寄生植物概述

对寄生植物的概念、寄生类型、寄生生物学特性和经济价值与危害进行了总结与概括,并对世界和我国广泛分布的重点寄生植物进行了着重介绍。对国内外目前关于寄生植物种子萌发刺激物质、吸器形成诱导物质以及寄生植物与寄主植物间形成寄生关系的作用机理进行了探讨。     

根寄生植物种子萌发刺激物研究进展

根寄生植物是被子植物中一类寄生在寄主根部, 以摄取寄主水分和营养物质为生的特殊植物类群, 其种子萌发需要寄主萌发刺激物的诱导。该文主要阐述根寄生植物种子萌发的特异性, 以及目前已发现的刺激种子萌发的信号物质及其调节机制和生物合成途径, 并就萌发刺激物的识别机制及其在根寄生植物或丛枝菌根真菌与寄主建立寄生关系过程中所起的作用进行综述, 提出根寄生植物种子萌发研究中存在的问题, 并对其研究前景进行了展望。     

根寄生植物吸器发育及其调控的研究进展

很多根寄生植物是重要的杂草,严重影响农业生产。吸器是寄生植物与寄主连接的特异性器官,是寄生植物与寄主间信号和物质交流的"桥梁",而吸器诱导因子在诱导吸器发育过程中扮演着重要角色。该文针对寄生植物吸器的发育过程,吸器诱导因子的种类、信号转导和基因调控,寄生植物如何避免寄主的防御这三方面的进展进行了综述,以期为进一步揭示寄生植物识别寄主的信号物质的分子机制提供参考。     

云南哀牢山两种常见半寄生植物的生态化学计量特征及其与寄主的关系

为深入探究半寄生植物与其寄主间的养分关系,在云南哀牢山徐家坝地区选取两种常见半寄生植物椆树桑寄生(Loranthus delavayi)和柳叶钝果寄生(Taxillus delavayi),研究其寄主枝条-吸器-寄生枝条-寄生叶片间的碳(C)、氮(N)、磷(P)生态化学计量特征关系。结果表明:1)两种半寄生植物的寄主枝条-吸器-寄生枝条-寄生叶片这一连续体的C、N、P生态化学计量特征变化趋势并不完全相同,具有物种自身的特性。2)同一半寄生植物的寄主枝条具有相似的C、N、P生态化学计量特征,寄主物种对半寄生植物的生态化学计量特征没有显著影响。3)寄主枝条的C、N、P生态化学计量特征间具有紧密的相关关系,吸器弱于寄主枝条,寄生枝条弱于吸器,寄生叶片的N、P含量相关关系不显著。4)寄主枝条与寄生叶片间的C含量存在极显著负相关关系。5)吸器与寄主枝条间的C、N、P生态化学计量特征存在紧密的相关关系,但在生态化学计量特征的数值上吸器与寄生枝条更为接近。吸器作为连接寄主与寄生植物的关键部位,其与寄主枝条极为显著的相关关系体现了寄主枝条养分对寄生植物的重要性,而吸器在元素含量、计量比的数值以及相互关系上与寄生枝条更为相似,则体现了吸器作为寄生植物器官具有与寄生枝条相似的生理功能。通过对寄主枝条-吸器-寄生枝条-寄生叶片C、N、P生态化学计量特征的分析,为深入研究半寄生植物的养分利用策略与生态适应性提供了重要的基础资料。     

云南哀牢山两种常见半寄生植物的生态化学计量特征及其与寄主的关系

为深入探究半寄生植物与其寄主间的养分关系, 在云南哀牢山徐家坝地区选取两种常见半寄生植物椆树桑寄生(Loranthus delavayi)和柳叶钝果寄生(Taxillus delavayi), 研究其寄主枝条-吸器-寄生枝条-寄生叶片间的碳(C)、氮(N)、磷(P)生态化学计量特征关系。结果表明: 1)两种半寄生植物的寄主枝条-吸器-寄生枝条-寄生叶片这一连续体的C、N、P生态化学计量特征变化趋势并不完全相同, 具有物种自身的特性。2)同一半寄生植物的寄主枝条具有相似的C、N、P生态化学计量特征, 寄主物种对半寄生植物的生态化学计量特征没有显著影响。3)寄主枝条的C、N、P生态化学计量特征间具有紧密的相关关系, 吸器弱于寄主枝条, 寄生枝条弱于吸器, 寄生叶片的N、P含量相关关系不显著。4)寄主枝条与寄生叶片间的C含量存在极显著负相关关系。5)吸器与寄主枝条间的C、N、P生态化学计量特征存在紧密的相关关系, 但在生态化学计量特征的数值上吸器与寄生枝条更为接近。吸器作为连接寄主与寄生植物的关键部位, 其与寄主枝条极为显著的相关关系体现了寄主枝条养分对寄生植物的重要性, 而吸器在元素含量、计量比的数值以及相互关系上与寄生枝条更为相似, 则体现了吸器作为寄生植物器官具有与寄生枝条相似的生理功能。通过对寄主枝条-吸器-寄生枝条-寄生叶片C、N、P生态化学计量特征的分析, 为深入研究半寄生植物的养分利用策略与生态适应性提供了重要的基础资料。     

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%...     

肉苁蓉寄生生长形态发育

肉苁蓉(Cistanche deserticola)是一种沙生根寄生濒危药用植物。利用光镜和电镜手段, 采用人工培养和诱导等方法详细观察并研究了肉苁蓉寄生生长过程中种子萌发、吸器产生以及植物体形态发育的过程。结果表明: (1)人工可以诱导肉苁蓉的种子萌发。肉苁蓉种胚具有明显的极性, 珠孔端细胞小于合点端, 珠孔端细胞分化、生长并产生白色的类胚根状结构。(2)有些化学物质可以诱导初生吸器的产生。用2, 6二甲氧基-对苯醌培养肉苁蓉24-26小时后, 其先端膨大并产生突起, 形成类似根毛状的结构, 即初生吸器。(3) 肉苁蓉属于主动寄生植物, 其在初生吸器与寄主幼根黏连后产生次生吸器。肉苁蓉与寄主梭梭(Haloxylon ammodendron)共培养, 其初生吸器主动与寄主幼根(0.1 mm左右)黏连, 穿过寄主根表皮和皮层后与维管束连通形成次生吸器, 肉苁蓉植物体分化、发育的基本组织形成。被寄生后的寄主根横向发育加快, 同时肉苁蓉植物体开始分化和发育。(4)肉苁蓉可以寄生在寄主幼根(0.1 mm左右)的任意部位。     

植物间挥发物信号的研究进展

植物VOCs信号是植物间进行信息交流的"语言",可由多种生物和非生物因素诱导产生。非寄生植物释放的VOCs信号可影响其所在群落中其它植物的种子萌发与幼苗生长;而寄主植物释放的VOCs信号却是诱导寄生植物种子萌发和幼苗生长的信号物质。VOCs作为植物间的伤害信息可以诱导临近的同种或异种植物做好防御准备,从而通过主动或间接防御以减少外界的伤害。植物间通过VOCs信号进行信息交流,从而实现其繁衍与防御。该文通过对VOCs信号的种类、诱导产生因素、传递及作用进行综述,以期对VOCs信号的研究有所帮助。     

植物间挥发物信号的研究进展

植物VOCs信号是植物间进行信息交流的“语言”, 可由多种生物和非生物因素诱导产生。非寄生植物释放的VOCs信号可影响其所在群落中其它植物的种子萌发与幼苗生长; 而寄主植物释放的VOCs信号却是诱导寄生植物种子萌发和幼苗生长的信号物质。VOCs作为植物间的伤害信息可以诱导临近的同种或异种植物做好防御准备, 从而通过主动或间接防御以减少外界的伤害。植物间通过VOCs信号进行信息交流, 从而实现其繁衍与防御。该文通过对VOCs信号的种类、诱导产生因素、传递及作用进行综述, 以期对VOCs信号的研究有所帮助。     

肉苁蓉寄生生长形态发育

肉苁蓉(Cistanche deserticola)是一种沙牛根寄生濒危药用植物.利用光镜和电镜手段,采用人工培养和诱导等方法详细观察并研究了肉苁蓉寄牛牛长过程中种子萌发、吸器产生以及植物体形态发育的过程.结果表明:(1)人工可以诱导肉苁蓉的种子萌发.肉苁蓉种胚具有明显的极性,珠孔端细胞小于合点端,珠孔端细胞分化、生长并产生白色的类胚根状结构.(2)有些化学物质可以诱导初生吸器的产生.用2,6二甲氧基-对苯醌培养肉苁蓉24-26小时后,其先端膨大并产生突起,形成类似根毛状的结构,即初生吸器.(3)肉从蓉属于主动寄生植物,其在初生吸器与寄主幼根黏连后产生次生吸器.肉苁蓉与寄主梭梭(Haloxylon ammodendron)共培养,其初生吸器主动与寄主幼根(0.1 mm左右)黏连,穿过寄主根表皮和皮层后与维管束连通形成次生吸器,肉苁蓉植物体分化、发育的基本组织形成.被寄生后的寄主根横向发育加快,同时肉从蓉植物体开始分化和发育.(4)肉苁蓉可以寄生在寄主幼根(0.1 mm左右)的任意部位.     

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.     

Parasites on parasites: hyper-, epi-, and autoparasitism among flowering plants

All organisms engage in parasitic relations, as either parasites or hosts. Some species may even play both roles simultaneously. Among flowering plants, the most widespread form of parasitism is characterized by the development of an intrusive organ called the haustorium, which absorbs water and nutrients from the host. Despite this functionally unifying feature of parasitic plants, haustoria are not homologous structures; they have evolved 12 times independently. These plants represent ca. 1% of all extant flowering species and show a wide diversity of life histories. A great variety of plants may also serve as hosts, including other parasitic plants. This phenomenon of parasitic exploitation of another parasite, broadly known as hyper- or epiparasitism, is well described among bacteria, fungi, and animals, but remains poorly understood among plants. Here, we review empirical evidence of plant hyperparasitism, including variations of self-parasitism, discuss the diversity and ecological importance of these interactions, and suggest possible evolutionary mechanisms. Hyperparasitism may provide benefits in terms of improved nutrition and enhanced host–parasite compatibility if partners are related. Different forms of self-parasitism may facilitate nutrient sharing among and within parasitic plant individuals, while also offering potential for the evolution of hyperparasitism. Cases of hyperparasitic interactions between parasitic plants may affect the ecology of individual species and modulate their ecosystem impacts. Parasitic plant phenology and disperser feeding behavior are considered to play a major role in the occurrence of hyperparasitism, especially among mistletoes. There is also potential for hyperparasites to act as biological control agents of invasive primary parasitic host species.     

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     

EXPERIMENTAL STUDIES OF HAUSTORIUM INITIATION AND EARLY DEVELOPMENT IN AGALINIS PURPUREA (L.) RAF. (SCROPHULARIACEAE)

In parasitic angiosperms the haustorium, an organ specialized for attachment and penetration of host tissue, functions in the transport of water and nutrients from the host to the parasite. In Agalinis purpurea (L.) Raf. (Scrophulariaceae) these organs are initiated laterally along its roots, opposite a primary xylem pole. Analyses of haustoria distribution and cellular root profiles show that the portion of the root which is most sensitive to haustorial elicitor molecules is the area distal to the zone of elongation and near the root meristem. Sectioned material supports this finding and, further, indicates that the cells which are the first to respond to haustorial elicitors are located in the inner cortex. Haustoria develop rapidly in response to a host root or to isolated chemical elicitors (xenognosins) normally contained in host root exudate. By 6 hr, vacuolation and radial cellular enlargement are observed in the cortex, and a lateral swelling along the root is visible. By 12 hr, cells of the epidermis divide anticlinally to establish a group of densely cytoplasmic cells at the apex of the haustorial swelling. Accompanying these divisions is the differentiation of specialized hair cells which elongate from epidermal cells flanking the presumptive haustorial apex. Next, the internal, radially enlarged cortical cells divide periclinally. Periclinal divisions are subsequently initiated in the pericycle as early as 18 hr post-induction. Cellular division and enlargement continue so that by 24–36 hr a mature pre-contact haustorium is formed. There is a reduction in root elongation concomitant with haustorial initiation. Depending upon the number of haustoria produced, elongation typically returns to the preinduction level within 2 or 3 days.     

Correlations between Haustoria Formation and Parasitic Development in Orthocarpus purpurascens (Scrophulariaceae)

Three lines of evidence correlate the parasitic performane ofOrthocarpus purpuruscens Benth. with numbers of haustoria produced:(i) the pattern of variation in numbers of haustoria producedin agar culture with different chemical stimuli correspondsclosely to the variation pattern of parasite vigour producedby a range of host plants; (ii) the progeny of plants demonstratingvigorous growth with hosts produce significantly more haustoriathan progeny from parents exhibiting weak parasitic development;(iii) conversely, seedlings that produce high numbers of haustoriain agar culture grow significantly better when transplantedwith hosts than do seedlings with low numbers of haustoria.Haustoria-forming potential is heritable, but highly influencedby environmental factors. Potential number of haustoria is aproduct of the concentration and/or quality of haustoria inducingstimuli, and the parasite's individual ability to respond. Intra-populationdifferences in parasitic development appear to be largely dueto the quantity rather than the quality of substrates receivedfrom host plants. haustoria, Orthocarpus purpurarcens, parasitic development     

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.     

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.