Studies on the Physiology of Host-parasite Relationship in Orobanche. III. Carbohydrate and Nitrogen Metabolism of Host and Parasite

Determination of different carbohydrate and nitrogen fractions was made in tomato (Lycopersicum esculentum Mill.) and mustard (Brassica campestris L.) serving as hosts for Orobanche cernua and O. aegyptiaca respectively. Shoots of Orobanche were also subjected to such analyses. Infection raised the level of total reducing and total sugar in the host with a simultaneous decrease in the level of acid-hydrolyzable and total carbohydrates in the constituent organs of infected hosts. This has been explained to be due to predominance of hydrolytic activity in the infected host. Infection also brought about a depression in the proportion of sucrose to the pool of total sugar in the host. This was also possibly due to predominance of hydrolytic processes and retardation in the synthetic processes. Higher concentration of acid-hydrolyzable and total carbohydrates in Orobanche than in the host indicated a high demand for sugars by the parasite. The insignificant differences between the relative proportions of different nitrogen fractions to the pool of total nitrogen in healthy and infected hosts indicated that nitrogen metabolism was not deranged in any way due to infection. Orobanche always had a lower concentration of total soluble and total nitrogen than the host root.     

Gene silencing of mannose 6-phosphate reductase in the parasitic weed Orobanche aegyptiaca through the production of homologous dsRNA sequences in the host plant

Orobanche spp. (broomrape) are parasitic plants which subsist on the roots of a wide range of hosts, including tomato, causing severe losses in yield quality and quantity. Large amounts of mannitol accumulate in this parasitic weed during development. Mannose 6-phosphate reductase (M6PR) is a key enzyme in mannitol biosynthesis, and it has been suggested that mannitol accumulation may be very important for Orobanche development. Therefore, the Orobanche M6PR gene is a potential target for efforts to control this parasite. Transgenic tomato plants were produced bearing a gene construct containing a specific 277-bp fragment from Orobanche aegyptiaca M6PR-mRNA, in an inverted-repeat configuration. M6PR-siRNA was detected in three independent transgenic tomato lines in the R1 generation, but was not detected in the parasite. Quantitative RT-PCR analysis showed that the amount of endogenous M6PR mRNA in the tubercles and underground shoots of O. aegyptiaca grown on transgenic host plants was reduced by 60%–80%. Concomitant with M6PR mRNA suppression, there was a significant decrease in mannitol level and a significant increase in the percentage of dead O. aegyptiaca tubercles on the transgenic host plants. The detection of mir390, which is involved with cytoplasmic dsRNA processing, is the first indication of the existence of gene-silencing mechanisms in Orobanche spp. Gene silencing mechanisms are probably involved with the production of decreased levels of M6PR mRNA in the parasites grown on the transformed tomato lines.     

Hatch and Reproduction of Globodera tabacum tabacum in Response to Tobacco,Tomato, or Black Nightshade

The effects of broadleaf tobacco, tomato, and black nightshade on juvenile hatch and reproduction of Globodera tabacum tabacum were determined in laboratory and greenhouse experiments. Root exudates from nightshade stimulated greater egg hatch than those from either ''Rutgers'' tomato or ''86-4'' tobacco. Hatch was greater at higher proportions of root exudates for all three plant species. Root exudates from plants greater than 3 weeks old stimulated more hatch than younger plants. No regression relationships existed between plant age and nematode batch. In other experiments, hatch from eggs in cysts was higher for tomato and nightshade after 10 weeks in greenhouse pots compared to tobacco and bare soil. Numbers of second-stage juveniles in eggs in cysts produced from a previous generation on the same host were highest on nightshade and less on tomato and tobacco. Cysts of variable age recovered from field soil had increased hatch in both root exudates or water compared to recently produced cysts from plants in growth chambers. Globodera t. tabacum may be subject to both host and environmentally mediated diapause.     

Studies on the Physiology of Host-parasite Relationship in Orobanche. I. Respiratory Metabolism of Host and Parasite

The respiratory rate of the roots of mustard (Brassica cam-pestris L.) and tomato (Lycopersicum esculentum Mill.) serving as hosts for the total root parasites Orobanche aegyptiaca Pers. and O.cernua Loefll. was measured using Warburg manometric technique. At the same time determinations were made of the respiration of the apical, basal and root regions of the parasites. The effects of sodium fluoride, malonic acid, sodium azide and DNP (2,4-dinitrophenol) on the rate of respiration of the host roots as well as of the parasites were studied. The Orobanche infection results in a marked increase in the respiratory rate near the host-parasite contact region. The damaging effect of infection seems to be due mainly to a continuous flow of water, minerals and metabolites from host to parasite. The haustorial invasion creates an obstruction in the translocation of metabolites. The respiration rate is lower in Orobanche than in the host, which might be related to its slower growth rate, inefficient oxidative processes and an escaping of certain energy-requiring interconversion processes. Roots of O. aegyptiaca are more well-developed and have higher rate of respiration. They can absorb more water and minerals from the soil. This fact might be connected with the specificity of the two species. NaF and malonic acid inhibit the respiration to a similar extent in healthy and infected roots. This indicates that the pathway of respiration does not change materially after infection. The EMP and Krebs cycle seem to operate at a lower intensity in Orobanche, which is proved by the lower inhibition of the respiration as compared to in the host. Azide causes a stronger reduction of the respiration in infected than in healthy roots. It would imply that the infection stimulates the activity of metal containing oxidases. The weaker inhibition of the respiration in Orobanche tissues indicates a mediation of other enzymes in the oxidation processes than in the host. The respiration is less stimulated by DNP in infected than in healthy roots. Contrary to the general effect of DNP, this substance decreases the O₂ uptake in the parasite tissues. This fact may be explained by the occurrence of exceptionally high amounts of endogenous phenolic compounds and an insufficient production of ATP in the parasite.     

Subcellular localization of proteins of Oryza sativa L. in the model tobacco and tomato plants

The cellular localization and molecular interactions are indicative of functions of a protein. The development of a simple and efficient method for subcellular localization of a protein is indispensable to elucidate gene function in plants. In this study, we assessed the feasibility of Agrobacterium-mediated transformation (agroinfiltration) of tobacco and tomato leaf tissue to follow intracellular targeting of proteins from rice fused to green fluorescent protein (GFP). For this, a simple in planta assay for subcellular localization of rice proteins in the heterologous host systems of tobacco and tomato leaf via transient transformation was developed. We have tested the applicability of this method by expressing GFP fusions of the putative antiphagocytic protein 1 (APP1) (OsAPP, LOC_Os03g56930) and ZOS3-18-C2H2 zinc-finger protein (OsZF1, LOC_Os03g55540) from Oryza sativa L. subsp. japonica in tobacco and tomato leaf tissues. Our results demonstrate the suitability of GFP as a reporter in gene expression studies in tomato cv. MicroTom. The use of GFP-fused proteins from rice for subcellular targeting in the heterologous hosts of tobacco and tomato plant systems has been confirmed.Key words: agroinfiltration, confocal microscopy, GFP fusion protein, tomato cv, microtom     

Variability of Orobanche ramosa populations in France as revealed by cross infestations and molecular markers

The dramatic spread of root holoparasite Orobanche ramosa L. (branched broomrape) has been observed in the last few years in France in fields of Brassica napus L., Nicotiana tabacum L., and Cannabis sativa L. In the face of this occurrence and considering its tremendous capacity to adapt to its host, the question about variability of O. ramosa populations arises. The virulence of broomrape populations was investigated by cross infestations performed under greenhouse conditions using the three host species mentioned above and broomrape seeds collected from Orobanche plants parasitizing them. After 45 days of co-culture, the intensity and kinetics of infestation were determined. In all cases, parasites developed but differences were observed among associations between the host species and the broomrape population. Differences in virulence of O. ramosa populations and greater affinity between a host species and “its own” broomrape population were demonstrated by statistical analyses (F-test, CFAs). These results led to the concept of host specificity and suggestions about the existence of three different pathovars in O. ramosa. Analysis of genetic variability of the three O. ramosa populations by PCR of RAPD markers confirmed that they may be considered as three distinct pathovars.     

Peroxidase Isozymes from Meloidogyne spp. and Their Origin

Two peroxidase isozymes (Ef 0.43 and 0.53) were detected by electrophoretic analysis in homogenates of Meloidogyne arenaria, M. hapla, M. javanica, and M. incognita females reared on tomato. No peroxidase isozymes were detected electrophoretically in homogenates of adult males, preparasitic larvae, or eggs. Peroxidase isozymes from females reared on bean, eggplant, or tobacco differed from those from females reared on tomato. Bean and eggplant populations had a single peroxidase isozyme each, respectively Ef 0.21 and 0.28. No peroxidase isozymes were detected in tobacco populations under the conditions used, although total activity assays did reveal low levels of peroxidase activity in homogenates of tobacco populations. The peroxidase isozymes detected in females reared on tomato or bean appear similar to the peroxidase isozymes present in root-knot galls, adjacent ungalled roots, and roots from uninoculated plants of the corresponding hosts. The probability is discussed that most of the peroxittase activity associated with Meloidogyne spp. females is of host origin.     

Induction of seed germination in <Emphasis Type="Italic">Orobanche</Emphasis> spp. by extracts of traditional Chinese medicinal herbs

The co-evolution of Orobanche spp. and their hosts within the same environment has resulted in a high degree of adaptation and effective parasitism whereby the host releases parasite germination stimulants, which are likely to be unstable in the soil. Our objective was to investigate whether extracts from non-host plants, specifically, Chinese medicinal plants, could stimulate germination of Orobanche spp. Samples of 606 Chinese medicinal herb species were extracted with deionized water and methanol. The extracts were used to induce germination of three Orobanche species; Orobanche minor, Orobanche cumana, and Orobanche aegyptiaca. O. minor exhibited a wide range of germination responses to the various herbal extracts. O. cumana and O. aegyptiaca exhibited an intermediate germination response to the herbal extracts. O. minor, which has a narrow host spectrum, showed higher germination rates in response to different herbal extracts compared with those of O. cumana and O. aegyptiaca, which have a broader host spectrum. Methanolic extracts of many Chinese herbal species effectively stimulated seed germination among the Orobanche spp., even though they were not the typical hosts. The effective herbs represent interesting examples of potential trap crops. Different countries can also screen extracts from indigenous herbaceous plants for their ability to induce germination of Orobanche spp. seeds. The use of such species as trap plants could diminish the global soil seed bank of Orobanche.     

Solute fluxes from tobacco to the parasitic angiosperm Orobanche cernua and the influence of infection on host carbon and nitrogen relations

Orobanche species are holoparasites which are very efficient sinks for host-derived solutes. Here, we report the use of direct measurements of xylem sap solute concentrations and water fluxes, together with a modelling procedure to calculate element fluxes within an association between Orobanche cernua and its tobacco host. Infection of tobacco by the parasite markedly influenced carbon acquisition and partitioning; net fixation of carbon was 20% higher in infected tobacco compared with controls. Orobanche cernua caused a 84% increase in net carbon flux moving downward from the tobacco shoot and 73% of this carbon was intercepted by the parasite, almost entirely through the phloem (>99%). Further, the parasite also exerted a large impact on the nitrogen relations of the plant, notably nitrate uptake was stimulated and the amino acid content of xylem sap was lower. The parasite also relied heavily on host phloem for the supply of other resources, with only 5 to 15% of N, and 16% of K, 23% of Na, 63% of Mg and 13% of S being derived from the xylem. Thus, we provide quantitative information on the phloem dependency of the parasite and show that host carbon and nitrogen metabolism is stimulated as a consequence of infection.     

Dehydrocostus lactone is exuded from sunflower roots and stimulates germination of the root parasite Orobanche cumana

The germination of the obligate root parasites of the Orobanchaceae depends on the perception of chemical stimuli from host roots. Several compounds, collectively termed strigolactones, stimulate the germination of the various Orobanche species, but do not significantly elicit germination of Orobanche cumana, a specific parasite of sunflower.Phosphate starvation markedly decreased the stimulatory activity of sunflower root exudates toward O. cumana, and fluridone - an inhibitor of the carotenoid biosynthesis pathway - did not inhibit the production of the germination stimulant in both shoots and roots of young sunflower plants, indicating that the stimulant is not a strigolactone.We identified the natural germination stimulant from sunflower root exudates by bioassay-driven purification. Its chemical structure was elucidated as the guaianolide sesquiterpene lactone dehydrocostus lactone (DCL). Low DCL concentrations effectively stimulate the germination of O. cumana seeds but not of Phelipanche aegyptiaca (syn. Orobanche aegyptiaca). DCL and other sesquiterpene lactones were found in various plant organs, but were previously not known to be exuded to the rhizosphere where they can interact with other organisms.     

Broomrape (Orobanche) seed germination inhibitors from plant roots

Two species (tomato and cucumber) which are not hosts to Orobanche crenata but which are hosts to other species of Orobanche not only failed to produce the compound required to trigger O. crenata to germinate but produced germination inhibitors which stopped germination even in the presence of a suitable stimulant. This suggested the possibility of using germination inhibitors to control at least some species of Orobanche. The question whether host species produce inhibitors as well as stimulants has not however been resolved.     

The potential of Ulocladium botrytis for biological control of Orobanche spp.

A strain of Ulocladium botrytis isolated from diseased Orobanche crenata shoots caused disease on the parasitic weed in pathogenicity tests. The potential of the fungus to be developed as a mycoherbicide for Orobanche spp. was further investigated. Although the fungus significantly decreased O. crenata germination in vitro by 80%, it did not generally lead to a decreased number of O. crenata shoots or tubercles in inoculated root chambers or pots. However, the number of diseased or dead tubercles and underground shoots was significantly increased compared to the noninoculated treatments. Postemergence inoculation of O. crenata shoots with a conidial suspension resulted in the death of almost all inoculated plants 14 days after application under greenhouse conditions. In preliminary host-range studies, the pathogen caused disease on Orobanche cumana on sunflower whereas on Orobanche aegyptiaca shoots parasitizing tomato only minimal disease symptoms could be detected after postemergence inoculation. Based on the results of our investigations, we conclude that Ulocladium botrytis has only a limited potential to be used as a biocontrol agent against Orobanche spp.     

Biochemical Aspects of Parasitism by the Angiosperm Parasites: Starch Accumulation

The investigation dealt with starch accumulation in four species of Cuscuta (Cuscuta campestris, C. indecora, C. planiflora and C. reflexa), a leafy mistletoe (Dendrophthoe falcata) and a chlorophyll-lacking root parasite (Orobanche aegyptiaca). The highest content of starch occurred In O. aegyptiaca, with a maximum of 45 per cent of dry weight Starch in Cuscuta filaments and mistletoe leaves showed a maximum of about 10 per cent of dry weight. The starch content varied along the length of the Cuscuta vine, with a maximum in the apical region. Orobanche had a higher starch content when it was still submerged than it was fully developed. Cuscuta vines did not show any marked diurnal alteration in the starch content. The content of ethanol-soluble carbohydrate was only a tenth of the starch in Orobanche, but was relatively higher in the other parasites. the neutral sugars in Cuscuta filaments were sucrose and glucose, whereas fructose was also present in mistletoe and Orobanche. Raffinose and stachyose were absent or present only ill traces in parasite tissue. Starch granules from Cuscuta and Orobanche bad ADPG/UDPG-starch synthetase activity and homogenates starch phosphorylase activity. The former enzyme appeared to be responsible for synthesis of starch and the latter for utilization. The four different species of Cuscnta, growing on alfalfa, had more or less the same activity of starch synthetase and also of phosphorylase activity. Hosts infected by Cuscuta had significantly less starch per plant than the controls. A characteristic feature of invasion by Cuscuta and Orobanche was increased phosphorylase activity in the host tissues. The protein content of the tissues of Cuscuta and Orobanche was of a lower level than that of the host shoot system or foliage, indicating that the parasite differed from the host in having a higher carbon (of starch) to nitrogen (of protein) ratio.     

Coexistence and weak amensalism of congeneric gall-forming aphids on the Japanese elm

Closely related species of gall-forming aphids are often associated with a single host species. SixTetraneura species coexist on the Japanese elm,Ulmus davidiana, in Sapporo, northern Japan. This paper describes the probabilities of coexistence on macro- and microgeographic scales (i.e., on host trees and host leaves) and examines whether coexistence with conspecific or heterospecific galls on leaves or shoots has any effect on the fecundity of each aphid species using multiple regression. A Monte Carlo simulation showed that the frequency distribution of the numbers of species on individual host trees differed significantly from that expected from the null model. There were significantly positive or negative associations between species.Tetraneura radicicola andsorini always coexisted with other species on trees they infested. Multiple regression revealed that the coexistence of conspecific or heterospecific galls on individual leaves had no influence on the fecundity ofradicicola andsorini, but had a negative influence on that of sp. O. On average,radicicola andsorini produced a smaller number of offspring in galls than sp. O, and obviously consumed only a small part of resources available on the galled leaves. Evidence available suggests that although amensalism does arise between sp. O and other species, its influence in not so strong as to exclude sp. O competitively from theTetraneura community.     

Use of the maize transposonsActivator andDissociation to show that phosphinothricin and spectinomycin resistance genes act non-cell-autonomously in tobacco and tomato seedlings

Cell-autonomous genes have been used to monitor the excision of both endogenous transposons in maize andAntirrhinum, and transposons introduced into transgenic plants. In tobacco andArabidopsis, the streptomycin phosphotransferase (SPT) gene reveals somatic excision of the maize transposonActivator (Ac) as green sectors on a white background in cotyledons of seedlings germinated in the presence of streptomycin. Cotyledons of tomato seedlings germinated on streptomycin-containing medium do not bleach, suggesting that a different assay for transposon excision in tomato is desirable. We have tested the use of the spectinomycin resistance (SPEC) gene (aadA) and a Basta resistance (BAR) gene (phosphinothricin acetyltransferase, or PAT) for monitoring somatic excision ofAc in tobacco and tomato. Both genetic and molecular studies demonstrate that genotypically variegated individuals that carry clones of cells from whichAc orDs have excised from either SPEC or BAR genes, can be phenotypically completely resistant to the corresponding antibiotic. This demonstrates that these genes act non-cell-autonomously, in contrast to the SPT gene in tobacco. Possible reasons for this difference are discussed.     

Sequestration of pyridine alkaloids anabasine and nicotine from Nicotiana (Solanaceae) by Orobanche ramosa (Orobanchaceae)

Plants of the root holoparasite Orobanche ramosa L. and four of its potential host species, Nicotiana glauca Graham, Nicotiana rustica L., Nicotiana sylvestris Speg. & S.Comes, and Nicotiana tabacum L., grown in the greenhouse in Kiel/Germany, were analyzed for their contents of pyridine alkaloids anabasine (1) and nicotine (2). All investigated samples contained both alkaloids in different amounts. The distribution of the alkaloids in the various plant organs of Nicotiana differed significantly between the species. The alkaloid contents of the Orobanche samples relative to the alkaloid contents of the roots of the respective host plants varied between 3.47 ± 1.08 and 28.8 ± 37.5%. Orobanche plants drain water and crucial nutrients from their hosts; also, some examples for the sequestration of specialized natural products have been reported. O. ramosa is not able to synthesize pyridine alkaloids anabasine (1) and nicotine (2) itself; therefore, the present study proves the sequestration of pyridine alkaloids by O. ramosa from the four investigated Nicotiana host species.     

A diffusible signal from germinating Orobanche ramosa elicits early defense responses in suspension-cultured Arabidopsis thaliana

In plant/parasitic plant interaction, little is known about the host plant response before the establishment of the parasite within the host. In the present work, we focused on host responses to parasitic plant, O. ramosa in the early stage of infection. We used a co-culture system of A. thaliana suspension cells and O. ramosa germinated-seeds to avoid parasite attachment. We showed that O. ramosa induced H₂O₂ generation and camalexin synthesis by A. thaliana followed by a drastic increase in cell death. We further demonstrated that a heat sensitive diffusible signal is responsible for this cell death. These data indicate that recognition of O. ramosa occurs before the attachment of the parasite and initiates plant defence responses.Key words: Orobanche ramosa, Arabidopsis thaliana, cell death, hydrogen peroxide, secondary metabolism     

Recognition of root exudates by seeds of broomrape (Orobanche and Phelipanche) species

Background and Aims

The long co-existence of broomrapes and their hosts within the same environment has culminated in a strong adaptation and effective parasitism. As a first step of specialization in the parasitic process, seed receptors of parasitic plant species vary in their ability to recognize compounds released by their hosts. This work aims to investigate potential patterns for the reception requirements needed to activate germination within Orobanche and Phelipanche species.

Methods

Induction of the germination of seeds of nine Orobanche and Pheliphanche species by root exudates of 41 plant species was studied and subjected to biplot multivariate analysis.

Key Results

A high level of specialization in root exudate recognition was found in Orobanche densiflora, O. gracilis and O. hederae, which germinated almost exclusively in contact with root exudates from the plants they infect in nature. At the opposite extreme, Phelipanche aegyptiaca, P. ramosa and O. minor were highly generalist, germinating when in contact with the root exudates of most plant species. Orobanche crenata, O. cumana and O. foetida showed intermediate behaviour.

Conclusions

A universal germination stimulant for all broomrape species has not being identified to date. The synthetic stimulant GR24 is active against most of the weedy broomrape species, but fails with the non-weedy species tested in this study and with the very recent weedy species O. foetida. In addition, germination behaviour of broomrape species depends on the crop plant tested. Weedy broomrapes with a broad host spectrum respond better to the different exudates released by a wide range of crops and wild species than do non-weedy broomrapes, which have a narrow host spectrum and are more restricted to their host range. Root exudates of many plant species were active in stimulating germination of seeds of Orobanche and Phelipanche species for which they are not described as hosts, representing interesting examples of potential trap crops.Key words: Xenognosis, broomrape, root exudate, germination, Orobanche, Phelipanche     

Gene flow between alien and native races of the holoparasitic angiosperm Orobanche minor (Orobanchaceae)

The holoparasitic angiosperm Orobanche minor parasitizes a diverse range of flowering plants from at least 16 orders in both the monocots and eudicots. However, populations of O. minor show host specificity at a local level. Our previous work identified the potential for host specificity to act as a catalyst for genetic divergence among populations of O. minor. Here we have extended this investigation by sampling populations from multiple hosts, across a broad geographic range. Sequence characterised amplified region (SCAR) data identified an exotic host-generalist lineage and a native host-specialist lineage of O. minor, suggesting genetic structure in this species is defined by both host specificity and geography. In addition, host-range overlap, discordant tree topologies, and cryptic morphology indicate the presence of gene flow between alien races and endemic populations. Therefore, repeated introductions of alien races of O. minor from disparate sources leading to introgression with native populations, and cryptic race formation, seem to have contributed to the taxonomic confusion associated with this species. We speculate that radiations associated with broad host range and divergent host ecologies may have promoted the unusually wide geographic distribution and diversification of this species. Finally, evidence of multiple shifts to exotic hosts, coupled with the predicted northward shift in climatic suitability, identify the potential for range expansion in alien races of O. minor, which may threaten nationally scarce native taxa with genetic assimilation. Our phylogenetic analysis provides a framework for identifying host races in Orobanche with a view to setting conservation priorities.     

Variation in Pathogenicity of Seventeen Isolates of Meloidogyne incognita

Pathogenicity of 17 isolates of Meloidogyne incognita collected in Tennessee was studied in the greenhouse on: Rutgers tomato, N.C. 95 tobacco, McNair 1032 cotton, Dixie Queen watermelon, California Wonder pepper and line M57-13N cowpea. Root-knot indices of the isolates on the different hosts differentiated six physiological races. The host reactions of each race are discussed.