|Keywords:||Inputs (agricultural); Disease/pest resistance; Herbicide/pesticide tolerance; Other disciplines than biotechnology; Grass root technologies; Genetic engineering.|
|Correct citation:||Verkleij, J.A.C. and Kuiper, E. (2000), "Various Approaches to Controlling Root Parasitic Weeds." Biotechnology and Development Monitor, No. 41, p. 16-19.|
Conventional methods, both individually and in combination, have a limited impact on controlling parasitic weeds. This suggests that transgenic plants could be used in an integrated approach to control them. However, research into transgenic parasitic resistance has a number of bottlenecks such as lack of knowledge of the physiology, biochemistry and molecular biology of parasitic weed and plant parasite interactions.
Parasitic weeds are a serious problem in many agricultural production systems. Unlike ‘normal’ weeds, that merely compete with the crop plants for nutrition and harbour diseases, root parasitic weeds damage the crops by attaching their own roots to the roots of the crop plant and taking their nutrition and water from it. They are especially hard to control because they cannot be treated as a separate plant and because they inflict much damage before emerging above ground (see box). Of all root parasites, Orobanche (broomrape) and Striga (witchweed) cause most damage to agricultural crops.
Orobanche species attack a wide variety of legumes and crop species such as sunflower, tomato and tobacco. Its main distribution is throughout the Mediterranean region, in Eastern Europe and India, and it can also be found in many other countries. It is broadly estimated that it affects over a million hectares of agricultural land with average yield losses as high as 40 per cent in some areas. The extent of economic damage caused by Orobanche is unknown.
Striga species are parasitic weeds found in Africa and Asia, mainly in India. They infest lands planted with maize, sorghum, millet, upland-rice and cowpea. They often destroy the crops completely. It has been estimated that Striga parasitism causes an annual yield loss worth several billion US dollars and affects the life of 300 million people in Africa. The Food and Agriculture Organization of the United Nations (FAO) therefore considers this parasitic weed a major biotic constraint on food production in sub-Saharan Africa.
|The life cycle of root parasites
Root parasites like Striga and Orobanche are difficult to control, partly due to their complex life cycle. Each parasitic plant may produce up to 200,000 extremely small seeds, which can remain viable for over 15 years. When shed, the seeds are dormant and require a period of after-ripening, which is usually completed before the end of the dry season. At the start of the rainy season the seed will absorb water, but it is still unable to germinate. This ‘conditioning’ period lasts for 5 to 21 days at a suitable temperature (30 to 40°C for Striga and 15 to 20°C for Orobanche). Conditioned seeds are capable of geminating if this is triggered by stimulating substances produced by potential host roots. Most of these substances belong to the strigolactones and are active in extremely low concentrations (1 ppb to 1 ppm). Their primary role in the host plants is unknown, however. Once the parasite makes contact with the host root, it develops a root-like structure known as haustorium. The haustorium has three functions: attachment, penetration of the host root, and nutrient acquisition from the host. When a successful connection has been made with the host, the parasite can grow rapidly using water and nutrients taken from the host, causing the inevitable negative effects on the host. After emergence the parasite will grow until it flowers, produces seed and dies. The complete cycle takes 10 to 15 weeks.
While Orobanche only takes nutrition and water from the host plant, Striga exerts direct, probably toxic effects, which completely alter the growth pattern of the host. This process is not yet understood. Symptoms such as stunting, wilting and leaf chlorosis (yellowing) are already exhibited by infected host plants several weeks before the parasite emerges above ground. Furthermore, a strong reduction of the shoot/root ratio can be observed in infected cereals. This effect is exerted within as little as 10 days after attachment of a single parasite to the host. When parasitic weeds emerge above ground, host plants often show symptoms of drought stress. This is caused by the higher transpiration rates in Striga than in the host plant, which ensures a flow of water and nutrients to the parasite.
There are a number of control strategies for parasitic weeds that can be used on their own or in combination.
Hand-pulling: This method is an option open to all farmers and is probably the most effective way to remove parasites, especially in fields with a relatively low infestation level. In the case of Striga, however, much of the damage to the host occurs while the parasite is still completely in the ground. Removing mature Striga plants from an infested field will reduce the amount of Striga seeds, but it will not significantly increase the field yield of the host during the first years of weeding. It will usually take more than three years before a substantial benefit is seen. In Kenya, for example, some farmers have effectively controlled the Striga problem in their field using this method. In the case of Orobanche, hand-pulling has a greater potential for damage control since this parasite causes less damage before emerging above ground.
Rotation, catch crops and trap crops: A fallow period of at least four years generally keeps parasite infestation within economically manageable bounds. However, especially in population-pressed African countries, such long fallow periods are not feasible, because the farmers simply do not have enough land and have limited access to non-farm incomes. As alternatives, crop rotation, catch crops and trap crops have been proposed as control measures. Catch crops are crops that are susceptible to the parasite and thus become infected. Before the parasite has a chance to flower and set seed, the farmer can destroy the catch drop and plough the fields thus killing the parasites. This method should diminish subsequent infection levels when another crop is grown. Trap crops such as cotton are crops that stimulate parasite germination but which cannot be infected by the parasite. This method should also diminish subsequent infection levels. In spite of several reports of successes, it has been shown over the years that these two methods only have a reasonable effect in areas where the parasite infestation level of the soil is very low. Unfortunately this is not usually the case. Experiments in Kenya have shown that even after eight years of trap-cropping, damaging levels of Striga seeds remained in the soil.
Furthermore, for economic reasons many small-scale subsistence farmers in Africa often want and need to grow their preferred crop, often a highly susceptible crop, every season. Moreover, in some regions the rainy season is too short to grow more than one crop.
Biological control: Biological control has mainly focused on two different organisms. The weevil Smicronyx forms galls in the fruits of Striga thus reducing the seed production. However, reductions are usually quite low and the application of Smicronyx alone will not be sufficient to control Striga. Fungi like Fusarium ssp. cause diseases in Striga and Orobanche and can therefore be used as so-called mycoherbicides. In a number of cases the parasite emergence has dropped to less than 10 per cent after inoculation of the soil with these fungi. To obtain such a significant reduction, however, large amounts of spores have to be applied to the soil and there is a chance that these fungi will also start to attack the crop plants. It is questionable whether such an ecological risk should be taken.
Chemical control: Several herbicides, both individually and in combination have been shown to provide good control of Striga and Orobanche. In the case of Striga, dicamba and 2,4-D are most widely used. Both chemicals also control most other broad-leaf weeds, but have little or no effect on grasses and cereals. Dicamba is a systemic herbicide that is applied to the foliage of the crop about 35 days after emergence of the crop. It is translocated through the host’s roots to the parasite, where it accumulates to a toxic level. 2,4-D is sprayed several times directly on the parasites during the growing season, because Striga seedlings which are still in their subterranean stage are unaffected by it. Many other chemicals have been tested for their effect on Striga and some provide good control of the parasite. However, because of their cost and the technology needed, none of these chemicals are accessible to small-scale subsistence farmers in Africa.
For Orobanche the situation is somewhat different. There are fewer effective herbicides available for Orobanche than for Striga. Until recently, the only effective control method was fumigation, a chemical disinfection of the soil. However, this method is very expensive and thus applied only in more developed countries. The application of the systemic herbicide glyphosate, a foliar contact herbicide, is widely used, even though it does not always reduce the damage done by Orobanche. During more recent years attention has shifted to sulfonylureas such as chlorosulfuron and imidazolines such as imazapyr. Initial studies with these chemicals have shown some promising results.
A completely different approach to chemical control of root parasites would be the application of synthetic germination stimulants to the soil in the absence of a host crop. This would induce ‘suicidal’ germination of the Striga and Orobanche seeds, because germinated seeds cannot survive without a suitable host. The application of ethylene gas as a germination stimulant to the soil has eradicated Striga asiatica in the USA. Unfortunately, its application is very expensive and needs specialized equipment, making it unsuitable for use in developing countries.
Two synthetic germination stimulants (GR-24 and a compound called Nijmegen-1) which are structurally related to the natural stimulants (see box) have also proven to be highly effective in inducing suicidal germination at low con- centrations, but as yet only bioassays have been performed. Field experiments have been hampered because of the difficulty of producing sufficient amounts of the germination stimulants. Nijmegen-1, however, can now be produced in large quantities at low cost and field trials are scheduled for the near future.
Resistant crop varieties: Resistance of the host plants is probably the best way of eradicating the parasites. Resistance to Striga has been documented in cowpea, upland-rice and sorghum and appears to be based on just a few genes that can readily be crossed into other cultivars to produce high yielding resistant cultivars. Over the past few years, several resistant crop varieties have come into use in various parts of Africa, but full immunity to Striga or Orobanche has not yet been found.
On its own, none of the conventional methods currently used proves to be very successful in controlling parasitic weeds in the field. There is a general agreement that combinations of several methods should be applied to achieve a higher degree of efficiency. Such an integrated control of Striga and Orobanche was developed by Parker & Riches in 1993, with successful control of the parasitic weeds being reported. However, these integrated programmes are still only used on a small scale in a number of countries in Africa and Europe. Large-scale applications are not expected in the near future, mainly because of management problems and lack of financial resources.
Biotechnological approaches could provide new possibilities for parasitic weed management. In principle there are different approaches possible within the various phases of the parasite’s life cycle.
Modified stimulant production: Conditioned Striga and Orobanche seeds germinate in response to extremely low concentrations of stimulants. In order to limit parasite infection, two possible strategies involve manipulating the synthesis of the stimulant in host plants.
Parasite resistance: Host resistance to the parasite might be the ultimate way of controlling the parasites. However, very little is known about the physiological processes involved in resistance. A lot more physiological and biochemical studies will be needed before molecular biology research on resistance mechanisms can begin. If immunity to Striga does indeed exist in wild grasses, it will take years before the physiological and genetic basis of immunity or resistance in grasses is identified.
Herbicide resistance: A cost-effective and selective control of weeds might be possible through the introduction of herbicide resistant crops to protect the crop plant from herbicide damage (see also the article by Nap in Monitor No. 38). Herbicide resistance can be achieved by several different approaches.
Glyphosate resistance is an example of target site resistance transferred to a number of crops to make them resistant to Monsanto’s herbicide Roundup. Effective control of Orobanche was obtained with transgenic oil seed rape. Crops carrying glyphosate resistance require one quarter of the normal application levels. Another example is the control of Striga on maize resistant to acetolactate synthase (ALS) inhibitors (imidazolinone herbicides). Recent investigations into herbicide resistance in transgenic tobacco plants confirm that the plant transformation technology with genes conferring resistance to the herbicides Basta and Glea can be successful in controlling Orobanche.
Other promising results were achieved by the generation of transgenic asulam-resistance (a target-site resistance trait) to facilitate eradication of parasitic Orobanche. The use of transgenic crops engineered with target site herbicide resistances is therefore a very promising solution for plant parasite infestations in many crops.
As a proportion of the total research activities in the field of Orobanche and Striga, biotechnological research is quite limited and mainly focused on generating genetically engineered herbicide resistant crops.
Genetically engineered target size herbicide resistance has been developed and experimentally tested in tobacco, oil seed rape, sugar beet and potato against Orobanche. National Agricultural Research Centers (NARC) in Israel and Bulgaria are involved in these studies. An imidazolinone resistant maize was developed and tested in the Striga research laboratory in the USA, and proved to be successful in controlling this parasite by dressing seed with concentrated herbicide in field experiments in Kenya. NARCs in Israel and USA and the International Maize and Wheat Improvement Center (CIMMYT) in Kenya participated in these studies.
More fundamental research using Arabidopsis thaliana as model species to study the molecular-level aspects of the host-parasite interaction has been started in Virginia Polytechnic Institute (USA) and more studies are ongoing in Israel and the Netherlands.
The main activities in non-biotechnological research are still concentrated on control measurements in which breeding for resistance, chemical and biological control play an important part. Cultural methods such as rotation or intercropping are practised in countries where Striga and/or Orobanche form a biological constraint. These research activities are mostly in collaboration with NARCs from developed countries, partially funded by organizations like the European Union (EU), and national research programmes. There is a strong involvement of scientists in Striga research that aims to benefit developing nations in Africa and Asia.
Although Striga and Orobanche have quite comparable life cycles, there are also very distinct differences in their effects on the host plants. Therefore many research teams in industrialized countries study both group of species. It is to be expected that basic research on the Orobanche-host interaction will increase at the expense of Striga because Orobanche is a more important weed on economic crops in Europe and Israel than Striga. However, the results of these studies will probably also help controlling Striga.
The application of herbicide resistant crops has limitations due to the expected evolution of herbicide resistant parasitic weeds. Specific population dynamic models suggest that imidazolinone-resistant Striga would appear after three to five years in uniformly sprayed fields. It will therefore be necessary to design management strategies to delay the evolution of resistant Striga and Orobanche populations by lowering the selection pressure. With respect to gene transfer from crops to weeds, there is indeed a small danger of engineered genes moving from crops to related weeds (see Monitor No. 38). The possibility of gene movement has been discussed at length and might be risky when a weed is closely related to an engineered crop. The probability of gene flow from cultivars to wild species is always considered and has been classified as ‘low’ by studies such as Gressel et al. (1994). Of the crops infested by Striga there will be no problem with transgenic maize in Africa, because no wild relatives of maize occur in this continent. The chance that transgenic crops will become volunteer weeds is also not very high and can be further diminished if different herbicides are used in different years and crop rotation is applied.
The development of transgenic crop plants to deal with root parasites is certainly a worthy research approach, which however will take some time before it can be applied as a real control measure. Biotech derived resistance should therefore be integrated with other parasite control measures.
J.A.C. Verkleij* & E. Kuiper**
*Department of Ecology and Ecotoxicology of Plants, Vrije Universiteit, De Boelelaan 1087, 1081 HV Amsterdam, the Netherlands.
Phone (+31) 20 4447054; Fax (+31) 20 4447123; E-mail email@example.com
Eplee, R.E. and Norris, R. (1995), "Control of parasitic weeds". In: M.C. Press and J.D. Graves (eds.), Parasitic Plants. London, UK: Chapman & Hall, pp. 256-277.
Gressel, J., Kleifeld, Y. and Joel, D.M. (1994), "Genetic engineering can help control parasitic weeds". In: A.H. Pieterse, J.A.C. Verkleij and S.J. ter Borg (eds.), Biology and Management of Orobanche. Proceedings of the Third International Workshop on Orobanche and Related Striga Research. Amsterdam, the Netherlands: Royal Tropical Institute, pp. 406-418.
Joel, D.M., Kleifeld, Y., Losner-Coshen, D., Herzlinger, G. and Gressel, J. (1995), "Transgenic crops against parasites". Nature No. 374, pp. 220-221.
Kuiper, E., Groot, A., Noordover, E.C.M., Pieterse, A.H. and Verkleij, J.A.C. (1998), "Tropical grasses vary in their resistance to Striga aspera, Striga hermonthica, and their hybrids". Canadian Journal of Botany No. 76, pp. 2131-2144.
Parker, C. and Riches, C. (1993), Parasitic weeds of the World: Biology and Control. Wallingford, UK: CAB International.
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