Localized hormone fluxes and early haustorium development in the hemiparasitic plant Triphysaria versicolor

Perhaps the most obvious phenotypes associated with chemical signaling between plants are manifested by parasitic species of Orobanchaceae. The development of haustoria, invasive root structures that allow hemiparasitic plants to transition from autotrophic to heterotrophic growth, is rapid, highly synchronous, and readily observed in vitro. Haustorium development is initiated in aseptic roots of the facultative parasite Triphysaria versicolor when exposed to phenolic molecules associated with host root exudates and rhizosphere bioactivity. Morphological features of early haustorium ontogeny include rapid cessation of root elongation, expansion, and differentiation of epidermal cells into haustorial hairs, and cortical cell expansion. These developmental processes were stimulated in aseptic T. versicolor seedlings by the application of exogenous phytohormones and inhibited by the application of hormone antagonists. Surgically dissected root tips formed haustoria if the root was exposed to haustorial-inducing factors prior to dissection. In contrast, root tips that were dissected prior to inducing-factor treatment were unable to form haustoria unless supplemented with indole-3-acetic acid. A transient transformation assay demonstrated that auxin and ethylene-responsive promoters were up-regulated when T. versicolor was exposed to either exogenous hormones or purified haustoria-inducing factors. These experiments demonstrate that localized auxin and ethylene accumulation are early events in haustorium development and that parasitic plants recruit established plant developmental mechanisms to realize parasite-specific functions.     

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%...     

檀香幼苗半寄生性初步研究

在不同寄主植物繁殖的基础上,研究了檀香(Santalum album L．)幼苗对寄主植物的半寄生性。檀香种子发芽及幼苗生长初期,并不需要寄主植物的参与,但随后的生长其根系必须寄生于适宜的寄主植物的根上。不同寄主植物对檀香幼苗的生长和吸器的发育影响不同,表现在根寄生吸器的数量、大小和结合的程度上。初步筛选了扶桑(Hibiscus rosa-sinensis)、烂头钵(Phyllanthus reticulatus)等优良的檀香幼苗寄主植物。檀香幼苗根系极不发达,细根缺乏根毛,但其导管非常发达,有利于从寄主根吸收养分和水分。此外还观察了檀香和寄主植物扶桑建立半寄生吸器的过程。     

檀香吸器维管组织的个体发育

为了解檀香吸器维管组织的发育过程,采用激光共聚焦显微镜、光学显微镜和透射电镜观察檀香吸器维管组织的个体发育。结果表明,檀香维管组织的分化分为两个时期:入侵前和入侵后。吸器维管组织发育始于盘状吸器时期,起源于吸器基部具有分生能力的细胞,后分为两束。侵入前无向顶的分化,处于吸器基部。侵入后随吸管深入寄主根与寄主根维管束连通,形成具有吸收功能的维管组织。成熟吸器维管组织呈倒烧瓶结构,仅处于吸器烧瓶核心两边,由木质部组成而无韧皮部。檀香的吸器维管组织发育有两个因素诱导,一个是遗传因素,另一个为寄主。这些为檀香半寄生性特性研究提供了形态解剖学基础。     

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.     

A species-specific recognition system directs haustorium development in the parasitic plant Triphysaria (Scrophulariaceae)

Parasitic plants use host molecules to trigger developmental programs essential for parasitism. One such program governs the initiation, development, and function of haustoria, parasite-specific organs responsible for attachment and invasion of host tissues. Haustoria development can be initiated by several different molecules produced by appropriate host species. We are interested in understanding how these signals are interpreted by two related facultative parasites, Triphysaria eriantha (Benth.) Chuang and Heckard, and T. versicolor Fischer and C. Meyer, to distinguish their own roots from those of potential hosts. We used an in vitro bioassay to determine what proportion of different Triphysaria populations formed haustoria in the presence and absence of closely related and unrelated host species. We found that the proportion of plants with haustoria was the same whether the plants were grown in isolation or with a conspecific host. In contrast, a significantly higher proportion of plants made haustoria when the host was a congeneric Triphysaria. Plants with haustoria neither enhanced nor inhibited other plants' propensity to form haustoria. Together these results indicate that qualitative differences exist in haustorium-inducing factors exuded by closely related species. The highest proportion of Triphysaria had haustoria when grown with Arabidopsis thaliana (L.) Heynh. Even in this case, however, some Triphysaria failed to develop haustoria. Interestingly, the percentage of haustoria that had vessel elements was higher when connections were made with Arabidopsis than with another Triphysaria. These results demonstrate that host recognition can be manifested at multiple points in haustorium development. Received: 18 December 1996 / Accepted: 14 February 1997     

The parasitic mechanism of the holostemparasitic plant Cuscuta

Cuscuta is a stem holoparasitic plant without leaves or roots, which develops a haustorium and sucks nutrients from host plants. The genus Cuscuta comprises about 200 species, many of which can cause severe problems for certain crops. The parasitic process in Cuscuta begins in finding and attaching to a host plant and then developing a haustorium. The process does not always require any chemical signal, but does require a light signal. Finding a host involves detecting the lower red light:far-red light ratio near a potential host plant by phytochrome. A contact signal is also necessary for haustorium induction. Apparently, cytokinin increase is downstream of the light and contact signal and is critical for haustorium induction. This pathway, however, appears to be slightly different from a standard pathway. The direct connection between Cuscuta and its host involves both the xylem and phloem, and mRNA and proteins can translocate. Several features indicate that Cuscuta is a useful model plant for parasite plant research as well as plant–plant interaction research. These include the simple anatomical structure and seedling development, no chemical requirement for haustorium induction, and the wide range of host plants.     

Evolution of the secondary haustoria to a primary haustorium in the parasiticScrophulariaceae/Orobanchaceae

In the parasiticScrophulariaceae andOrobanchaceae, two types of contact organs exist: secondary and primary haustoria. Secondary haustoria are lateral organs, developing in large numbers and only when the seedling is fully established. In contrast, a primary haustorium represents the first developmental stage of the seedling itself. In the root system of the parasiticLesquereuxia syriaca (=Siphonostegia syriaca) there are only secondary haustoria, but a few of them apparently develop in a terminal position. This is achieved by transferring the haustorial initiation region closer to the root apex. One can interpret this as a transformation of the apical meristem into a meristematic haustorial tissue. On the condition that an extreme shortening (abbrevation) of the primary root could happen, we discuss the transformation of the terminal secondary into a primary haustorium.     

EXPERIMENTAL INITIATION OF HAUSTORIA IN AGALINIS PURPUREA (SCROPHULARIACEAE)

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

Metabolite changes with induction of Cuscuta haustorium and translocation from host plants

Cuscuta is a stem holoparasitic plant without leaves or roots, parasitizing various types of host plants and causing major problems for certain crops. Cuscuta is known as a generalist and, thus, must have unique parasite strategies to cope with different host plants. For elucidating metabolic responses and mechanisms of parasitization, metabolomic approaches using GC/MS were applied. We compared five stages of Cuscuta japonica: early stage seedlings, with far red light (FR) cue, with contact signal, haustorium induced seedlings by both signals and adult plant parasites on host plants. Sugars, amino acids, organic acids, nucleic acids, and polyols were identified from the polar phase fraction. The apical part contained metabolite profiles different from the haustorium induced part or the basal part. Amino acid and some organic acids were up-regulated for haustorium induction but decreased after parasitization. After attachment to different host plants, metabolite profiles of Cuscuta japonica changed dramatically due to the absorption of specific host plant metabolites such as pinitol. Cuscuta seedlings attached to pinitol rich host plants contained more pinitol and showed different profiles from those attached to plants having less or lacking pinitol.     

锁阳和肉苁蓉寄生方式的区别

锁阳和肉苁蓉都是中医药里重要的补益类药材,但由于过度采挖和采挖方式不当,目前它们的野生资源已濒临枯竭。肉苁蓉和锁阳分别是我国濒危和易危珍稀植物,研究二者寄生方式的特点与区别不仅可以促进锁阳和肉苁蓉的人工栽培,从而使野生药材得到一定的保护,而且对了解寄生植物在荒漠地区等极端严酷环境中的适应机制具有重要的生态学意义。该研究采用形态学观察结合常规石蜡切片法,对锁阳和肉苁蓉分别在各自寄主植物上的寄生方式进行了研究。结果表明：(1)锁阳的营养繁殖体在寄主植物根部呈串状分布,与寄主植物的连接方式属于非末端寄生;锁阳的吸器侵入寄主根系韧皮部和木质部的一部分区域,但是韧皮部和木质部大部分区域未被锁阳吸器占据,即有部分营养物质被锁阳“截取”。(2)肉苁蓉在其肉质茎基部长出新的芽体,与寄主植物的连接方式属于末端寄生;肉苁蓉的吸器侵入寄主根韧皮部和木质部全部区域。因此,锁阳寄生后,被寄生的寄主根依然能够向前生长,具有正常的功能;肉苁蓉寄生后,寄生点的寄主根失去根系的正常功能,成为一个为肉苁蓉生长发育提供营养物质的“输送通道( Transport channel)”。     

An anatomical study of the haustoria of Rhinanthus minor attached to roots of different hosts

Roots of a range of potential hosts responded differently when Rhinanthus minor attempted to form haustoria. Roots of Fabaceae show the weakest reaction as apart from slight lignification, no reaction was observed at the interface between the endophyte and the cortical tissue of the host root. Grass roots react with strong lignification of all cells within the stele with the exception of a small number of phloem cells whilst the endodermis fully enters the tertiary stage. In the case of Phleum bertolonii the cortical cells also become lignified. The lignification is even observed in the host root tissue in a distance of about 1 mm from the haustorium (both apically and basipetally). In the case of Leucanthemum vulgare, strong suberisation can be observed in the cell walls of the interface between endophyte (tip of the sucker) and host. Plantago lanceolata exhibits the strongest reactions against the haustorial tissues. Cells of the interface between the endophyte and the host cortex are completely destroyed, as well as a few cell layers outside the central xylem cylinder, even in some distance from the haustorium. Thus, host xylem is completely isolated from the haustorium in this case. Extraction of sap from xylem vessels is likely to be drastically impaired in such a situation.     

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。     

Comparative haustorium morphology and vegetative reproduction in the Old World genus Scurrula L. (Loranthaceae) from the Central Nepal Himalayas

In a comparative morphological study of haustoria in the four Scurrula species occurring in the Annapurna Conservation Area (ACA) of Nepal, only one basic type of haustorium was found, regardless of host and elevation, namely a primary haustorium (leading to a wood rose) with epicortical root. The endophytic system of the primary haustorium, however, showed differences, which allowed classification into (1) flanging endophyte, (2) flanging endophyte with radial shaft, and (3) sinker endophyte. Scurrula elata and Scurrula gracilifolia showed marked differences in haustorium morphology on different hosts. Scurrula elata is most successful in establishing over a wide range of hosts due to its specific haustorium morphology and endophyte system. Epicortical roots of the genus Scurrula resemble those of other Old World Loranthaceae morphologically and functionally, but differ significantly from New World species. Vegetative reproduction by means of epicortical roots, which develop secondary haustoria and shoots, is very common in all four Scurrula species. The number of root-borne shoots produced by secondary haustoria varied among Scurrula species, depending on the age of the mistletoes as well as the length and vigour of the roots.     

An haustoria inducer for the root parasite Agalinis purpurea

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