Haustorium parasitic Plants Newsletter



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HAUSTORIUM 51 July 2007



HAUSTORIUM

Parasitic Plants Newsletter

Official Organ of the International Parasitic Plant Society


July 2007 Number 51




IPPS

Dear IPPS Members,

Last June we have enjoyed the very successful 9th World Congress on Parasitic Plants, which was carefully organized by our American colleagues in Virginia. Both the scientific program and the venue were perfectly prepared, and allowed both oral and poster presentations with fruitful discussion on key issues in parasitic plants research and parasitic weed management. All Congress abstracts can be found at http://www.cpe.vt.edu/wcopp/Abstracts_Final.pdf. A review of the scientific presentations, kindly prepared by Chris Parker, is given below.

Experts and students from more than twenty countries attended the Congress, and the pleasant venue plus the weather conditions also allowed enjoying the pleasant atmosphere of downtown Charlottesville. The tour to Monticello and the visit to a local winery added a glimpse into the US history and the local wine industry.

This is an opportunity to thank, again, the Local Organizing Committee chaired by Mike Timko, who did an excellent job in preparing all Congress details. We are also grateful to Jim Westwood, Lytton Musselman and Mike Timko for putting together an excellent scientific program with the aid of the International Scientific Advisory Committee, which represented various aspects of research and development related to parasitic plants.

Please visit http://www.cpe.vt.edu/wcopp/photos.html for a selection of photographs taken during the Congress.

We are happy to announce that the next IPPS Conference will be in Turkey during the first half of June 2009. We are presently negotiating the details, and will send you the first Circular as soon as we have more details.
This is my last message to you as president of the IPPS. Jim Westwood, who currently serves as Vice-President, will become the new IPPS President, and we will soon have elections for a new Executive Committee. A detailed announcement on the elections will be given separately.

Daniel M. Joel

IPPS President

9th World Congress on Parasitic Plants
A total of 80 delegates from 24 countries gathered at the Omni Hotel in Charlottesville, Virginia, USA from 3 to 7 June for the 9th International meeting of the parasitic plant community. A total of 37 oral papers and 39 posters were presented. A list of titles is included under MEETINGS below. There will be no published proceedings but the abstracts of all contributions

are available at the conference website (http://www.cpe.vt.edu/wcopp/index.html).


An opening address by Klaus Wegmann set the scene with a truly historic review tracing the subject back to Theophrastus in about 300 BC and noting many other very early literature references.
In the next invited paper Jeffrey Palmer reviewed the topic of horizontal gene transfer (subject of a Literature Highlight in Haustorium 48). The genes transferred (often several at once) are mostly in the mitochondrial genome and rarely in nuclear or plastid genomes. Agents of transfer are believed to include lichens, pollination, fungi or insects, but about half the known occurrences involve parasitic plants and their hosts.
A series of papers on genomic studies and evolution of parasitic plants included a masterly review by Dan Nickrent on the different modes of aerial and root parasitism in the Santalales, and the evidence for 5 separate evolutionary origins of parasitism in that group, based on intensive DNA and fossil studies. Further papers explored the evolution and phylogenetic relationships within other groups of parasitic plant - by Funk et al. (presented by Kirsten Krause) on Cuscuta; by Sasa Stefanovic and Costea also on Cuscuta; and by Schneeweiss et al. on Orobanche. Chris Thorogood presented preliminary evidence for host specificity leading to speciation in British Orobanche spp.
Another invited paper dealt with the newly established link between germination stimulants for parasitic plants, and the arbuscular mycorrhizae, for which these substances area a vital signal (topic of a Literature Highlight in Haustorium 47). In a wide-ranging and detailed review Maria Harrison described the morphology and development of the mycorrhiza and the fact that about 80% of all plant species can be infected. She also explored the molecular events and specific genes that underlie development and functioning of the symbiosis.
Related papers on germination included one by Plakhine et al. (presented by Danny Joel) which surprisingly showed that there appear to be genes responsible for suppressing spontaneous germination and a certain combination of genes in hybrids from Orobanche cernua and O. cumana could result in high levels of spontaneous germination. Koichi Yoneyama presented a paper by Xie et al. in which the full range of strigolactones was reviewed, 6 new structures described, and the structure of ‘alectrol’ re-investigated, showing it to be an acetate of ‘orobanchol’. They conclude that most host species (and many unaffected by parasitic plants) exude several different strigolactones. Another important finding was the effect of reduced phosphorus in increasing stimulant exudation in both legumes and in sorghum. Harro Bouwmeester described work on the biosynthesis of the strigolactones from carotenoids, suggesting that they should be referred to as ‘apocarotenoids’ rather than ‘sesquiterpene lactones’. A step in the biosynthetic pathway could be blocked by the herbicide fluridone, and low doses applied to rice could be shown to reduce attack by Striga hermonthica. Yukihiro Sugimoto described the use of aseptic plant tissue cultures of several species for copious production of stimulant substances, identified mainly as strigol or 5-deoxystrigol. The latter proved equal to GR24 for stimulation of S. hermonthica and O. crenata and 10-fold more active on O. minor.
Among several papers on post-germination events, one presented by Andrew Palmer, suggested that elevated cytoplasmic calcium is among the very earliest responses to exposure of S. asiatica to the xenognosin DMBQ, occurring within 15 minutes. The involvement of hydrogen peroxide and NADPH oxalate were also discussed. Ralf Kaldenhoff had explored gene expression in Cuscuta reflexa and its host tomato and shown the importance of cysteine protease production in Cuscuta tissue. He had shown that application of a polypeptide inhibitor of cysteine protease activity could lead to death of the Cuscuta. This concept is the subject of a patent. John Yoder presented detailed work by Tomilov et al. which explored the genes involved in haustorial development in Triphysaria. With nearly 12,000 gene sequences now generated from Triphysaria, analyses of haustorial initiation are producing intriguing findings, such as a connection between touch and haustorial formation.
Jay Bolin introduced many of us to the interesting structure and physiology of the African Hydnora spp., plants with an almost totally subterranean habit, without stomata and extremely resistant to desiccation. Isotope studies confirm that all carbon derives from the host, while levels of P and K are much higher in the parasite than in the host. Studies on the mistletoe, Viscum album, presented by Michiel de Mol confirmed direct vessel to vessel connections at the host-parasite interface, allowing mass transport of water and nutrients. Philippe Simier presented work by Draie et al. exploring in detail the enzyme systems involved in sucrose metabolism in the tubercles of Orobanche ramosa. Mike Timko then described the latest studies exploring the range of biotypes of Striga gesnerioides, their specificity to particular host species or cowpea varieties, and mapping the relevant resistance and avirulence genes in host and parasite.
A further invited paper from Julie Scholes discussed the molecular basis of susceptibility and resistance to Striga, describing detailed exploration of the up- and down-regulation of genes in both host and parasite and seeking to relate these to the various types of resistance mechanism observed. Many hundreds of genes appear to be involved.

There were then 3 papers presented by Ms Gunathilake on Triphysaria, Jim Westwood (for Roney et al.) on Cuscuta, and Radi Ali on Orobanche, describing the intriguing phenomenon of ‘trafficking’ of double-stranded RNA molecules between host and parasite. Ali et al. showed how the phenomenon could perhaps be exploited for ‘silencing’ key metabolic genes in the parasite – mannose 6-phosphate reductase could be inhibited and significant reduction of O. aegyptiaca achieved on suitably transformed tomato.

Moving on to a more ecological level, Duncan Cameron described how species richness could be enhanced by the introduction of Rhinanthus minor into a plant community, thanks to selective suppression of susceptible grass species, while non-grasses showed resistance based on a hypersensitive response or host lignification. Darryl Miguel described the problem of O. ramosa in S. Australia, which has caused some 100,000 ha to be placed under quarantine. Studies suggest very slow loss of viability in the seed bank, and the need for chemical treatments to enhance seed loss and/or prevent new seed production. A commercial pine oil product has given up to 95% reduction but results are variable and the large volumes of water required lead to very high costs of application. Studies by Janice Alers-Garcia suggest that Cuscuta gronovii tends to select and grow more successfully on larger individuals of the host plant Pilea pumila. Alistair Murdoch reviewed work on the influence of temperature on the after-ripening, conditioning and germination of Orobanche and Striga spp. and discussed the potential design and use of predictive models based on these data.

Alex Pérez de Luque reviewed the topic of resistance mechanisms and incidentally made a plea for the term ‘haustorium’ to be reserved for the organ once it had made vascular connection with the host: prior to that it should be referred to as the ‘appressorium’. Yasutomo Takeuchi then presented a paper by Kusumoto et al. on the induction of resistance in Trifolium and rice via salicylic acid-mediated defences. Application of BTH to clover reduced attack by O. minor and application of tiadinil to rice reduced attack by Aeginetia indica. Julien de Zélicourt described work exploring the resistance of sunflower var. LR1 to race E of O. cumana, which suggested a major role for the peptide defensin.

A final session on Management and Control began with an invited paper from Fred Kanampiu who gave a detailed description of the development and use of the herbicide imazapyr as a seed-dressing for control of Striga spp. on naturally herbicide-resistant maize. The treatment has been commercialised since 2005 and is proving successful in East Africa. It can be used in conjunction with inter-planted legumes, provided they are at least 12 cm from the maize row. Hanan Eizenberg then described a minirhizotron technique for monitoring underground development of Orobanche tubercles as a means of validating a Growing Degree Days model, designed to ensure optimum timing of herbicide application. Hilary Sandler described the problem from Cuscuta gronovii in cranberry and described valuable results on the germination behaviour of seeds over the years following shedding, allowing more effective timing of flooding and chemical control methods. A further paper on the O. ramosa problem in Australia was presented by Anna Williams who described the development of a Growing Degree Days model on which to base the optimum timing of herbicide treatments. Djibril Yonli reported on studies with a range of Fusarium isolates for suppression of S. hermonthica in Burkina Faso. Promising results were obtained even when innoculum was placed up to 10 cm away from the sorghum planting hole. Simon Shamoun provided an update on the development of Colletotrichum gloeosporioides and Neonectria neomacrospora for biocontrol of Arceuthobium tsugense in Canada, which is of increased concern with the prohibition on clear-cutting and consequent persistence of old heavily infected trees. Both organisms had proved partially effective, though some wounding of host tissue may be needed for maximum effect. Combinations of the two are to be tested. A final presentation by Charlie Riches described the successful results achieved with green manure crops, especially Crotalaria ochroleuca for control of S. asiatica in rice and maize in Tanzania. Crop yields were often doubled following treatment, compensating for the lost season of food-crop cropping, and farmers were adopting the practice with enthusiasm.

Among the 39 posters there was a study on Cuscuta spp. in Taiwan (Chiang et al.); demonstration of the increased production/exudation of stimulant at reduced phosphate levels, a phenomenon not previously well-documented (Lopez-Raez et al.; Yoneyama et al.); identification of orobanchol as the major Orobanche stimulant exuded by Arabidopsis thaliana (Goldwasser et al.); an effect of trehalose in increasing germination of Orobanche minor (Okazawa et al.); evidence for suberization and protein cross-linking in the cell walls of the sunflower variety HE-39999 resistant to race F of O. cumana (Echevarria-Zomeño et al.); a series of studies on the virulence of different accessions of Medicago trunculata on a range of hosts and the associated variations in resistance mechanism and germination stimulation activity, all of potential value in the study of host-parasite interaction (Fernández-Aparicio et al.; Lozano-Baena et al (x2).; Castillejo et al.); identification of a Streptomyces isolate in Jordan, with potential for control of Orobanche cernua (Saadoon et al.); a study suggesting no correlation of tocopherol levels with carotenoid or chlorophyll content in Cuscuta spp., suggesting an unrelated function (van der Krooj et al.); a study showing generally excellent but not completely reliable control of Cuscuta spp. by glyphosate in Roundup Ready alfalfa (Lanini et al.); evidence for reduced hydraulic conductivity in the stems of spruce trees infected by Arceuthobium pusillum (Dunlavey et al.); detailed study of the site of production of hydrogen peroxide in S. asiatica and evidence for its importance in the initiation of the haustorium (Palmer et al.); identification of 2 lignin biosynthesis genes in S. asiatica with presumed roles in the development of the vascular connection with the host (Liu et al.); an update on the development of Alternaria destruans as a biocontrol agent for Cucuta gronovii in cranberry, which is hopefully close to commercial release (Bewick and Cascino); identification of a sunflower variety AO-548 resistant to a new highly virulent race of O. cumana in Romania, based on two independent dominant genes (Pacureanu-Joita et al.); updates on the development of Fusarium oxysporum (Foxy 2) for control of Striga in cereals, application by seed treatment, and synergism with resistant cultivars (Heller et al.; Elzein et al.).

Social events included a welcome reception on the first evening, a field trip to the home of President Jefferson at Monticello which was rounded off with a visit to the Jefferson Winery, and a banquet, at which a number of elder statesmen of the community were honoured. Bob Eplee, Doug Worsham and Chris Parker were presented with ‘Legacy Awards’, while Yasutomo Takeuchi, Binne Zwannenburg, Jose Ignacio Cubero (not present), Klaus Wegmann and Patrick Thaluarn (not present) were recognised as ‘Significant Contributors’.

Jim Westwood, Mike Timko and others on the organising committee are to be congratulated on a superbly planned meeting and an excellent choice of venue - the hotel and its setting were ideal.

Chris Parker.
A different kind of parasitic plant: a brief history of myco-heterotrophy and epiparasitism
Debunking the myth of saprotrophic plants

Some 400 species of plants, termed myco-heterotrophs (Leake 1994), lack chlorophyll but do not form haustorial connections to other plants and are nourished instead by forming (parasitic) associations with fungi (Smith & Read 1997). Most of these plants have commonly been referred to as “saprophytes” on the assumption that they obtain carbon directly from decaying soil organic matter. Indeed the myth of the “saprophytic” plant has been perpetuated by floras through to the current day; even the New Atlas of the British Flora (Preston et al., 2002) describes the myco-heterotrophic Neottia nidus-avis, Corallorhiza trifida, Epipogium aphyllum and Monotropa hypopitys as “saprophytic perennial herbs of rotting vegetation” despite the absence of evidence for this (Leake 2005).


Myco-heterotrophy has evolved in both lower plants such as the myco-heterotrophic liverwort Cryptothallus mirabilis and on at least five separate occasions in higher plants in the dicotyledonous families; Monotropaceae (and the closely related Pyrolaceae), Polygalaceae and Gentianaceae which combined represent 12% of myco-heterotrophic species (Leake 1994). The remainder belong to two orders of the monocotyledons; Triuridales and the Orchidales (Leake 1994). Job Kuijt highlighted this disparity commenting on the abundance of (myco)heterotrophs in the monocotyledonous plants and the extreme contrast with haustorial parasitism which occurs exclusively in dicotyledonous plants (Kuijt 1969).
Myco-heterotrophy in the orchids

Perhaps the most studied of all plant families with myco-heterotrophic species are the orchids. There are estimated to be around 200 species of achlorophyllous or largely achlorophyllous orchids but all orchids in fact begin their lives with a myco-heterotrophic growth phase. Like many haustorial parasitic plants, orchids produce prodigious numbers of minute dust seeds, typically in excess of 100,000 seeds per plant that do not have sufficient seed reserves to germinate unaided, instead orchids engage in a symbiosis with fungal partners where by the fungus supplies the developing orchid seedling with all of the carbon and mineral nutrients it requires for establishment (Smith & Read 1997). Whilst some orchids never produce chlorophyll, the majority of adult orchids are green and putatively photosynthetic. As green adults, orchids were believed to continue in this parasitic habit throughout their lives but recent evidence has cast doubt on this dogma demonstrating that the green orchid Goodyera repens can, as an adult, supply its fungal symbiont, Ceratobasidium cornigerum, with carbon (Cameron et al. 2006) in return for mineral nutrients (Cameron et al. 2006 & 2007) suggesting the potential for mutualism in the symbiosis. Thus, as with the haustorial parasites, it appears there is a continuum from autotrophy to heterotrophy (holoparasitism) in the orchids.


Epiparasitic myco-heterotrophs

The source of carbon for fungi parasitised by myco-heterotrophic plants falls into two distinct categories. Firstly, myco-heterotrophs may form associations with fungi which gain their carbon saprotrophically from organic matter. It is important to make the distinction that we do not imply that myco-heterotrophs are directly saprophytic, they parasitise fungi, but their fungal partners may gain carbon saprotrophically, and/or be weakly parasitic on other plants. Typically but not exclusively, these fungi belong to the polyphyletic Rhizoctonia complex. Secondly, some myco-heterotrophic plants are associated with fungi that obtain their carbon through forming mutualistic mycorrhizal symbioses with other autotrophic plants and are thus in tripartite symbiosis with the myco-heterotroph connected to an autotrophic plant through a shared fungal network (Bidartondo et al. 2004). These plants are referred to as the epiparasites (Bidartondo et al. 2002), the “epi” prefix referring to the indirect nature of the parasitism of the co-associated plant (and being distinct from epiphytic plants which rely on structures such as other plants for mechanical support). Moreover, using radioactive 14C tracers, carbon transfer has been directly demonstrated from green plant (Betula pendula) though an ectomycorrhizal fungal network to the largely achlorophyllous orchid Corallorhiza trifida (McKendrick et al. 2000). This epiparasitic mode of nutrition has underpinned the convergent morphology of myco-heterotrophic and haustorial parasitic plants.


Parallels and contrasts between epiparasitic plants and haustorial holoparasites

Heterotrophic plants, such as the enigmatic “Ghost orchid” Epipogium aphyllum, have long been considered botanical curiosities and in the past they have even been inaccurately included in parasitic genera. Indeed, before being described by Linnaeus in 1753, Monotropa hypopitys (Monotropaceae) was considered to be an Orobanche (Leake 1994)! At one level such confusion is not surprising given the striking convergent morphology of myco-heterotrophs and haustorial holoparasites. Both exhibit highly reduced leaves, often to scale leaves or bracts, contain little or no chlorophyll and produce prodigious numbers of seeds that depend upon host-derived cues to initiate germination as they cannot establish in the absence of a host plant (for haustorial holoparasites) or fungus (for myco-heterotrophs).


In summary, there is no doubt that the epiparasitic myco-heterotrophs and haustorial holoparasitic plants are physiologically very different in terms of their carbon acquisition strategies from other plants. In the case of the epiparasites the connection to the host plant is a fungal “bridge” whereas in the haustorial holoparasites the physiological bridge is the haustorium, but beneath this they are functionally the same, they are parasitic on other plants!
References:

Bidartondo, M.I., et al. 2004. Proceedings of the Royal Society London B 271: 1799-1806.

Bidartondo, M.I., et al. 2002. Nature 419: 389-392.

Cameron D.D., et al. 2007. Annals of Botany 99: 831-834

Cameron D.D., et al. 2006. New Phytologist 171: 405-416.

Kuijt, J. 1969. The biology of parasitic flowering plants. Berkeley and Los Angeles: University of California press.

Leake, J.R.1994. New Phytologist 127: 171-16.

Leake, J.R. 2005. Mycologist 19: 113-122.

McKendrick, S.L, et al. 2000. New Phytologist 145: 539-548.

Preston, C.D, et al. 2002. New Atlas of the British Flora. Oxford University Press.

Smith, S.E. and Read, D.J. 1997. Mycorrhizal Symbiosis. Academic Press London and New York.
Duncan D. Cameron and Jonathan R. Leake

Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK.



E-mail: d.cameron@shef.ac.uk
(Editors’ note: in the light of the discussion above, Haustorium will in future include reference to

literature on at least some ‘saprophytic’ higher plants.)

OFFICIAL LAUNCH OF PUSH-PULL TECHNOLOGY
The ‘push-pull’ technology that has been under development by ICIPE (International Centre for Insect Physiology and Ecology) for the past 10 years was given an official launch at ICIPE’s Mbita Point station on the shores of Lake Victoria, Kenya, in early July 2007 (see Butonyi, 2007 a,b). The technique, as described in Haustorium 37, was developed for control of stem-borers (Busseola and Chilo species) in maize, Desmodium spp. being grown as an intercrop to repel the adult moths and Napier grass (Pennisetum purpureaum) grown around the field edges to attract the moths to lay their eggs which hatch, but fail to develop on this species. After some years it was noticed that Striga hermonthica was being suppressed by the Desmodium and extensive trials confirmed that the parasite was massively reduced while soil fertility was enhanced and maize yields greatly increased. The technique has been used successfully by over 10,000 farmers in Kenya and Uganda and has now been shown to work well also with sorghum. The aim now is to extend the technique to reach 20,000 farms by the end of 2009. Apart from benefiting from improved cereal yields, farmers find they have sufficient fodder from the Desmodium and Napier grass to be able to keep a cow and improve their nutrition and farm income. Dr Zeyaur Khan and his colleagues at ICIPE are to be congratulated on this highly successful and promising development. See the following item for news of further efforts to understand how it works.
Chris Parker.



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