaches to its host, using haustoria to extract nutrients and water. We analyzed the transcriptome of six C. campestris tissues and identified a key gene, LATERAL ORGAN BOUNDARIES DOMAIN 25 (CcLBD25), as highly expressed in prehaustoria and haustoria. Gene coexpression networks from different tissue types and laser-capture microdissection RNA-sequencing data indicated that CcLBD25 could be essential for regulating cell wall loosening and organogenesis. We employed host-induced gene silencing by generating transgenic tomato (Solanum lycopersicum) hosts that express hairpin RNAs to target and down-regulate CcLBD25 in the parasite. Our results showed that C. campestris growing on CcLBD25 RNAi transgenic tomatoes transited to the flowering stage earlier and had reduced biomass compared with C. campestris growing on wild-type (WT) hosts, suggesting that parasites growing on transgenic plants were stressed due to insufficient nutrient acquisition. We developed an in vitro haustorium system to assay the number of prehaustoria produced on strands from C. campestris. Cuscuta campestris grown on CcLBD25 RNAi tomatoes produced fewer prehaustoria than those grown on WT tomatoes, indicating that down-regulating CcLBD25 may affect haustorium initiation. Cuscuta campestris haustoria growing on CcLBD25 RNAi tomatoes exhibited reduced pectin digestion and lacked searching hyphae, which interfered with haustorium penetration and formation of vascular connections. The results of this study elucidate the role of CcLBD25 in haustorium development and might contribute to developing parasite-resistant crops.

Impact of Trichoderma harzianum and bacterial strains against Striga hermonthica in Sorghum

A series of laboratory and green house experiments were conducted to investigate the efficacy of Trichoderma harzianum fungi, bacteria ( Flavobacterium, Bacillus megatherium var. phosphaticum (BMP) and Azomonas ) on Striga hermonthica early developmental stages , incidence and sorghum growth under laboratory and greenhouse conditions. The first laboratory experiment results showed that application of BMP + Flavobacterium significantly inhibited S. hermonthica seeds germination during and after conditioning in response to GR24 concentrations as compared to medium control. In the second laboratory experiment, application of the filtrate of T. harzianum alone or in combinations with bacteria significantly inhibited S. hermonthica germination and haustorium initiation as compared to the corresponding control. From greenhouse experiment results, S. hermonthica emergence significantly reduced by T. harzianum and insignificantly by the combination of Flavobacterium + BMP + T. harzianum . T. harzianum followed by the combination of Flavobacterium + BMP gave the highest increment in plant height. T. harzianum significantly increased sorghum number of leaves as compared to the infested control. The combination of Flavobacterium + BMP + T. harzianum gave the highest number of leaves. The combination Flavobacterium + BMP gave the highest sorghum shoot dry weight, followed by T. harzianum . While the highest sorghum root dry weight was obtained from the combination of Flavobacterium + BMP + T. harzianum . Generally, the combination of T. harzianum + Flavobacterium + BMP reduced S. hermonthica infestation and enhanced sorghum growth in comparison to the infested control.

Cytokinin treated microcalli of Phelipanche ramosa: an efficient model for studying haustorium formation in holoparasitic plants

The ability of Orobanchaceae to establish a parasitic relationship is based on the development of a specific organ called haustorium. Previous studies in Phelipanche aegyptiaca and P. ramosa have underlined the interest of producing calli from germinated seeds as an efficient genetic transformation system, but they also pointed the need to improve the attachment rate of infectious calli to the host plant as well as the seed yield of the regenerated plants following attachment. It was previously shown that haustorium formation in P. ramosa is triggered by cytokinins. The present work demonstrates that one month-old microcalli produced from germinated seeds displayed a response to c/tZ similar to germinated seeds, as shown by the temporal expression profile of markers genes of cytokinin-related haustorium initiation, including PrRR5, PrCKX2, PrCKX4, PrTRN2 and PrZFP6. In addition, a 48 h treatment of microcalli with c/tZ (10−7 M) before infestation triggered a 4-fold increase in the attachment rate of microcalli to tomato roots after 18 days in minirhizotrons when compared to untreated microcalli. Finally, it also outlines that increasing the aggressiveness of microcalli with a c/tZ treatment followed by the transfer of parasitized tomato plants into pots significantly improved seed yield of regenerated P. ramosa plants. Indeed, high amounts of viable seeds which germinated at more than 90% in response to GR24 were harvested after a 10 weeks-co-cultivation period. According to these results, cytokinin treated P. ramosa microcalli thus appear to be a good tool for further functional studies in holoparasitic plants, especially on haustorium formation. Cytokinin treated microcalli show a high ability to attach to host roots and to regenerate seed-producing plants. They are of great interest for functional studies especially for studying haustorium formation.

Risk versus reward: host dependent parasite mortality rates and phenotypes in the facultative generalist Triphysaria versicolor

BackgroundParasitic plants engage in a complex molecular dialog with potential host plants to identify a host and overcome host defenses to initiate development of the parasitic feeding organ, the haustorium, invade host tissues, and withdraw water and nutrients. While one of two critical signaling events in the parasitic plant life cycle (germination via stimulant chemicals) has been relatively well-studied, the signaling event that triggers haustorium formation remains elusive. Elucidation of this poorly understood molecular dialogue will shed light on plant-plant communication, parasitic plant physiology, and the evolution of parasitism in plants.ResultsHere we present an experimental framework that develops easily quantifiable contrasts for the facultative generalist parasitic plant, Triphysaria, as it feeds across a broad range of diverse flowering plants. The contrasts, including variable parasite growth form and mortality when grown with different hosts, suggest a dynamic and host-dependent molecular dialogue between the parasite and host. Finally, by comparing transcriptome datasets from attached versus unattached parasites we gain insight into some of the physiological processes that are altered during parasitic behavior including shifts in photosynthesis-related and stress response genes.ConclusionsThis work sheds light on Triphysaria’s parasitic life habit and is an important step towards understanding the mechanisms of haustorium initiation factor perception, a unique form of plant-plant communication.

Host Specificity of Sudan Witchweed (Striga Hermonthica [Del.] Benth.) Seed Germination and Haustorium Initiation in Response to Sorghum Root Exudates and Extracts

Field surveys and laboratory experiments were conducted during the season 2009/10 and 2010/11 in witchweed (Striga hermonthica [Del.] Benth.) endemic areas in Sudan to investigate the host specificity of witchweed populations collected from different locations with respect seed germination and haustorium initiation in response to sorghum root exudates and extracts. Field surveys were conducted to collect seeds from witchweed plants growing under their respective hosts; sorghum and millet. A total of fifteen witchweed populations were collected. Tow in vitro experiments were conducted at the Phytopathology Center and Biology Laboratory, Faculty of Agricultural Sciences, University of Gezira, Sudan to study the effects of root exudates and root extracts of sorghum cv. Abu-70, cv. Wad Ahmed and cv. Hakika on percentage of seed germination and haustorium initiation. Treatments (fifteen witchweed populations and three sorghum cultivars) were arranged in a factorial completely randomized design with three replicates. Data were collected and subjected to analysis of variance. Means were separated for significant using Duncan's Multiple Range Test (at p £ 0.5). The results showed that, there were significant differences in seed germination and haustorium initiation of witchweed in response to root exudates and root extracts among sorghum cultivars and among the witchweed populations. However, the highest seed germination and haustorium initiation percentages attained by each of the witchweed population were on their respective hosts. This study suggests two levels of physiological specialization in witchweed in Sudan: intercrop specialization and intra-crop specialization. Moreover, two strains of witchweed are suggested, one specific to sorghum and the other, to millet. The existence of variability and host specificity within witchweed populations are suggested to be based almost entirely on differential response of Striga isolates to root exudates and root extracts from host.

Effects of powder and aqueous extracts of Euphorbia hirta on Phelipanche ramosa germination and haustorium initiation

Phelipanche ramosa, a root parasitic weed, is a copious seed producer. A series of laboratory experiments was undertaken to investigate the effects of powder and aqueous extracts from Euphorbia hirta on germination, and haustorium initiation in P. ramosa. P. ramosa seeds conditioned in water and subsequently treated with diluted E. hirta extract (10–25% v/v) displayed considerable germination (47–62%). Increasing extract concentration to 50% or more reduced germination in response to the synthetic germination stimulants GR24 and Nijmegen−i in a concentration dependent manner. P. ramosa germlings treated with diluted Euphorbia extract (10–75% v/v) displayed haustorium initiation comparable to the synthetic haustorium factor 2, 6-dimethoxybenzoquinone (DMBQ) at 20 µM. Euphorbia extract applied during conditioning reduced haustorium initiation in a concentration dependent manner. E. hirta extract or air-dried powder, applied to soil, induced considerable P. ramosa germination. The results indicate the plausibility of using E. hirta air-dried powder or extract as spot treatments to induce suicidal germination of Striga hermonthica.

Fast and abundant in vitro spontaneous haustorium formation in root hemiparasitic plant Pedicularis kansuensis Maxim. (Orobanchaceae)

Haustorium formation is the characteristic feature of all parasitic plants and a vital process for successful parasitism. Previous investigations on haustorium initiation and development are constricted to induced processes by host-derived signals or synthetic analogs. Spontaneous haustorium formation in the absence of host signals, a process representing an early stage in the evolution of parasitic plants, remains largely unexplored. Lack of fast and frequent formation of spontaneous haustoria greatly hinders full understanding of haustorium formation in root hemiparasites. In this study, seedlings of Pedicularis kansuensis Maxim., a facultative root hemiparasitic species in Orobanchaceae observed to produce many spontaneous haustoria, were grown in autoclaved water agar in the absence of any known haustorium-inducing stimulants. We aimed to test the temporal and developmental pattern of spontaneous haustorium formation. Also, effects of sucrose supply and root contact on spontaneous haustorium formation were tested. Spontaneous haustoria were observed starting from six days after germination, much earlier than previously reported root hemiparasites. A majority of the spontaneous haustoria formed on lateral roots. Percentage of seedlings with spontaneous haustoria was 28.8% when grown on water agar plates, with a mean of four haustoria per seedling two weeks after germination. Haustorium formation by seedlings grown in water agar amended with 2% sucrose was more than twice of those without sucrose amendment. Singly grown seedlings were able to develop spontaneous haustoria at similar levels as those grown with another conspecific seedling. In view of the fast and abundant formation of spontaneous haustoria, P. kansuensis may be developed as an excellent experimental system in future investigations for unraveling endogenous regulation of haustorium initiation and development in root hemiparasitic plants.

Genetic analysis of the maximum germination distance of Striga under Fusarium oxysporum f. sp. strigae biocontrol in sorghum

Maximum germination distance (MGD) is an important component of Striga resistance in sorghum. The objective of this study was to determine gene action influencing MGD of Striga hermonthica and Striga asiatica among selected sorghum lines treated with a biocontrol agent, Fusarium oxysporum f. sp. strigae (FOS) for effective breeding with Striga resistance, and FOS compatibility. Twelve sorghum genotypes were selected based on their Striga resistance, FOS compatibility, and superior agronomic performance. Selected genotypes were crossed using a bi-parental mating design to generate six families for genetic analysis. Agar-gel assays were used to determine low haustorium initiation factor (LHF) using the 12 parental lines, their F1 progenies, backcross derivatives, and F2 segregants in two sets. One set had S. hermonthica seed and the other one had S. asiatica seed. Both were treated with and without FOS. Genotypes were evaluated using a split-plot design with three replications and MGD data were recorded followed by generation mean analysis. FOS reduced MGD by 1 cm under both S. hermonthica and S. asiatica infestations. Additive, dominance, and epistatic gene actions were involved in the control of MGD of the two Striga species in the evaluated populations. On average, the relative contribution of additive, additive×additive and dominance×dominance genetic effects on the MGD of S. hermonthica and S. asiatica, with FOS, were 20, 33, and 36%; and 21, 32, and 35%, respectively. Breeding methods exploiting these genetic effects may provide enhanced response to selection for Striga resistance and FOS compatibility in integrated Striga management (ISM) programmes.

Haustorium initiation in the obligate parasitic plant Phelipanche ramosa involves a host-exudated cytokinin signal.

The heterotrophic lifestyle of parasitic plants relies on the development of the haustorium, a specific infectious organ required for attachment to host roots. While haustorium development is initiated upon chemodetection of host-derived molecules in hemiparasitic plants, the induction of haustorium formation remains largely unknown in holoparasitic species such as Phelipanche ramosa. This work demonstrates that the root exudates of the host plant Brassica napus contain allelochemicals displaying haustorium-inducing activity on P. ramosa germinating seeds, which increases the parasite aggressiveness. A de novo assembled transcriptome and microarray approach with P. ramosa during early haustorium formation upon treatment with B. napus root exudates allowed the identification of differentially expressed genes involved in hormone signaling. Bioassays using exogenous cytokinins and the specific cytokinin receptor inhibitor PI-55 showed that cytokinins induced haustorium formation and increased parasite aggressiveness. Root exudates triggered the expression of cytokinin-responsive genes during early haustorium development in germinated seeds, and bio-guided UPLC-ESI(+)-/MS/MS analysis showed that these exudates contain a cytokinin with dihydrozeatin characteristics. These results suggest that cytokinins constitutively exudated from host roots play a major role in haustorium formation and aggressiveness in P. ramosa.

Induction of Striga hermonthica germination and haustorium initiation by Allelochemicals produced by millet cultivars

Induction of Striga hermonthica germination and haustorium initiation by Allelochemicals produced by millet cultivars

Effects of bacterial isolates and strains on Phelipanche ramosa (L.) Pomel haustorium initiation

Laboratory experiments were conducted to study the efficacy of bacterial isolates and strains on Phelipanche ramosa haustorium initiation at the Bio-pesticides and Bio-fertilizers Department, Environment, Natural Resources and Desertification Research Institute (ENDRI), NCR, Khartoum, Sudan. Fifteen soil borne bacteria isolates (9 organic nitrogen users and 6 mineral nitrogen users) and 3 bacterial strains (Bacillus circulans, B. megatherium var. phosphaticum and Azospirillum brasiliense) were tested. All tested bacterial isolates and strains caused a significant (P≤ 0.5) reduction in haustorium initiation as compared to control in response to DMBQ (2, 6 dimethoxybenzequinone). Organic nitrogen using bacterial isolates ISO5M and ISO22M reduced haustoria by 32 to 54% as compared to the corresponding control. While the mineral nitrogen using bacterial isolate ISO1S and Bacillus circulans strain reduced haustorium by 37 to 45% compared to medium control. The most efficient isolates were identified as Serratia odorifera (ISO22M) with probability 95% and Rhizobium radiobacter (ISO5M) with probability 99%.

Effect of Seed Priming on Early Development of Sorghum (Sorghum bicolor L. Moench) and Striga hermonthica (Del.) Benth.

Striga hermonthica is an obligate, root parasite, that limits cereal production in sub-Saharan Africa. Successful control depends on eliminating its seed reserves in soil, thereby preventing parasitism. Two experiments were conducted to evaluate the effects of salinity on germination traits and seedling growth of sorghum (cultivar Wad Ahmed) and S. hermonthica. The experiments were conducted in a factorial arrangement on the basis of completely randomized design (CRD) with 4 replications. In the first experiment, sorghum height, leaf area, and shoot and root dry weights were examined. The results displayed that, with increasing salinity level, leaf area and dry biomass were increased, while the height was decreased. In the second experiment, Striga germination and haustorium initiation percentages were examined. Among all salts, C2H4O2·NH3 inhibited Striga germination (0–15%) during conditioning or (0–25%) at germination compared to the control (75%). However, salt MgSO4·7H2O improved germination during conditioning up to 70%, while during germination CH3COONa·3H2O recorded 65% germination. Regarding haustoria initiation, results showed that C2H4O2·NH3 at all concentrations inhibits haustorium formation by 100%, while CH3COONa·3H2O at 10 µM improved haustorium formation up to 64% but still below the control (70%). Osmotic potential may significantly affect germination and radicle elongation of the parasitic weed.

The differentially regulated genes TvQR1 and TvPirin of the parasitic plant Triphysaria exhibit distinctive natural allelic diversity

BackgroundPlant parasitism represents an extraordinary interaction among flowering plants: parasitic plants use a specialized organ, the haustorium, to invade the host vascular system to deprive host plants of water and nutrients. Various compounds present in exudates of host plants trigger haustorium development. The two most effective haustorium inducing factors (HIFs) known for the parasitic plant Triphysaria versicolor (T. versicolor) are peonidin, an antioxidant flavonoid, and 2,6-dimethoxybenzoquinone (DMBQ), an oxidative stress agent. To date, two genes involved in haustorium initiation in T. versicolor have been identified: TvQR1, a quinone oxidoreductase that generates the active HIF from DMBQ, and TvPirin, a transcription co-factor that regulates several other DMBQ- responsive and –non-responsive genes. While the expression of these genes in response to DMBQ is well characterized, their expression in response to peonidin is not. In addition, the pattern of polymorphisms in these genes is unknown, even though nucleotide changes in TvQR1 and TvPirin may have contributed to the ability of T. versicolor to develop haustoria. To gain insights into these aspects, we investigated their transcriptional responses to HIFs and non-HIF and their natural nucleotide diversity.ResultsHere we show that TvQR1 and TvPirin are transcriptionally upregulated by both DMBQ and peonidin in T. versicolor roots. Yet, while TvQR1 also responded to juglone, a non-HIF quinone with toxicity comparable to that of DMBQ, TvPirin did not. We further demonstrate that TvPirin encodes a protein shorter than the one previously reported. In the T. versicolor natural population of Northern California, TvQR1 exhibited remarkably higher molecular diversity and more recombination events than TvPirin, with the highest non-synonymous substitution rate in the substrate recognition and catalytic domain of the TvQR1 protein.ConclusionOur results suggest that TvQR1 and TvPirin have most likely evolved highly distinct roles for haustorium formation. Unlike TvPirin, TvQR1 might have been under diversifying selection to maintain a diverse collection of polymorphisms, which might be related to the recognition of an assortment of HIF and non-HIF quinones as substrates for successful haustorial establishment in a wide range of host plants.

Inoculation with arbuscular mycorrhizal fungi suppresses initiation of haustoria in the root hemiparasite Pedicularis tricolor.

Plant parasitism and arbuscular mycorrhizal (AM) associations have many parallels and share a number of regulatory pathways. Despite a rapid increase in investigations addressing the roles of AM fungi in regulating interactions between parasitic plants and their hosts, few studies have tested the effect of AM fungi on the initiation and differentiation of haustoria, the parasite-specific structures exclusively responsible for host attachment and nutrient transfer. In this study, we tested the influence of AM fungi on haustorium formation in a root hemiparasitic plant. Using a facultative root hemiparasitic species (Pedicularis tricolor) with the potential to form AM associations, the effects of inoculation were tested with two AM fungal species, Glomus mosseae and Glomus intraradices, on haustorium initiation in P. tricolor grown alone or with Hordeum vulgare 'Fleet' (barley) as the host plant. This study consisted of two greenhouse pot experiments. Both AM fungal species dramatically suppressed intraspecific haustorium initiation in P. tricolor at a very low colonization level. The suppression over-rode inductive effects of the parasite's host plant on haustoria production and caused significant growth depression of P. tricolor. AM fungi had strong and direct suppressive effects on haustorium formation in the root hemiparasite. The significant role of AM fungi in haustorium initiation of parasitic plants was demonstrated for the first time. This study provides new clues for the regulation of haustorium formation and a route to development of new biocontrol strategies in management of parasitic weeds.

Screening of maize genotypes for low <i>Striga Asiatica</i> stimulant production using the 'agar gel technique'

One of the best characterized mechanisms of resistance to the parasitic weed ( Striga ) is low germination stimulant exudation by host plants. Using a simple and rapid agar gel assay, 112 maize genotypes were tested for low stimulant production and haustorium initiation of Striga asiatica (L.) Kuntze. Most of the genotypes induced high level of germination of Striga seeds at distances more than 1.3 cm, from the crop seedling root to the furthest germination Striga seed. Germination distance highly correlated with rate of germination and was used as a quick way of determining the level of haustorial germination induced by the test genotypes. Some breeding lines such as PR 91A 496-13 and PR 91A 496-25 induced moderately low germination. B 37.91-952-1 induced the least germination distance (0.87cm). This is comparable to that of highly resistant sorghum varieties. The commercial varieties induced high level of germination. A similar procedure was followed to observe the rate of haustorium initiation. Despite the tendency to stimulate considerably high germination, some entries permitted very low haustorium initiation. The best three lines in terms of low haustorium initiation were PR 496-45, PR 91B 5323 33x34 and PR 91A 496-17. There was no relationship between germination distance and haustorium initiation.

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.

Powdery mildew (Podosphaera xanthii) resistance in melon is categorized into two types based on inhibition of the infection processes

Physiological races of powdery mildew (Podosphaera xanthii) cause different symptoms in eight melon lines. Infection by races 1, 2, and 5 was examined in different melon lines. After a compatible reaction, conidia germination, haustorium initiation from the germ tube, germ tube branching, and sporulation occurred within 12, 24, 48, and 120 h, respectively, and the conidia matured within 240 h. In contrast, type i and ii inhibition were identified through incompatible reactions. The germ tube and haustorium were initiated from conidia, but no germ tube branching occurred in the lines with type i resistance within 48-240 h. In type ii resistance, germ tube branching was observed within 120 h, but no sporulation was observed within 240 h. The number of fluorescing epidermal cells was higher within 24 h in type i, and within 48-120 h in type ii resistance lines than in susceptible lines. Callose accumulation around the haustorium was detected in type ii resistance lines within 48-120 h. This suggests that the rapid hypersensitive response (HR) within 24 h has an important role in the type i response, while HR and callose accumulation in the type ii response occur slowly between 48 and 120 h. Of the resistant lines, PMR 45 and WMR 29 showed a type i incompatible response; the PI 414723 response was entirely type ii; and PMR 5, PI 124112, and MR-1 showed different responses depending on the race. Therefore, the two types of incompatible responses were intermixed in the same germplasm.

Characterization of theOrobanche–Arabidopsissystem for studying parasite–host interactions

Parasitization by Orobanche is a complex process, one that is mediated by host-derived chemical signals that control parasite seed germination and haustorium initiation and one that ultimately results in the union of two plant species. Experiments were conducted to characterize Orobanche parasitization of the model plant Arabidopsis thaliana and to begin to explore the role of host flavonoid metabolism in the interaction. Arabidopsis thaliana stimulated seed germination and allowed tubercle development of O. aegyptiaca and O. ramosa but did not significantly stimulate seeds of O. crenata, O. minor, or O. cernua. However, if Orobanche seeds were artificially stimulated, O. crenata and O. minor successfully established tubercles on A. thaliana. When compared to the recognized crop hosts, Daucus carota and Nicotiana tabacum, A. thaliana stimulated less O. aegyptiaca germination but allowed for formation of equivalent numbers of tubercles. These findings indicate that A. thaliana is not a large-scale producer of germination stimulant but is highly susceptible to the parasite once Orobanche seeds have germinated. Experiments comparing wild-type A. thaliana plants to mutant lines deficient in flavonoid biosynthesis revealed no differences in the ability to stimulate germination or to allow tubercle formation, indicating that host flavonoid production is not essential for Orobanche parasitization. The results of this work support the use of A. thaliana as a valuable host in understanding Orobanche parasitization.

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.

Light and electron microscope studies of the interactions of host and non-host plants with cowpea rust— Uromyces phaseoli var. vignae

A comparison of infection of host plants and eight non-host angiosperm species by cowpea rust revealed that uredospores usually germinated equally well on hosts and non-hosts. On most of the latter, however, fewer germ tubes contacted stomata, apparently because of the reduced efficiency of directional growth. Compared with both resistant and susceptible host cultivars, at least twice as many hyphae in non-host leaves stopped growing during the very early stages of intercellular development. Nevertheless, the majority of infection hyphae continued to grow to the stage at which the first haustorial mother cell would be expected to form. However, further intercellular growth was rare and few, if any, infection hyphae formed haustoria. Ultrastructural investigation of four non-hosts suggested that haustorium formation could be inhibited by at least three mechanisms: deposition of osmiophilic material on adjacent non-host walls ( Phaseolus spp.), loss of contact between haustorial mother cell and non-host cell ( Vicia faba), or fungal death prior to haustorium initiation ( Pisum sativum). If haustoria were formed, the death of both haustorium and invaded cell rapidly followed although the initial response to the one haustorium observed ultrastructurally resembled that described previously for susceptible interactions. The diversity of cellular responses induced in resistant hosts and non-hosts by cowpea rust, described in this and earlier work, suggests that there are a number of stages during infection at which an interaction between this fungus and the higher plant can take place. It is proposed that each stage represents a “switching point” and that responses at each determine the subsequent progress of infection in both compatible and incompatible associations.