December 11th, 2007
Orobanche spp. are one part of the broomrape family Orobanchaceae and are parasitic plants that germinate in response to chemical stimulants referred to as xenognosins. There is little known about the germination stimulants of Orobanche spp., but what is known is investigated through experimentation and through the geological distribution of the plants, methods of control of broomrape, and through the biology of the parasite. Basically what will be covered includes, what induces germination of Orobanche essentially what attracts it to its host, how Orobanche is classified and the current the most effective ways of controlling Orobanche.
The plant genus Orobanche exists as an obligate angiosperm, establishing a holoparasitic connection with a desired dicotyledonous host (Westwood, 2000). There are greater than one hundred species of Orobanche, of which only four taxa will be discussed in further detail. It is thought that the older Scrophulariales evolved from a hemiparastic orgin into the now holoparasitic family Orobanchaceae (Cooke, 2002). The genus Orobanche are holoparsitic in that they lack chlorophyll (Koskelva, Mutikainen, & Salonen, 2001) and are completely dependent on their host for nutrients (Cooke, 2002). A holoparasite connects to its host’s phloem using a haustorium to extract the nutrients, water, and organic compounds, which has a negative effect on various forms of the growth, reproduction and photosynthesis of the host. The parasite becomes a nutrient sink absorbing the nutrients causing depletion of the host (Koskelva et al., 2001).
The parasitic plant is also affected in this relationship. The host has a defensive system that helps protect it against the parasitic plant. The parasitic plant has to adapt to be able to overcome this immune response. (Koskelva et al., 2001). There are different species of Orobanche of which all have host-specific preferences. The species that will be discussed are Orobanche minor, Orobanche ramosa, Orobanche cernua, and Orobanche crenata. Members of the Orobanchaceae family are found in Mediterranean, Asian, and African distributions mainly in temperate and in some subtropical regions (Liu, Randle, Steiner, & Wolfe, 2005).
The taxonomy of Orobanche is complicated, there are as many as 150 species of broomrape (Musselman, 1980).Classifying Orobanche in the Plante Kingdom, it belongs to the Subkingdom Tracheobionta, the Superdivision Spermatophyte, in the Division Magnilophyta which is in the Class Magnoliopsida, and the Subclass Asteridae, the Order Scrophulariales, and in the Family Orobanchaceae which is essentially Genus Orobanche (USDA, 2007). Table 1 and figure 1 show the phylogeny and biogeography of Orobanchaceae. Table 1 includes the distribution of the genus of Orobanche and the general number of species that inhabit that area. Figure 1 is the phylogeny of Orobanchaceae and also includes the distributions of the different parasitic plants (Liu, Randle, Steiner, & Wolfe, 2005).
The common name for Orobanche is broomrape, the word Orobanche comes from the Greek translation “orobos” which translates “to strangle,” the word strangle seems a more appropriate clarification because only certain species of Orobanche attack broom (Mussselman, 1980). Orobanche crenata was first discovered in Bulgaria in 1935 infecting sunflowers (Kreutz, 1995). Orobanche romosa was first described in 1753 in Europe (Cooke, 2002). Orobanche belong to the family Orobanchaceae which include hemiparasites as well as holoparasites. Hemiparasites only partially rely on their host, while holoparasites completely rely on their host for nutrients. The holoparasite shows evidence of a hemiparasitic origin through their early independence at development to their complete dependence at germination, and growth. Evolution of a holoparasite from a hemiparasite could have been from a suppression of the need for chloroplasts until the process of photosynthesis was unnecessary (Cooke, 2002).
Broomrapes show a wide range of flexibility in the environmental conditions they can endure. They prefer disturbed areas with constant movement and an open habitat that is sunny and favors increased transpiration (Musselman, 1980). Orobanche need a warm, wet environment for their incubation periods, allowing the passage of water through the plant from the roots into the plant’s vascular system and into the atmosphere (Cao, Jin, Joel, Song, Takeuchi, & Yoneyama, et al.,2005). Orobanche species have certain germination requirements, Orobanche crenata for example, requires conditioning by exposure to moisture at temperatures between fifteen to twenty degrees Celsius for at least eighteen days. Orobanche ramosa is at twenty one degrees Celsius for seven days (Cooke, 2002). In some cases, broomrape parasitism often increases with increased temperature (Colquhoun, Eizenberg, & Mallory-Smith, 2004).
Biology of Orobanche
Orobanche can be annuals or perennials; they come through the soil forming an asparagus-like shape. The roots of Orobanche do not have root hairs and the stem is a fleshy brown-yellow due to lack of chlorophyll. They can have up to twenty flowers and produce around five hundred to five thousand seeds per capsule. Each broomrape plant can produce thousands of seeds which may remain dormant in the ground creating a seed bank that can be activated by stimulants at any time (Chae, Joel, Iso, Rungmekarat, Sato, & Takeuchi, et al.2004). Seeds from Orobanche cernua are yellow but turn a dark brown when ripe. Like any seeds, they have ways of travel through water, or birds, the surface of Orobanche seeds allows air to be trapped making it easier for them to float. When buried in the soil they can stay dormant from five to twelve years and only germinate when presented with a suitable host (Yang-han, 1981).Figure 2 shows a depiction of an Orobanche cernua seed.
After the parasite has depleted its host’s nutrients it withers away back into the ground, but sometimes a part of the parasite remains lodged in the plant, thus making it a perennial when the plant is able to provide germination stimulants again (Kreutz, 1995).
The parasitic plant has to undergo host recognition in order to determine if a host is capable of supporting the plant. The parasite begins development and germination in response to reception of a chemical stimulus from host roots. Specific xenognosins regulate the seed germination, development of the haustorium, and later stages in host penetration (Keyes, Kim, Lynn, & O’Malley, 2000). Xenognosins are germination stimulants that trigger the transition of the plant from autotrophic to heterotrophic growth by signaling haustorium development. Currently three germination stimulants have been extracted from plant secretions and they are termed strigolactones in an experiment conducted by Okuno et. al 1998, which collectively studied these processes. The three germination stimulants in the experiment by Okuno and colleagues were termed strigol, alectrol, and sorgolactone. Strigol came from a false host, alectrol came from a genuine host, and sorgolactone came from a genuine host. This experiment was based solely on the germination stimulants that were taken out of the secretions of a plant. There was no certainty that these were the actual stimulants that allow for germination of the parasite into the host. The experiment shows that alectrol later named orbanchol and an unknown stimulant which came from the genuine host red clover may be involved in the parasitism of Orobanche minor. It is then thought that strigolactones are most likely a germination stimulant for Orobanche but there is no conclusive evidence that just because these results occurred in lab that would also occur in natural surroundings (Okuno, Sakai, Takeuchi, Yakota, & Yoneyama, 1998).
Life Cycle of Parasitism
In order for host recognition to take place the seeds need to be conditioned first (Chae et al., 2004). The germination process of the holoparasite Orobanche is an intricate and complex system of stimulants and connections. The first stage of the germination process is referred to as after-ripening. It is the period of time between when the seeds are shed from the plant and when the seed goes through conditioning to prepare for host recognition (Musselman, 1980). Figure 3 shows the steps and processes in the seed phase of the life cycle of Orobanche spp.(Dhanapal, Struik, Timmermans, & Ubayalkumar, 1996). Conditioning is the second step, it is also referred to as pre-treatment. This stage can be halted for an extended period of time, because the seeds need the correct environment in order to germinate. Conditioning can occur as little as two weeks for specific species such as Orobanche crenata and Orobanche minor.
Haustorial initiation is a key component in the germination process. The Orobanche seed detects a chemical signal that is emitted from the host’s roots which in turn triggers the germination process (Westwood, 2000). A radicle or germ tube emerges in response to the stimulant. The radicle has to develop a haustorium, which is the organism through which all the transfer is done between the host and parasite (Musselman, 1980). The haustorim is the multicellular organ which all parasitic plants rely on, it is a formation of a tube through which the nutrients of the host are connected through the phloem and brought into the parasite. Figure 4 shows the steps and processes in the germination phase of the life cycle of Orobanche (Dhanapal et al., 1996). The haustorim penetrates the host through the roots and establishes connections developing a tubercle. The tubercle is the structure that emerges from the soil to disperse the seeds for the next generation of parasites (Westwood, 2000). Figure 5 shows the various tubercle stages (Colquhoun, Eizenberg, & Mallory-Smith, 2004). Figure 6 shows the steps and processes in the parasitic/reproductive phase of the life cycle of Orobanche (Dhanapal et al., 1996).
The germination stimulants of Orobanche have yet to be identified. There have been experiments to extract the stimulant, but it is inconclusive evidence whether or not the extracted stimulants actually occur in nature or not (Westwood, 2000). There is information supporting effective and artificial germination stimulants, the natural initiation of host stimulation still remains unclear. The stimulant that the host gives out contains growth inhibitors which allow the parasite to detect the presence of the host and directs the growth of the parasite toward the host roots (Musselman, 1980). After the parasite develops a connection with the host, not only does the parasite react with the host, the host reacts with the parasite in that it triggers a defense mechanism to fight the parasite. This allows the parasite to adapt and evolve against the resistance of the host (Westwood, 2000). Figure 7 shows the three main phases in the life cycle of Orobanche spp.(Dhanapal et al., 1996).
Parasitization by Orobanche involves many complex processes. Many experiments have been conducted to understand the process that Orobanche goes through to attach to its host. One of the experiments was conducted by a scientist Westwood, who uses a model plant Arabidopsis thaliana to understand the initiation of parasitism. Figure 8 shows the germination rates of five Orobanche spp. in the presence of Arabidopsis thaliana (Westwood, 2000). It was assumed by Westwood that the germination sequences are specific to the parasite through interactions in the tissues and roots of the host. The four species of Orobanche (cernua, crenata, ramosa, and minor) underwent three treatments. The first was being introduced to the Arabidopsis thaliana roots, the second was being introduced to the Arabidopsis thaliana roots along with a synthetic stimulant labeled GR-24, this was known as the positive control. The third treatment was termed the negative control and didn’t have the Arabidopsis thaliana roots or the stimulant. The results concluded that Orobanche ramosa only recognized the host, while only a few seeds of the other species germinated slightly. This could be due to the specificity of Orobanche cernua, crenata and minor in their host preferences (Westwood, 2000).
Host range and Impact and Effect on Economy
Orobanche includes a broad host range, mainly focusing on dicotyledonous plants. Some of the families of hosts infected include Fabaceae which is the legume, pea, bean and pulse families (Mahn, Jones, & Wojciechowski, 2007), Solanaceae or the potato family (USDA, 2007), and Asteraceae which is the aster, daisy or sunflower families (Wikipedia, 2007). Each species has its own host preferences which pertain to certain families. The parasite’s ability for host recognition is thought to function on multiple levels. If a parasite comes into contact with a plant that is not usually a host the host plant produces germination stimulants, but they have no effect on the parasite, therefore the plant cannot support the growth of the parasite. This is one example of the complex levels that go into host recognition and the process of parasitism.
Orobanche crenata, which mainly affects the faba bean ((Vicia faba), lentil (Lens culinaris), carrot (Daucus carota), pea (Pisum sativum), chickpea (Cicer arietinum), vetches (Vicia spp.) and other crops in Mediterranean countries was studied under certain geological conditions to determine why Orobanche prefers the habitat that is does. The only limiting factor was that unlike other species of Orobanche, Orobanche crenata has never deviated outside of its geological area; it has not spread as the other types of Orobanche have. The results of this experiment showed that Orobanche crenata can only complete its life cycle, where soils dry out during a period with high temperatures, and where there also is a wet-warm period. Such conditions are highly expected in regions that have distinct precipitation seasonally, including the complete Mediterranean areas where most genus of Orobanche inhabit (Grenz, & Sauerborn, 2007).
Each species of Orobanche is distinct in that it will only germinate when in contact with certain hosts. Certain germination stimulants can be used to allow the host recognition to occur. Orobanche can be stimulated, even if it cannot use the plant as a host. Many of the important affected crop species are found in the Solanaceae, Fabaceae, Asteraceae, and Apiaceae families. Orobanche crenata prefers Fabaceae which is the legume, pea, bean and pulse family, as well as the Compositae family and the Umbelliferae family (Westwood, 2000). Orobanche cernua is specific to the sunflower plant (Helianthus annus L.)(Musselman, 1980). It also favors the potato family or Solanaceae. Orobanche ramosa is found on tobacco (Nicotiana tabacum L.), tomato (Lycopersicon esculentum milli.), hemp (cannibus sativa), and maize (zea mays) (Kreutz, 1995). Orobanche minor is familiar to the Fabaceae family as well along with the compositae family (Westwood, 2000).
The extent of Orobanche’s destruction can yield from no damage up to 1-100 % crop loss. In the case of minor damage the crops can become wilted and their growth stunted and barely yield any profitable crops. Farmers may have to choose their crops according to the susceptibility of Orobanche infestations, which may result in less profitable seasons (Andreasen, Bernhard, & Jensen, 1998). High crop losses have to do with the parasite’s diversion of water and nutrients from the host (Westwood, 2000). Orobanche crenata is significant in all Mediterranean countries with losses due to this species from 50 – 80%. Orobanche ramosa parasitizes various cultivated plants such as tobacco, tomato, hemp and can cause reductions of up to 40% (Chae et al, 2004). Several factors play a role in crop losses, such as host susceptibility, level of infestation, and environmental conditions (Abang et al., 2007). The level of infestation is determinant on however many seeds germinate. For example, a single Orobanche minor plant in a field of Trifolium pretense in the Netherlands was a significant enough infestation for the ministry of agriculture to close out the whole crop (Kreutz, 1995).
In one experiment Alcántara and colleagues observed the infestation of sunflower plants with Orobanche cumana which is similar to Orobanche cernua. Sunflower plants were grown in pots maintaining a pH of 7.3 and they were monitoring the growth, development and mineral nutrition of the plants during the infestation. Two treatments were followed, with broomrape and without broomrape which was the control. For each treatment there were twelve pots, with one sunflower per pot. Temperatures were regulated from 15 to 30 degrees Celsius, the plants received tap water and two hundred milliliters of nutrient solution and at certain specified days 57, 67 and 77 days after sowing four plants from each treatment were collected. The plants were examined using several techniques and the results are shown in figure 9. The plant height in centimeters, the number of leaves per plant, the dry weight of the plant in grams per plant, and the head diameter in centimeters were measured from this experiment (Alcántara, Díaz-Sánchez, & Morales-García, 2006). .
Distribution of Orobanche
The Orobanche genus is widespread in many geographic areas. They are found in the Mediterranean region, East and South Africa and the Middle East (Abang, Abu-Irmaileh, Bayaa, & Yahyaoui, 2007). There are reports in France and Russia where Orobanche ramosa is affecting oilseed rape (Brassica napus). In the United States Orobanche minor infests the clover population. In China there are about twenty Orobanche species including Orobanche ramosa (Chae et al., 2004). Table 1 shows the distribution of some of the species of Orobanche. Figure 1 shows the parsimonious tree which involves the phylogeny and biogeography of some of the species of Orobanche (Liu et al., 2005).
Control and Prevention of Orobanche Parasitism
There are various methods of control when dealing with Orobanche. Orobanche must be controlled before production of a host crop (Colquhoun, Mallory-Smith, & Ross, 2004). Known measures which have been experimented with in using would be for example, hand weeding. Hand weeding can be effective but it is a very time consuming task and not completely worth the effort. Another method would be adjustment of the sowing date. By adjusting the sowing date of certain crops has showed that broomrape parasitism significantly decreases under the lower temperatures. For example, growing broad bean (Vicia faba) in the winter instead of autumn reduced Orobanche crenata. In Spain, the adjustment of sunflower sowing dates into the winter reduced Orobanche cumana parasitism (Colquhoun, et al., 2004).
Utilizing an experiment on the adjustment of sowing dates along with crop resistance using Orobanche crenata yielded significant results. Just using one method of crop control does not encompass complete control. Legumes are normally used to help the adjustment of sowing dates although they have a long life cycle and delayed sowing can decrease their yield because of the shortened crop season, which is why resistant crops are usually incorporated as well. The results of an experiment conducted by Cubero and colleagues showed that the higher the rainfall during the season, the more infected the faba beans became, due to the fact that Orobanche thrives under moist conditions. Infection decreased with the adjustment of sowing and throughout the season the intensity varied. The resistant portion of the experiment showed the same results, around October there was little attachment in the susceptible plants. But the attachments in November and December increased. Essentially, later sowing dates reduce the amount of emerging shoots in the host and the resistant plants were least infected (Cubero, Moral, Pérez-de-Luque, Rubiales, & Sillero, 2004).
Crop rotation is a method that is commonly used to avoid parasitism. By rotating the areas in which you plant your crops it avoids the buildup of pathogens and pests such as the parasitic plants that often occur when one species is continuously used. One of the main methods of control of Orobanche has to do with the use of biological control agents. The use of biological control of parasitic weeds with insects and some other agents has been reviewed and several insects are known to attack Orobanche. There include the Dipteran Sciara sp. that eats the seedlings and stems of some Orobanche. Another insect is the agromyzid fly (Phytomyza orobanchia), it has long been attacking certain species of Orobanche so much that populations of Orobanche are decreasing in Yugoslavia (Musselman, 1980). It is a pest of the Orobanche species and it helps reduce the seed bank in the soil by boring into the bulb and stem of Orobanche and eating most of the seeds. It doesn’t eat all of the seeds however, so some seeds survive and are still capable of growing. There are other options that could yield better results (Abang et al., 2007).
The use of herbicides is also another means of control, however it is an expensive means, and the areas in which Orobanche are populated, there isn’t a high economy in which most farmers can afford the herbicides let a lone have time to apply them continuously. Demonstrations on how to apply certain herbicides like glyphosate were offered in certain areas, but farmers wouldn’t attend (Abang et al., 2007). Another problem with herbicides is that most herbicides cannot differentiate between crop and parasite except for transgenic crops that are targeted for specific herbicide resistance, but that type of technology has yet proven to be effective (Amsellem, Barghouthi, Cohen, Goldwasser, Gressel, & Hornok, et al., 2001). Solarization is another method of control. It involves placing transparent plastic sheeting over the tilled soil during the warmest months of year. The plastic traps the heat of the sun. Under plastic sheeting, the top few inches of soil can be as much as 20 degrees warmer than uncovered soil. In this extra-warm environment, the heat eliminates weed seeds and seedlings. When possible, standard techniques of phytosanitation may help to control the spread of certain species of Orobanche. It is known that clean clover seed has restricted the spread of Orobanche minor in some situations (Musselman, 1980). Solarization has been proven to be effective but it is partly banned due to environmental risks (Amsellem et al., 2001).
Trap and cash crops are also another means of reducing Orobanche numbers in the wild. These false hosts can be used to help decrease the Orobanche seedbank in the soil. False-host crops decreased the Orobanche crenata seedbank by around thirty percent in one cropping cycle (Colquhoun et al., 2004a).Essentially False host plants help to stimulate the parasitic seed germination but do not promote tubercle formation, the non-host plants do not stimulate parasitic seed germination or attachment with the parasite. The false hosts release a stimulant that only promotes germination (Colquhoun et al., 2006).
Trap crops are plants that are planted around the original crop to help lure away the parasite, in this case Orobanche. Trap crops usually stimulate germination but do not support parasitism. Catch crops are plants that are grown quickly between plantings of a main crop to lure out the parasitic plants. Catch crops are usually hosts that both stimulate germination and support parasitism; they are usually plowed under to prevent parasite maturation (Musselman, 1980). In an experiment that was conducted to find a useful trap crop for Orobanche minor to help reduce the seedbank in the United States to protect certain crops, wheat was found to be a successful false host. Wheat has the potential to be a useful trap crop for Orobanche minor, if it is utilized correctly in the future it could be a useful control method (Colquhoun, Lins, & Mallory-Smith, 2006).
A big amount of research has gone into the selection and breeding of resistant crops. An example of breeding plant resistance occurred in the Soviet Union in the 1920’s. Orobanche cernua resistant sunflowers were developed, but with time, resistnace was lost and new host strains were developed. That is one problem with breeding resistant crops, the parasites may eventually adapt and develop a new “super” strain which may be more effective and harder to maintain and get rid of. A similar situation occurred in France with Orobanche ramosa (Musselman, 1980).
Parasitic broomrapes are an uncontrolled weed mainly encompassing the Mediterranean areas. There are certain measures that have to be done to keep them under control, of which there have been many attempts to discover remedies that are not too costly as well as efficient to save crops that many countries depend on. In order to develop these antidotes to the problem I believe more work needs to be done to discover the germination stimulants which cause Orobanche to originate. Throughout the articles discussed I believe there was sufficient knowledge in the extent of Orobanche’s destruction yet there was little knowledge about what chemicals generate from the actual parasite or the host even to allow for the propagation of broomrape.