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- 1. See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/229607231 Parasitic Plants Chapter · February 2011 DOI: 10.1002/9780470015902.a0021271 CITATIONS 26 READS 16,496 2 authors, including: Diego Rubiales Spanish National Research Council 591 PUBLICATIONS 18,905 CITATIONS SEE PROFILE All content following this page was uploaded by Diego Rubiales on 01 February 2018. The user has requested enhancement of the downloaded file.
- 2. Parasitic Plants Diego Rubiales, Institute for Sustainable Agriculture, CSIC, Córdoba, Spain Henning S Heide-Jørgensen, University of Copenhagen, Denmark Parasitic flowering plants exploit other flowering plants for water and nutrients by specialised structures called haustoria. Part of the haustorium, the intrusive organ, penetrates host tissue to establish contact with the conductive tissue of the host. Parasitic plants occur throughout the world in all types of plant communities except the aquatic. Generally, the parasite weakens the host so it produces fewer flowers and viable seeds or the value as timber is reduced. However, some para- sites, mostly annual root parasites belonging to Oroban- chaceae, may kill the host and cause considerable economic damage when attacking monocultures in agri- culture, and much effort is done to control these harmful parasites. Parasitic Flowering Plants: Numbers, Types and Distribution Known parasitic plants comprise about 4500 species in about 280 genera belonging to 20 families (Figure 1). However, a recent revision of Santalales based on molecular data has added four newfamilies earlier included in Santalaceae (Nickrent et al., 2010). The majority, about 4100 species, are hemiparasites covering most or all their needs for carbon by their own photosynthesis. In adult Rhinanthus minor about 50% of the carbon is derived from the hosts (T?šitel et al., 2010). About 390 species are holoparasites lacking chlorophyll and photosynthesis; hence carbon must be obtained along with water and other nutrients from the host. Both hemiparasites and holoparasites may be either root or stem parasites. Root parasites are attached to host roots and water and other nutrients are supplied partly from the soil through normal roots and partly from the host through haustoria. Stem parasites obtain all water and inorganic nutrients from the host. Parasite ‘look alikes’ are known among green orchids, bromelias and ferns sitting on tree branches, but they neither develop haustoria nor obtain nutrients or water from the branches supporting them. Such plants are called epiphytes. Other ‘look alikes’ have lost all or nearly all chlorophyll and therefore look like holoparasites, but they have a three-part relationship where a mycorrhizal fungus interconnects the chlorophyll-free plant with a normal green plant having photosynthesis. Such plants used to be called saprophytes, but are now called myco-heterotrophic plants or mycotrophic plants. The gymnosperm Para- sitaxus usta has been suggested as a parasite but may better be classified as a mycotrophic plant (Heide-Jørgensen, 2008). Other examples are Monotropa, Sarcodes, some Pyrola and orchids such as Neottia nidus-avis and Corallorhiza trifida. Parasitic plants occur in all climatic zones from northern Greenland to Tierra del Fuego and on all continents except the Antarctica. They can be found in all plant communities except aquatic environments. Water, and particularly shortage of water, is a major driving force in evolution of land plants. However, there is no advantage in being a parasite in an environment where water is no limitation to plant growth. This may explain why parasitic plants are absent from aquatic environments. Within a plant com- munity distribution of parasites may be patchy due to variation in the effect of dispersal vectors. Dispersal of mistletoes by birds is strongly correlated with the behav- iour of the birds which prefer free standing trees, hedges and wood edges but avoid the interior of woods. Root parasites having small seeds are relatively rare in rain- forests, where the air is calm and humid and not in favour of wind dispersed seeds (Kuijt, 1969). The distribution of acceptable/preferred hosts is another factor influencing the distribution of parasites. If a parasite is absent from a certain species this species may still be an acceptable host. The reason for the absence may be ecological such as the lack of a suitable dispersal agent (e.g. birds), or the light conditions may be insufficient for the parasite Advanced article Article Contents . Parasitic Flowering Plants: Numbers, Types and Distribution . Haustoria . Systematic Overview . Parasitic Plants as Weeds . Management Online posting date: 15th February 2011 ELS subject area: Plant Science How to cite: Rubiales, Diego; and Heide-Jørgensen, Henning S (February 2011) Parasitic Plants. In: Encyclopedia of Life Sciences (ELS). John Wiley & Sons, Ltd: Chichester. DOI: 10.1002/9780470015902.a0021271 ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net 1
- 3. (e.g. hemiparasites are light demanding; hence they are rare in the deep shade of forests). Host range varies from one acceptable host (e.g. the dwarf mistletoe Arceuthobium minutissimum on Pinus griffithii) to several hundreds for certain members of Loranthaceae, Viscaceae and Oro- banchaceae (Kuijt, 1969). In general holoparasites have fewer hosts than hemiparasites. Genetics and biochemical tissue incompatibility determine the maximum number of acceptable hosts, but in practice host range is mainly influenced by geographical (host distribution) and eco- logical (dispersal biology and environmental factors) relationships. Haustoria The haustorium is the key element in plant parasitism. There are two types of haustoria. The primary haustorium develops directly from the primary root apex, and it is the only haustorium which functions throughout the lifetime of the parasite. When only a primary haustorium is present the parasite is considered evolutionary more advanced. Evolution of the primary haustorium made holoparasitism possible, since the generally small seeded holoparasites need water and nutrients from a host immediately after germination (Kuijt, 1969). Secondary haustoria develop on lateral and adventitious roots. They are often short lived, sometimes only a few months, but may occur up to several hundreds per plant. It may be an advantage to have these haustoria placed on roots from several hosts since different hosts absorb various nutrient-ions in varying amounts. In all investigated stem parasites and many root para- sites the haustorium consists of an outer part, the holdfast, with an adhesive surface used for attachment to the host. Within the holdfast an intrusive organ develops which at first penetrates the outer layers of the holdfast and then penetrates the host by a combination of enzymatic dis- solution of cell walls (pectin methyl esterase dissolves middle lamellae) and mechanical compression of host cells. Compression occurs after disruption of the cell membrane and loss of turgour pressure caused by the release of acid phosphatase and possibly other lysosomal enzymes from cells in the tip of the intrusive organ (Toth and Kuijt, 1977b; Heide-Jørgensen, 1989; Losner-Goshen et al., 1998). When the intrusive organ reaches the conductive tissue of the host, a bridge of xylem cells differentiates and connects host xylem with parental xylem of the parasite. The boundary between the intrusive organ and the host is called the interface. Along the interface, cell walls are often thicker and in some cases labyrinth-like and com- parable to transfer cells (Gedalovich-Shedletzky and Kuijt, 1990; Heide-Jørgensen and Kuijt, 1993; Fineran and Calvin, 2000). Apoplastic markers have demonstrated an apoplastic continuum across the interface (Coetzee and Fineran, 1987). Haustorial cells, particularly along the xylem bridge, are enriched in organelles including mitochondria (Visser et al., 1984; Kuo et al., 1989; Heide-Jørgensen and Kuijt, 1995). Increased enzyme activity has been demonstrated in the haustorium of Striga Olacaceae 14 Schoepfiaceae 1 Opiliaceae 10 Loranthaceae 73 Misodendraceae 1 Eremolepidaceae 3 Viscaceae 7 Krameriaceae 1 Lauraceae (Cassytha) 70 Orobanchaceae 20 (Cuscuta europaea) Cynomoriaceae 1 Holoparasites (100%) Hemiparasites (90%) Stem parasites 40% Root parasites 60% Lennoaceae 2 Apodanthaceae 3 Cytinaceae 2 Mitrastemonaceae 1 Rafflesiaceae 3 Hydnoraceae 2 Balanophoraceae 17 Convolvulaceae/Cuscuta (Cascuta reflexa) Santalaceae 35∗ Figure 1 Parasitic families arranged according to parasitic types. Family names are followed by number of genera. Santalaceae has recently been split into five families (see text). Broken lines indicate a few exceptions from main type. Colour codes: Black, nonparasites. Green, hemiparasites. Brown, holoparasites. Dark green, several species of Cuscuta almost or completely lack chlorophyll and hence are holoparasitic. Percentages are in relation to the total number of parasitic plants. Apodanthaceae, Cytinaceae and Mitrastemonaceae used to be in Rafflesiaceae. Orobanchaceae includes parasitic Scrophulariaceae. Copyright Henning S. Heide-Jørgensen. Parasitic Plants ENCYCLOPEDIA OF LIFE SCIENCES 2011, John Wiley Sons, Ltd. www.els.net 2
- 4. hermonthica (Ba and Kahlem, 1979). Furthermore, analy- sis of xylem sap in the host, haustorium of for example Striga and Viscum album, and the parasite shoot system has shown differences in composition of carbohydrates, amino acids and organic acids. These results indicate that nitrogen and carbon compounds are metabolised after entering cells of the haustorium and before they are further distributed in the parasite (Richter and Popp, 1987; Mallaburn et al., 1990). Since parasites have high transpiration rates and always maintain a lower water potential than the host, water and inorganic ions may passively move from host to parasite following the transpiration stream through the apoplastic continuum while the uptake of organic molecules involves active transport in haustorial cells of the transfer cell type (Pate et al., 1990; Ehleringer and Marshall, 1995). In the most advanced holoparasites, the intrusive organ comprises all vegetative tissue of the parasite. It splits into cellular strands, which penetrate large parts of the host, but only in a few cases, such as Arceuthobium douglasii (Figure 2), does it reach the shoot tips (Lye, 2006). This internal tissue is called the endophyte as opposed to the exophyte for external parts such as shoots and flowers. Systematic Overview The order Santalales comprises at least eight families (Figure 1) of which Loranthaceae is the largest with more than 900 species of hemiparasitic stem parasites and three root parasites mainly from tropical and subtropical regions. Among the root parasites are up to the 10 m high Australian Christmas tree (Nuytsia floribunda). The hold- fast of its numerous secondary haustoria often completely encircles the roots of grasses and the intrusive organ develops a knife-like cutting device which is pushed through the host roots cutting the vascular bundle (Fineran and Hocking, 1983; Calladine and Pate, 2000). The most advanced stem parasites have only a primary haustorium while the majority has both kinds with the secondary haustoria located on epicortical roots. These roots run parallel with the branches of the host (Kuijt, 1969; Calvin and Wilson, 2006). Host range is generally high. In India Dendrophthoe falcata causes enormous damage in plan- tations of teak (Tectona grandis) and the parasite may lead to death of entire trees. In West Africa, some of the larger loranths have become real pests in plantations of cocoa, teak and rubber trees (Parker and Riches, 1993). Many species have large, showy, bisexual, nectar pro- ducing flowers adapted to bird pollination (Kuijt, 1969, 2009). Some species show remarkable co-evolution with birds (Polhill and Wiens, 1998). Their flowers are explosive and some, such as two species of Peraxilla endemic to New Zealand (Ladley and Kelly, 1995), need to be touched by the beak of the pollinator in order to open explosively. Neotropical loranths have nonexplosive flowers but otherwise show similar co-evolution with pollinating hummingbirds (Kuijt, 2009). The fruit is fleshy including a viscid layer which serves to glue the seed to host branches when wiped off the beak, regurgitated or dropped after passing the digestive canal of certain birds (Kuijt, 1969, 2009; Polhill and Wiens, 1998; Watson, 2001). Santalaceae comprises both herbs and woody species and most species are root parasites (Figure 1) with many secondary haustoria as general for root parasites. Some are very well investigated anatomically (Fineran, 1963, 1991; Toth and Kuijt, 1977a; Tennakoon and Cameron, 2006). The distribution is similar to Loranthaceae except it extends farther to the north. The flowers are usually less than 1 cm in diameter, regular and insect pollinated, whereas the fruits are dispersed by birds as in Lor- anthaceae. Of the 35 genera, Thesium is by far the largest with 250 species. Santalum is the most important genus since several species have a positive economic value as a source of hard timber and essential oils. S. album is grown on larger scales in India and has been introduced to a number of Pacific Islands (Kuijt, 1969). The largest known parasitic plant Okoubaka aubrevillei, an up to 40 m high tree from tropical Africa, belongs to Santalaceae. It has a wide host range and causes clearings in the vegetation, presumably through weakening of the hosts and shadow- ing from its canopy (Veenendaal et al., 1996). All members of Viscaceae are hemiparasitic stem para- sites. The distribution is similar to Loranthaceae but with more species in the northern temperate zone. Only a pri- mary haustorium is present and the most advanced genera have a widely distributed endophyte. The flowers are less than 3 mm across, and the viscid fruits are dispersed by birds except in Arceuthobium which has self-dispersal by explosive fruits (Hinds et al., 1963). Arceuthobium (Figure2) species are the most harmful parasites on conifers in North America but the maximum dispersal distance is 20 m from the mother plant and long distance dispersal rarely occurs. There are only seven genera and Viscum and the European mistletoe, V. album, is one of the most investigated and best known parasitic plants (Calder and Bernhardt, 1983). Figure 2 Male inflorescences of Arceuthobium douglasii, a hemiparasitic stem parasite on Pseudotsuga menziezii in western North America. The endophyte extends to the shoot tips of the host making the wood useless as timber. Copyright Henning S. Heide-Jørgensen. Parasitic Plants ENCYCLOPEDIA OF LIFE SCIENCES 2011, John Wiley Sons, Ltd. www.els.net 3
- 5. Phoradendron with 234 species is the largest genus (Kuijt, 2003). About the remaining five families in Santalales (Figure 1) see Kuijt (1969), Hiepko (1982), Pate et al. (1990), Fineran (1991), Vidal-Russell and Nickrent (2007), and Heide-Jørgensen (2008). Krameriaceae is a small New World family. Krameria is the only genus and all 18 species are root parasites and small shrubs or semi-shrubs mainly from semi-arid to arid plant communities. The fruits have spines that adhere to the fur of mammals (Kuijt, 1969; Simpson, 1989). Convolvulaceae and Lauraceae are the only two families where the majority of species are autotrophic plants and only one genus in each family is parasitic, Cuscuta (dodder) and Cassytha, respectively. These two genera are so much alike regarding vegetative morphology and mode of para- sitism that they represent a classical example of convergent evolution (Kuijt, 1969). They are winding, stem parasites with only secondary haustoria. Host range is high for most species but often difficult to determine, since the haustoria attach to any subject within reach. However, many haus- toria only develop a holdfast and no intrusive organ or endophyte. Cuscuta species are fast growing. This may in part be explained by more efficient nutrient translocation since the xylem bridge is accompanied by phloem, a unique feature, elsewhere only reported from one species of Orobanche (Dörr, 1972; Dörr and Kollmann, 1995). Cuscuta species are annuals and can be troublesome in agriculture. The North American C. campestris is invasive in many countries (Parker and Riches, 1993). Cassytha is perennial but grows rather slowly. Orobanchaceae is a large family with about 90 genera (Figure 1). Seventy genera representing about 1800 species are hemiparasitic root parasites transferred from Scro- phulariaceae. The remaining genera include about 270 holoparasitic root parasites and a nonparasitic genus Lindenbergia (Bennett and Mathews, 2006). The family is represented in all climatic zones and on all continents except Antarctica. Orobanchaceae contains some of the most troublesome parasites to be discussed later. Most species have numerous secondary haustoria and many hosts, but some advanced species such as Striga her- monthica and all holoparasites have only a primary haus- torium (Kuijt, 1969; Dörr, 1997). Flowers are bilaterally symmetrical and insect pollinated. Seeds are small and numerous, remain viable for years, and mostly dispersed by wind or rain water. Orobanche is the largest genus with about 150 mostly annual holoparasites. Lathraea was once assumed to be carnivorous due to the glands in the hollow scale leaves of the rhizomes (Heide-Jørgensen, 2008). Seven small families with just 1–3 genera each represent some of the most remarkable holoparasites regarding reduction of the exophyte, dissection and wide distribution of the endophyte, and unusual flower construction. In Cynomoriaceae flowers are so reduced that the two species earlier were considered being fungi (Lanfranco, 1960). Lennoaceae is interesting by having root dimorphy. Pilot roots search for host roots and when found they develop short haustorial roots connecting to the host (Kuijt, 1966, 1969). In Mitrastemonaceae about 16 stamens form a mitre- shaped tube which after pollen release is pushed off by the growing pistil and self-pollination is prevented (Kuijt, 1969; Meijer andVeldkamp,1993).Apodanthaceaerepresentsthe tiniest parasites. The exophyte consists only of the 2–3 mm large flowers. There are three genera of stem parasites (Kuijt etal.,1985;Blareretal.,2004).Theothersixfamiliescontain root parasites. In Cytinaceae, the exophyte consists only of theinflorescenceofmaleandfemaleflowers(Kuijt,1969).In the Southeast Asian Rafflesiaceae, a few species of Rafflesia (Figure3) may occasionally occur as stem parasites (Fig. 292 in Heide-Jørgensen, 2008). Here too all vegetative parts are embedded in the host (Tetrastigma) and flower capacity transferred to the endophyte, that is flower buds develop from the endophyte inside the host and break through host tissue at the time of flowering (Kuijt, 1969). Nonetheless Rafflesia arnoldii pollinated by calliphorid flies produces the largest flower in the plant kingdom with a diameter of almost a metre (Meijer, 1984; Bänziger, 1991). The African-South American Hydnoraceae has pilot and haustorial roots as in Lennoaceae and partly or completely subterranean flowers and fruits (Kuijt, 1969; Tennakoon et al., 2007). The last holoparasitic family Balanophoraceae com- prises 17 genera of mainly tropical root parasites (Hansen, 1972). The primary haustorium is only present. It trans- forms into a tuber (occasionally up to 60 cm) which partly consists of host tissue. The inflorescence develops from the tuber and specialised conductive cells connect the vascular system of the inflorescence with the endophyte (Gedalovich- Shedletzky and Kuijt, 1990; Hsiao et al., 1995). The flowers are highly reduced and several species were earlier con- sidered to be fungi. Parasitic Plants as Weeds Only a few out of the many parasitic plant species have evolved to parasitise cultivated plants becoming weeds. Figure 3 Rafflesia keithii, a holoparasitic root parasite on Tetrastigma spp. in Borneo. The exophyte consists only of the flower with a diameter up to 90 cm. Photo courtesy of Thomas Læssøe. Parasitic Plants ENCYCLOPEDIA OF LIFE SCIENCES 2011, John Wiley Sons, Ltd. www.els.net 4
- 6. They pose a tremendous threat to world economy, mainly because they are at present almost uncontrollable (Joel et al., 2007; Parker, 2009). They belong to various plant families, and attach to host roots, shoots or branches. Accordingly we find mistletoes like Viscum and Arceutho- bium that parasitise trees, climbers like Cuscuta that para- sitise shoots, and parasites like Striga and Orobanche that connect to host roots. The weedy root parasites, which are often host specific, exert their greatest damage before their emergence; there- fore the majority of crop/yield loss may occur before diagnosis of infection. In spite of intense efforts during the twentieth century, effective means to selectively control the various species of parasitic weeds are still scarce or lacking. A wide variety of approaches – physical, cultural, chemical and biological – have been explored against root parasites, but most of them are not effective, or not selective to the majority of susceptible crops (Joel et al., 2007; Rubiales et al., 2009a, b). The most damaging weedy root parasites belong to the Orobanchaceae. The broomrapes (Orobanche and Pheli- panche spp.) are widespread in Mediterranean areas in Asia and Southern and Eastern Europe, attacking dicoty- ledonous crops and depend entirely on their hosts for all nutritional requirements. Only a few species are weedy, but their impact on agriculture is tremendous. The areas infested by the seven most important species of Orobanche are vast and ever-growing, because farmers do not have the resources and the education needed for sanitation meas- ures in agricultural fields, and since quarantine services do not have the means to monitor broomrape seeds in seed testing stations. Nevertheless, sanitation measures are essential in order to limit Orobanche distribution. Orobanche crenata is mainly restricted to the Medi- terranean basin, Southern Europe and the Middle East. It is an important pest in grain and forage legumes, as well as in some Apiaceous crops like carrot and celery. Phelipanche aegyptiaca (syn. O. aegyptiaca) is important on many crops in Mediterranean countries in the Middle East and Africa, extending eastwards to central Asia, India, China and southern Russia. P. aegyptiaca attacks crop plants belonging to a variety of families, including Solanaceae, Fabaceae, Cucurbitaceae, Brassicaceae and Asteraceae. It attacks tomato, potato, tobacco, cabbage, oilseed rape, sunflower, parsley, watermelon, faba bean, common vetch, chickpea, lentil and (Parker and Riches, 1993). P. ramosa (syn. O. ramosa) attacks many different crop plants; its host range resembles that of P. aegyptiaca. In early times, it was known to attack cannabis and various solanaceous crops, but currently its host range is very wide, and differs in the various countries. It attacks potato, tomato, eggplant and tobacco, it also severely attacks brassicas, melon, water- melon and cucumbers, and to a lesser extent it may cause damage to parsley, celery, parsnip, some legumes and let- tuce. O. foetida has recently been described to severely attack legumes in North Africa. O. foetida seems to be more aggressive on faba bean and on common vetch than on other legumes. O. minor has a wide host range among forage legumes in temperate climates. Though known as a nonimportant weed in most regions, including Australia, Japan and Europe, it has recently become a serious prob- lem on red clover in Oregon, USA. O. cernua is known in native habitats to parasitise members of the Asteraceae, but as a weed it is typically attacking solanaceous crops, mainly tobacco, tomato and potato. O. cumana has spe- cialised as a pest of sunflower and is an increasing problem in sunflower production in Mediterranean countries and eastern Europe (Parker and Riches, 1993; Joel et al., 2007; Parker, 2009; Figure 4, Figure 5 and Figure 6). In tropical Africa the most damaging parasitic weeds are the witchweeds (Striga spp.), obligate root parasites of grain grasses and legumes, which endanger food supply in many developing countries. The most destructive species on cereals are S. hermonthica and S. asiatica, followed by S. aspera and S. forbesii, parasitising important food crops like rice, pearl millet, sorghum and maize in much of Africa and some parts of Asia. S. gesnerioides is an important pest of Fabaceae, especially cowpea (Parker, 2009). Similar root parasites belonging to the genera Alectra, Buchnera and Rhamphicarpa attack agricultural crops in Africa. A. vogelii causes considerable yield losses of grain legume crops, particularly cowpea, throughout semi-arid areas of subSaharan Africa (Parker and Riches, 1993). Its main hosts are cowpea in Southern, East and West Africa and groundnut in East and West Africa. Soyabean, bam- bara groundnut, common bean, mung bean and many legume fodder crops are also parasitised (Riches et al., 1992; Rubiales et al., 2006). Management Rather than being controlled, the parasitic weed problem is increasing both in intensity and in acreage. With climate change, these invasive parasites are spreading further north in Europe and further south in Africa. Recent studies (Mohamed et al., 2006; Grenz and Sauerborn, Figure 4 Orobanche crenata infecting pea. Parasitic Plants ENCYCLOPEDIA OF LIFE SCIENCES 2011, John Wiley Sons, Ltd. www.els.net 5
- 7. 2007) suggest that very large areas of new territory are at risk of invasion if care is not immediately taken to limit the introduction of parasitic weed seeds and to educate farmers and others to be on alert for new infestations. Only few crops can be protected from parasitic weeds by resistances or herbicides, therefore the main current means for controlling parasitic weeds should be focusing on (a) reducing soil seedbank, (b) preventing seed set and (c) inhibiting seed movement from infested to noninfested areas, that is sanitation (Joel et al., 2007; Rubiales et al., 2009b). Seedbank demise can be efficiently achieved by fumigation or solarisation, however, this is only eco- nomically feasible in high-value crops. There is promise in a number of strategies such as rotations with trap or catch crops (Rubiales et al., 2009b) or intercropping what has shown useful in Striga management (Pickett et al., 2010) and could also be applied for Orobanche (Fernández- Aparicio et al., 2007). Biological control has also shown promise (Sauerborn et al., 2007). Insects like the broomrape fly (Phytomiza orobanchia) and the Striga gall-forming weevils (Smicronyx spp.) and some fungal species, particularly some Fusarium spp. have shown potential for parasitic weed control and are currently under field evaluation (Klein and Kroschel, 2002; Müller-Stöver et al., 2004; Dor and Hershenhorn, 2009). Several classes of plant secondary metabolites are known to induce seed germination of root parasitic weeds, including dihydrosorgoleone, sesquiterpene lactones and strigolactones (Yoneyama et al., 2009). New compounds with stimulatory seed germination activity on particular parasitic weeds are continuously been reported, such as peagol, peagoldione or peapolyphenols A-C (Evidente et al., 2009, 2010). Stimulatory activity of these metabolites on parasitic weed germination could be exploited in suicidal germination control strategies by synthesising and directly applying them to the field. Attempts have been made with the application to the soil of synthetic strigolactones, such as GR24 or Nijmegen-1 with no definitive results. Other compounds of natural origin, such as fungal metabolites (Fernández-Aparicio et al., 2008), natural amino acids (Vurro et al., 2009) or plant or algae extracts (Economou et al., 2007) or chemical agent inductors of systemic acquired resistance such as BTH (1,2,3-benzothiadiazole-7-carbothioic acidS-methyl ester) (Sauerborn et al., 2002; Pérez-de-Luque et al., 2004) have shown promise for parasitic weed management. Also, root colonisation with symbiotic Rhizobium (Mabrouk et al., 2007) or arbuscular mycorrhizal fungi (Lendzemo et al., 2005; Fernández-Aparicio et al., 2010) can provide pro- tection against parasitic weeds. However, all these sug- gested methods are currently still under development, and need further verification in the field before registration. Figure 6 P. ramosa infecting tobacco. Figure 5 Phelipanche ramosa infecting tomato. Parasitic Plants ENCYCLOPEDIA OF LIFE SCIENCES 2011, John Wiley Sons, Ltd. www.els.net 6
- 8. Chemical control strategies have been developed for a small number of crops, but these are not always applicable for economic and environmental issues (Joel et al., 2007; Pérez-de-Luque et al., 2010). The herbicides that are cur- rently in use for parasitic weed control are (a) glyphosate, inhibitor of 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase – a key enzyme in the biosynthesis of the aromatic amino acids, and (b) imidazolinones and sulfonylureas, inhibitors of acetolectate synthase (ALS, or acetohydrox- yacid synthase – AHAS), a key enzyme in the biosynthesis of branched-chain amino acids. Sulfonylurea herbicides are selective systemic herbicides that can be absorbed through foliage and roots with a rapid acropetal and basipetal translocation (Schloss, 1995). The imidazolinone herbicides are translocated by the host to meristematic tissues, where the enzyme is highly active. Target site herbicide resistance might be a promising solution for controlling broomrape that is being explored in some crops (Gressel, 2009), particularly with nontransgenic imidazo- linone target-site resistant sunflowers which are now being released (Tan et al., 2004). Alternatively, nanoencapsula- tion of herbicides could be used to solve problems regarding phytotoxicity on the crop. Lower doses of herbicides would be needed because they will not be degraded by the crop, and they will accumulate preferen- tially in the parasitic weed due to the sink effect (Pérez- de-Luque and Rubiales, 2009). One of the most suitable control options is the devel- opment of resistant crop varieties (Pérez-de-Luque et al., 2009; Rubiales et al., 2009a). Breeding for broomrape resistance is a difficult task considering the scarce and complex nature of resistance in most crops. Only incom- plete resistance has been identified in most crops. However, these resistances have been successfully accumulated by breeding in many cases, allowing the release of cultivars with some degree of resistance (Rubiales et al., 2006; Ejeta, 2007; Pérez-de-Luque et al., 2009). In a few instances, such as in the systems sunflower/O. cumana and cowpea/ S. gesnerioides resistance of simple inheritance has been identified and has been widely exploited in breeding. The integration of information obtained from QTL analysis with gene and protein expression analysis currently per- formed for both Orobanche (Die et al., 2007; Castillejo et al., 2009; Dita et al., 2009) and Striga (Swarbrick et al., 2007; Li et al., 2009) can shortcut conventional breeding or marker-assisted selection in identifying candidate genes. Therefore, increased efforts in delivering control by resistant cultivars can be more effectively made, and the tools of modern plant breeding and of heterologous gene transfer (Rispail et al., 2007; Yoder et al., 2009) will be valuable. Means to limit the development of parasitic weed in the field may not only reduce the direct damage of the crop but also limit the production of additional seeds, which replenish and enrich the local seedbank and at the same time also increase the risk that noninfested areas will also be contaminated by parasitic weed seeds. Preventing the movement of parasitic weed seeds from infested areas into uninfested areas is therefore a crucial component of control that requires in most cases significantly less resources than controlling the parasite once it is established. Both sani- tation and quarantine are required to prevent the dispersal of parasitic weed seeds. References Ba AT and Kahlem G (1979) Mise en évidence d’activatés enzy- matiques au niveau de l’haustorium d’une phanérogame para- site; Striga hermonthica (scrophulariaceae). Canadian Journal of Botany 57: 2564–2567. 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- 11. Further Reading Bhandari NN and Mukerji KG (1993) The Haustorium. New York: Wiley. Curtis’s Botanical Magazine 26(4) 2010. – Special edition on parasitic plants. Fineran BA (1985) Graniferous tracheary elements in haustoria of root parasites. Botanical Review 51: 389–441. Heide-Jørgensen HS (2011) Parasitic plants. In: Simberloff D and Raymánek M (eds) Encyclopedia of Biological Invasions, pp. 504–510. University of California Press. Irving LJ and Cameron DD (2009) You are what you eat: inter- actions between root parasitic plants and their hosts. Advances in Botanical Research 60: 87–138. Kuijt J (1979) Host selection by parasitic Angiosperms. Symbolae botanicae Upsalienses 22: 194–199. Leake JR (2004) Myco-heterotroph/epiparasitic plant inter- actions with ectomycorrhizal and arbuscular mycorrhizal fungi. Current Opinion in Plant Biology 7(4): 422–428. Mathiasen RL, Nickrent DL, Shaw DC and Watson DM (2008) Mistletoes: pathology, systematics, ecology, and management. Plant Disease 92: 988–1006. Nickrent DL The parasitic plant connection. http://www. parasiticplants.siu.edu/ Press MC and Phoenix GK (2005) Impacts of parasitic plants on natural communities. New Phytologist 166: 737–751. Stewart GR and Press MC (1990) The physiology and bio- chemistry of parasitic Angiosperms. Annual Review of Plant Physiology and Plant Molecular Biology 41: 127–151. Visser J (1981) South African Parasitic Flowering Plants. Cape Town: Juta. Weber HC (1978) Schmarotzer: Pflanzen die von anderen leben. Stuttgart: Belser Verlag. Weber HC (1993) Parasitismus von Blütenpflanzen. Darmstadt: Wissenschaftl. Buchge. Parasitic Plants ENCYCLOPEDIA OF LIFE SCIENCES 2011, John Wiley Sons, Ltd. www.els.net 10 View publication stats