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 haustoriuminducing 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.     

Chemical Induction and Repression of Haustoria in Orthocarpus Purpurascens (Scrophulariaceae)

A preliminary survey of compounds that induce haustoria in thehemiparasite Orthocarpus purpurascens suggests that cellulosefibre is one source of the active, water soluble factor(s).Haustoria are induced by extracts of soya bean seeds, flax seeds,plant roots, cotton string, cotton, paper towel, tissue paperand yeast extract. Sugars, hormones, nitrogenous compounds,microbial exudates, and citric and ascorbic acid do not inducehaustoria. Citric acid has a positive synergistic affect inthe presence of an inducing factor. Terramycin applied to seedlings 3 days of age had a detrimentaleffect on root development, while penicillin did not. Penicillinreduced the numbers of haustoria formed between 19 and 25 daysof age. The combined use of penicillin and inducing factorsmay provide an excellent experimental system for determiningthe temporal and developmental pattern of haustoria formation. haustoria, chemical induction and repression, Orthocarpus purpurascens, hemiparasite     

Morphological Response of Witchweed (Striga asiatica) to In Vitro Culture

Excised sorghum root segments (5–10 mm in length) werecultured for 50 d in four different liquid media containingmineral salts, vitamins, amino acids, glucose, and IAA. Theroots were removed and the remaining medium was solidified withan equal volume of warm 1–6% water agar. Dry unconditionedor conditioned Striga asiatica seeds were transferred to themedium. Some of the seeds germinated and developed into parasitic-typeseedlings. These seedlings had haustoria, tubercles, dense roothairs, branched shoots, and multiple shoot-borne adventitiousroots. The plumule pole developed into a shoot, but the radiclepole displayed only rudimentary development. On the same media,but which had not previously been used to grow sorghum roots,the seedlings displayed a well-developed radicle-derived rootsystem, but the plumule did not grow. Shoots began to appearon the roots only after 35–50 d of culture. These seedlingshad no haustoria, no tubercles, few or scattered root hairs,no shoot-borne adventitious roots and few shoot branches, andappeared to be non-parasitic-type seedlings. Shoots grew ina medium supplemented with IAA and kinetin, but did not in amedium containing NAA plus IBA. On replacement of glucose andIAA with sucrose and 2,4-D, respectively, Striga seeds germinated,and the heart-shaped embryos dedifferentiated into calli. Thecalli have been maintained by subculturing for over 9 months.The results demonstrated that a host signal, in addition tothose for germination and haustorium formation, is requiredfor further development. Moreover, morphogenesis of culturedS. asiatica is influenced by exogenous growth regulators. Key words: Striga asiatica, parasitic weeds, haustoria, Sorghum bicolor, in vitro culture     

Growth, Resource Allocation and Haustorial Biology of the Root Hemiparasite Olax phyllanthi (Olacaceae)

Olax phyllanthi was found to parasitize a wide range of taxain the native habitat in coastal heath, South-West Australia.All major life and growth forms were regularly exploited, includingmost woody dicotyledons (except members of the Myrtaceae), afew monocotyledons and cohabiting root hemiparasites. Initiationof haustoria occurred mostly in autumn (southern hemisphere),with some senescing the following summer, and others survivinguntil replaced by a new generation of haustoria the followingautumn. Seedlings increased in dry matter and contents of N,P, K, Mg and Ca during preparasitic development, but did notsurvive beyond 6 months if failing to establish haustoria ona suitable host. Plant dry matter, mineral content and haustorialnumber increased exponentially during subsequent parasitic development.Mean shoot:root d. wt ratios of 1st to 3rd-year plants lay withinthe range 1.2–1.4. Root systems were laterally extensiveand restricted to the top 40 cm of rooting substrate. Mean totalroot lengths of 2nd- and 3rd-year plants were 7.1m (n = 5) and60.9 m (n = 5), respectively. Haustoria comprised 0.7–3.5%of plant d. wt, with a mean of 7.9 haustoria (n = 10, 2nd- and3rd-year plants) per metre of root length. Comparisons of mineralconcentrations in dry matter of O. phyllanthi and of a rangeof commonly parasitized hosts showed the parasite to be muchricher on average than its hosts in K, P, and to a lesser extentin N, but not noticeably different in Mg and Ca. Olax, root hemiparasite, mineral nutrition, haustoria, resource allocation     

Does legume nitrogen fixation underpin host quality for the hemiparasitic plant Rhinanthus minor?

The high quality of leguminous hosts for the parasitic plantRhinanthus minor (in terms of growth and fecundity), comparedwith forbs (non-leguminous dicots) has long been assumed tobe a function of the legume's ability to fix atmospheric nitrogen(N) from the air and the potential for direct transfer of compatibleamino compounds to the parasite. Using associations betweenRhinanthus minor and Vicia faba (Fabaceae) that receive N eitherexclusively via symbiotic associations with rhizobia supplyingorganic N fixed from N₂ or exclusively through the supply ofinorganic nitrate to the substrate, the underlying reasons forthe quality of legumes as hosts for this parasite are unravelled.It is shown that sole dependence of the host, V. faba, on Nfixation results in lower growth of the attached parasite thanwhen the host is grown in a substrate supplied exclusively withinorganic N. In contrast, the host plants themselves achieveda similar biomass irrespective of their N source. The physiologicalbasis for this is investigated in terms of N and abscisic acid(ABA) partitioning, haustorial penetration, and xylem sap aminoacid profiles. It is concluded that legume N fixation does notunderpin the quality of legumes as hosts for Rhinanthus butrather the well-developed haustorium formed by the parasite,coupled with the lack of defensive response of the host tissuesto the invading haustorium and the presence of sufficient nitrogenouscompounds in the xylem sap accessible to the parasite haustoria,would appear to be the primary factors influencing host qualityof the legumes. Key words: ABA, haustorium, legume, nitrogen fixation, nodules, parasitic plant Received 14 November 2007; Revised 7 January 2008 Accepted 8 January 2008     

Nutritional studies of Hemiparasitic Orthocarpus (Scrophulariaceae)

Excised shoot tips from Orthocarpus attenautus and O. purpurascenswere cultured in vitro to ascertain whether the stem tips ofthese hemiparasites require complex organic substances for theirdevelopment, and to determine if the capacity for haustoriaformation is retained by the resulting plantlets. Explants consistedof the apical meristem plus the four smallest leaf primordia,having a volume of less than 0.5 mm3. A variety of mineral media,sucrose concentrations, root extract, soil extract, yeast extract,and malt extract were tested for effects on growth. Both speciescompleted development in sterile culture on simple media. Orthocarpusattenuatus grew best on Knop‘s minerals with Ball’smicroelements + 0.1 g ferric citrate 1–1+ 1% (w/v) sucrose,while MurashigeSkoog‘s minerals+ 2% sucrose provided thebest growth of the media tested for O. purpurascens. Haustoriaformed on the roots of all plantlets chemically induced by cottonstring. The mean number of haustoria per plantlet was abouthalf that of control plants raised from seed. Growth of intactO. Purpurascens seedlings was also compared on mineral agar,mineral agar supplemented with yeast extract, and in soil culturessupplemented with yeast extract and a host. While yeast extracthas variable effects on the growth of shoot tip explants andintact plants raised under axenic conditions, it is highly stimulatoryto the autotrophic growth of intact plants in soil culture.     

Host influence on floral variability inOrthocarpus densiflorus (Scrophulariaceae)

This study investigates the influence of host plants on the expression of floral variability in the hemiparasiteOrthocarpus densiflorus (Scrophulariaceae), to determine if interaction with host plants can facilitate the expression of normally hidden genes. Style extension inO. densiflorus is highly uniform in natural populations, with less than 1% of the individuals exhibiting an aberrant extended-style condition. Both seed treatment with giberellic acid and parasitic development with host plants significantly facilitates the expression of extended-style in progenies that show only minimal expression when grown autotrophically. Artificial selection experiments demonstrate the range of potential phenotypic variation to be under genetic control, and differentially influenced by the host environment. Although the biochemical basis of the host influence is unknown, some host plants produce stimuli that apparently disrupt buffering inO. densiflorus and expose normally unexpressed genetic variability to selection. If variation in other traits is similarly affected, the influence of host chemistry may provide parasitic plants with a greater than normal opportunity for rapid selectional change.     

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

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

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     

寄生植物对寄主植物的化学识别

植物间寄生关系的研究近年来受到了广泛的重视。大量的研究表明,寄主释放的次生物质对植物间寄生关系的建立和维持起了重要的调节作用。寄主植物的次生物质对寄生植物的化学防御和昆虫授粉等生态功能起重要的作用,寄主植物次生物质对寄生植物生理与生态的调节作用是受寄生植物基因调节的。更为重要的是寄主植物释放的次生物质成为寄生植物的种子萌发和吸器发生的异源识别物质。能够刺激寄生植物种子萌发的次生物质主要是倍半萜和氢醌类物质,而诱导吸器发生的物质则是酚酸、醌和黄酮类化合物,诱导吸器发生的核心结构是对苯醌。这些异源识别物质大多是寄主植物释放的化感抑制物质,显示寄生植物在化学防御方面要比寄主植物高级。异源识别化合物的活性与其氧化潜力显著相关。由于寄生植物中存在一抑制异源识别物质诱导吸器发生的调节过程,因此吸器的产生与寄生植物根部接触异源识别物质的浓度与时间呈正相关关系,这一调节过程对寄生植物准确识别寄主并寄生其上是十分重要的。对寄生植物和寄主植物间的化学识别关系的揭示有助于人们防治有害寄生植物和开发利用有价值的寄生植物资源。     

OBSERVATIONS ON ROOT-PARASITISM IN CORDYLANTHUS (SCROPHULARIACEAE)

Eight species of Cordylanthus (Scrophulariaeeae: tribe Euphrasieae) representing the three major sections of the genus were successfully grown both with and without a host plant, demonstrating that the plants are facultative hemi-parasites capable of completing their life cycle without a host under the favorable conditions of greenhouse culture. The plants are almost certainly parasitic in the wild. Even though haustorial connections were formed by Cordylanthus on the roots of all host plants provided, most species were more vigorous when growing with certain hosts (Quercus, Pinus, Helianthus) than with others (Plantago, Phleum). This indicates a difference in the ability of the parasite to obtain materials successfully from the various plants used as hosts. Natural hosts are inferred for 15 species of Cordylanthus from the presence of associated species in nature. The haustoria are exogenous in origin, being formed by dedifferentiation of several cells in the hypo-dermal or subhypodermal layers of the rootlets. Anatomically the haustoria differ from those reported for other members of Euphrasieae in having secondary vascular structure and abundant connecting strands. The vascular structure of haustoria is composed mainly of specialized vessel elements. Root-parasitism gives Cordylanthus the ability to grow in arid areas during the summer after most other annuals have died, thus successfully equipping the plants for survival and expansion in western North America.     

Santalum album: Kernel Composition, Morphological and Nutrient Characteristics of Pre-parasitic Seedlings under Various Nutrient Regimes

Omission of all nutrients or individual minerals (N, K, P, S,Ca) from the growth medium of pre-parasiticSantalum album seedlingsproduced significant morphological effects. Compared with fullyfertilized seedlings, shoot and root length, leaf area, leaflength and width, leaf number, internode length, shoot length/rootlength ratio, chlorophyll levels, fresh mass and dry mass wereall reduced; leaf thickness increased and stomatal numbers,petiole lengths, root lengths and haustorial numbers showedvariable but significant responses. Mineral-specific deficiencysymptoms were evident in all omission treatments after 3 months.Treatments in which N, K or all minerals were omitted had thegreatest dwarfing effect and increase in height and leaf numberceased after 3 months: only seedlings with no added nutrients(dependent on kernel nutrition) were healthy. The mineral contentof no-nutrient seedlings approximated that in kernels. The distributionof minerals between plant parts for each treatment is given.K concentrations and K/Ca ratios were relatively high in leavesof seedlings which received treatments containing K, especiallywhen Ca was absent. A high K/Ca ratio may be inherent inS. albumseedlings rather than acquired after subsequent host connection.All fully fertilized seedlings developed haustoria within 3months suggesting readiness for early host attachment. Seedlingsin no-nutrient, no-N, no-K or no-P treatments had fewer haustoria. Santalum album ; Indian sandalwood; sandalwood kernels; sandalwood seedlings; nutrient omission; healthy characteristics; deficiency characteristics     

蔗糖对列当科两种根部半寄生植物吸器发生的促进作用

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

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 seedlingsof Nuytsia floribunda were assessed after a 12 month periodof association singly with each of a range of potential woodyhost species. One species,Adenanthos cygnorum , of similar sizeto most parasitized hosts, served as measure of response ofNuytsia in a non-benefiting situation. Rated on this basis,all 23 parasitized hosts elicited greater mean dry weights ofNuytsia than when on Adenanthos, and seven of these instanceswere highly significant. Numbers and weights of penetratingand presumably functional haustoria formed on a host were broadlycorrelated with growth benefit to Nuytsia, but there were notableinstances of unusually poor or great benefit from a host relativeto the complement of haustoria involved. Experiments in whichhaustoria-bearing associations of Nuytsia partnered with nodulatedAcacia hosts (Acacia acuminata and A. cyclops) were fed¹⁵N₂showedsignificant transfer of¹⁵N to the parasite, but failed to determinewhether the label had been acquired through haustoria or directlyby Nuytsia roots following turnover of nodule and root residuesof the host in the rooting medium. A parallel study using theunusual non-protein amino acid, djenkolic acid, as a markerof benefit from the djenkolic acid-containing host A. cyclops,showed appearance and progressive build-up of the compound infoliage of Nuytsia over a 6 month period after partnering thespecies in pot culture. Presence of the compound at final harvestin xylem sap of both partners but not in soil solution of thecultures strongly indicated xylem transfer via haustoria asthe principal avenue for N benefit to the parasite. Resultsare discussed in relation to a recent evaluation of haustorialstructure and functioning of N. floribunda. Copyright 2000 Annalsof Botany Company Root hemiparasite, Nuytsia, Loranthaceae, growth benefit, haustorial production, nitrogen transfer from hosts     

Haustoria of Cuscuta japonica, a Holoparasitic Flowering Plant, Are Induced by the Cooperative Effects of Far-Red Light and Tactile Stimuli

When seedlings of Cuscuta japonica were grown with Vigna radiata(the host plant) in a flower pot for 6 d under white light andthen irradiated with far-red or blue light (ca. 6 µmolphotons m^–2 s^–1), the seedlings parasitized V. radiata.However, no parasitism of the seedlings was observed under redor white light or in darkness. The parasitic behavior of seedlingsof C. japonica was observed even if an acrylic rod was usedas a substitute for the host plant. Upon incubation under far-redlight, the seedling twined tightly around the rod and developedhaustoria towards it. Haustoria also developed when apical andsubapical regions of seedlings were held between two glass platesthat were about 0.7 mm apart and were irradiated with far-redlight. However, no haustoria were induced by either the holdor irradiation alone. These results indicate that parasitismof Cuscuta japonica is controlled by the cooperative effectsof two physical signals, far-red light and appropriate tactilepressure. Our findings suggest that parasitism by the genusCuscuta involves a novel strategy. (Received April 10, 1996; Accepted August 21, 1996)     

Tropism and Morphogenesis of Striga Seedlings in the Host Rhizosphere

Using an agar culture technique, studies have been made on thegrowth and development of Striga asiaticaseedlings in the rhizosphereof a host plant(Sorghum Vulgare). The host root exerts a definitepositive chemotropic influence over the growth of the parasiteradicle in the immediate vicinity of the host root, but notfurther. The host root also produces a morphogenic factor whichinduces striking changes in the parasite seedlings. The mechanismof tropism and the significance of the morphogenic factor inthe host rhizosphere are discussed.     

植物寄生对生态系统结构和功能的影响

寄生植物是生态系统中的特殊类群之一。植物寄生可以驱动生态系统中生物与非生物因子的变化,在生态系统结构与功能中起关键作用。寄生植物可以通过对寄主营养的集聚、改变凋落物的质量与数量、改变根的周转与分泌物格局、改变土壤水势,从而影响土壤理化特性。寄生植物会改变寄主的行为,改变寄主与非寄主植物之间的相互作用,从而影响植物群落的结构、多样性和动态,进而影响植被演替和植被生产力等。寄生植物与寄主均可被消费者取食,可直接或间接地影响生态系统的食草动物,包括草食昆虫等。寄生植物与寄主的其它寄生物存在竞争关系,可以直接或间接地影响寄主的其它寄生植物或病原真菌。寄生植物可以明显地改变土壤地球化学循环,将固有的不可动的成分转变为可利用的营养成分,改变土壤生物群落的结构与功能,从而显著影响地下生物群落。这些表明,植物寄生对生态系统的结构和功能有重要影响。针对特殊的被入侵的植物群落,该地寄生植物可以通过影响入侵植物寄主的生长、繁殖、生物量分配格局,改变土壤的理化特性,促进非寄主的非优势本地植物的生长,从而改变被入侵植物群落结构与多样性,达到生物防治及生态恢复的目的。     

Mineral Nutrition of Sandalwood (Santalum spicatum)

Acacia acuminata is a preferred host of the root hemiparasitictree, Santalum spicatum (sandalwood). Comparison between nutrientcontent of adult trees of sandalwood and results for an earlierstudy of the mistletoe, Amyema preissii, on the same host species,A. acuminata, showed similar high levels of K and Na and lowlevels of Zn in both parasites compared with the host plants.Differences in K, Ca, N and Cu levels between parasitized anduninfected Acacias imply that the host plant contributes tothe nutrition of sandalwood. The high K/Ca ratio in sandalwoodconfirms that K uptake in preference to Ca is a general featureof all categories of angiosperm parasites. Patterns of distribution of nutrients between various partsof sandalwood and A. acuminata depend on the type of nutrient,but levels are usually highest in leaves of both species andthe haustoria. Although K, Ca and Na are much lower in the kernelsthan in vegetative parts of the parasite, only seedlings withoutsupplementary Ca in a nutrient omission experiment failed togrow at all in the absence of hosts. Growth is not dependenton the level of K in the unattached plants but other evidenceindicates it may have a role in water uptake in the attachedplant. Calcium supply has a marked effect on internal Ca levelsand growth of unattached plants. Compared with field plants,levels of Ca, and to a lesser extent Zn, were much higher inplants of the Ca/K treatment that produced greatest growth over34 weeks. Haustorial formation is enhanced by the presence of A. acuminataroots. However, competition for nutrients, especially Ca, fromco-planted A. acuminata seedlings results in suppression ofgrowth of young sandalwood compared with their growth in theabsence of the host species. Key words: Mineral nutrients, Santalum spicatum, Acacia acuminata, hemiparasites, K/Ca nutrition, seeds