LATE BLIGHT POTATO
What is late potato blight?
In about 1800, potato was introduced in Europe from the South and Central America, and it was a well established crop in Ireland (Agrios G, 2004). But when the weather over Northern Europe and Ireland became wetter and cooler during growing season of 1845, the potato crop began to show blighted leaves and shoots (Agrios G, 2004). Potato blight can be referred to two types of blight diseases which are late blight caused by OomycetePhytophthora Infestans and early blightcaused by the fungusAlternariasolani.Of all the potato diseases, late blight is the most destructive disease. In the late 19th century the late blight disease spreads quickly and caused the Great Potato Famine. This epidemic disease caused a serious starvation that leads to up to one million death of people in Ireland alone and forced a number of people to migrate to Europe and USA (Song et al., 2003). Potatoes are the world's fourth largest food crop, afterrice,wheat, andmaize. Currently, late blight is responsible for multibillion-dollar loss in potato production annually. Moreover, in developing countries, where funds for purchasing fungicides are limited, late blight can completely eliminate potato crops since many developed countries (e.g. US) still rely extensively on fungicide applications (Song et al., 2003). The disease has been around for more than 150 years and is still the worst disease problem for the potato growers.
Control of Late Blight
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Late blight is one of the plant diseases that are hard to control, but since it has caused mass destruction on potato crops and a great loss a few control mechanisms has been used to overcome it. The major approach to prevent late blight development is by using fungicides. A number of broad spectrum (e.g. Copper) and systemic fungicide (e.g. Metalaxyl) are used to control late blight (Agrios, 2004). For example in 1971, Northeastern of USA was sprayed with fungicides (Fry W.E, 1976). Traditionally, it relied on cooper based fungicides such as Bordeaux mixture. Fungicides are most effective if they are applied to foliage before any infection occurs or when the disease is still in the early stages (Johnson D. E, 2001). Fungicides action against late blight is by inhibiting germination of spores, sporulation, and lesion formation as well as development. In England and Wales, during the period of 1978-1992, penylamide (Metalaxyl) fungicides has been used with the dithiocarbamate fungicides and were tested for the potato late blight in 51 separate fields in England and Wales (Bradshaw and Vaughan, 1996). The mixture of phenylamide and dithiocarbamate showed a better control of foliage blight than usage of the dithiocarbamate alone. Instead, haulm destruction is also applicable. The destruction of potato haulm is important to reduce the late blight and virus spread by reducing harvest interference, improving set skin and to control the tuber size. If there is an establishment of P.Infestans on potato foliage, there are chances that the sporangia will be flowed down by water and infect the tuber. Sanitary measures such as only disease free seeds can be used to seed, any potato dumps need to be burn before planting time in the spring, and all volunteers of potato plant need to be demolished are among control actions that has been applied (Agrios G.N., 2004). Alternatively, sexual crossing and resistance breeding can also act as one of the disease control. Report made in 1984 claimed that A2 mating types of P.Infestans were introduced in Western Europe (Fry et al., 1993). Previously only A1 mating type had been detected outside Mexico due to migration (Fry et al., 1993). Sexual reproduction will generate a large number of genotypes that can overcome the attempts to control late blight. Breeding is one of the major ways to produce stable potato cultivars with good resistance to unfavorable climate conditions, fungal and bacterial diseases, virus infection and pest. There are two most important resistance genes (R genes) that have been introduced to the commercial cultivars that will be explained later.
The Pathogens-Phytophtera Infestans
Members of the genus Phytophthora pose threats to several fields especially in agriculture and food production, causing serious diseases in many plant hosts (Judelson and Blanco, 2005). P.Infestans, pathogenicity and biology still raised some controversies up until today (Judelson and Blanco, 2005). This oomycetes belong to the Kingdom Straminipilia, with groups like brown algae and diatoms (Baldauf, 2000). Oomycetes are a group of organisms in a kingdom separate from the true fungi, plants, or animals. Which include them in the Kingdom Protoctista or Chromista. This group of organisms is characterized by the absence of chitin in the cell walls. It is a heterothallic oomycete under natural or agriculture condition (Fry, 2008). It acts by parasitizing the potato plant under suitable conditions. The life cycle of P. infestans has been well studied. It follows three basic steps, formation of mycelium in the host plant, spatial expansion of the affected area lesion in the host plant and formation and dispersal of spores. This oomycetes will generate spores in two ways - asexual reproduction (vegetative mycelium in infected tubers) and sexual reproduction (in the form of resting spores). Massive efforts have been developed over the last ten years that provides detailed analysis of the Phytophthora life cycle stages. Sporangiophores are the spores produce by Phytophtera by asexual reproduction (Erwin et al 1983). These branch and multinucleate spores developed at the termini of specialized hyphae are then called sporangia when they release six or more zoopores. Since the sporangia can detach from the hypha for dispersal (term caduceus), wind or water can easily cause sporangia to travel in several kilometers (Aylor, 2003). These flaking sporangia actually allow researchers to easily purify them and further use them as study materials. Asexual reproduction is divided into two different pathways -direct and indirect germination. The asexual life cycle begins as the sporangia are carried away by the wind, as they landed on the plant tissues, zoopores will be released and enter host through several opening such as wounds or stomata (Judelson and Blanco, 2005). Temperature above 14oC allows direct germination to occur. Hyphae will emerge through the sporangial wall and penetrate the host tissue (formation of germ tubes), thus have a chance to gain nutrients from the host. In contrast, the indirect germination (zoosporogenesis) takes place in temperature below 14oC, zoopores are released, encyst and produce germ tubes. At the same time, zoopores can also infect the stems and tubers when they are washed down by water flow. Comparing both reproduction ways, sexual reproduction differ from the asexual reproduction in just a few steps at the beginning. The presence of two mating types (antheridium and oogonium) - allow the sexual reproduction to occur. Male and female gametagia are formed when normal vegetative growth and asexual reproduction are repressed. Nuclei from both types will fuse together as the antheridium enters the oogonium in response to hormones. Germinated oospores are generated with diploid oospore. Then, the diploid oospores will grow to become sporangium and follow the asexual cycle as before.
Development of the disease.
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It is possible for the late blight to be blown from nearby gardens, allotments and commercial crops miles away and easily infect the host. After the sporangia landed on the plant, about 6 to 12 zoopores are released with the help of water drops on the plant surface. Further development of the fungus after infection is mainly determined by climate conditions. Germination involves the cleavage of the sporangial cytoplasm by nuclear envelope and the existence of two flagella that helps the zoopores swim out of the sporangia (Hardham, A. R. & Hyde, G. J, 1997). After the cleavage of sporangia, proline concentration will be increased and built up turgor pressure which then needed to expel the zoopores out of the sporangium (Ambikapathy J et al, 2002). Later, zoopores lose their flagella and form cysts that initiate the formation of germ tube that bind tightly to the host plant. Within a few hours, hyphae grow and penetrate the host tissues through haustoria which are the hyphal tip by forming biotrophic interaction. In hours or even days, zoopores can remain motile by moving more than 6 cm through by water (Ristaino, J. B. & Gumpertz, M. L, 2000). The infection processes usually take place on both sides of the leaf although; the lesions usually appear first on the lower leaves.
In the incubation period-the period between penetration of the host and the first appearance of lesion that lasts between 2-3 days, then only few symptoms start to appear. It is hard to detect the appearance of initial infections, since the potato leaves tend to develop various brown spots in vegetative period. On the potato leaves, late blight appears as pale green at the leaf edges. Healthy tissues will be merged by pale yellowish border that surrounds the leaf lesions (Agrios, 2004). As the plant tissues die following the growth of the lesions, brown spots surrounded with the yellowish green margin start to spread within plant tissues. In high humidity, a cottony and white mold growth is able to be seen on the lower leaf surfaces (Randall et al, 2001). In contrast with the dry conditions, white molds growth will disappear when the leaf surfaces has been dried up. When there is moist, potato stems will show brown color and turn to black. Whereas, potato tubers shown coppery brown granular spread from the surface to the outer part of the tissues. In the late blight disease cycle, tuber blight is a critical stage in the late blight disease cycle. Later, secondary bacteria often usually will enter late blight lesion and cause a breakdown of the entire tubers. If the surrounding temperature is low; the lesion development will be retarded.
Resistance genes in Potato
There are few events where, P.Infestans begun to developed some degree of resistance. In between 1940-1950 in Mexico , an A2 type was discovered where the pathogen originates and spread through the USA, Europe, Asia and North Africa in the 1970s and 1980s (Fry et al. 1993). A2 type can then mate with A1 type sexually and produce strain that overcome the pathogen infection. There are few efforts that have been taken to increase the potato resistance to the new mating strain type, by focusing on the race non-specific resistance that end up with a little progress . Genes conferring race-specific resistance often control disease resistance in plants. Gene-gene interaction is when R genes interact with the Avr genes in the pathogen. In the last ten years, more than 55 plant disease R genes related to monogenic resistance have been isolated from a number of plants that nine of the R genes are from potatoes (Van Oijen, 1995).
R genes-Rpi-blb1 & Rpi-blb2
While fungicides spraying over the season requires huge amount of money to the farmers, it also pose a danger to the environment as well as pathogens will have more chances to develop resistance. There is an environmental way, by providing cultivars with the resistance genes that act against broad spectrum of P.infestans that is much more economically and environmentally sustainable. Resistance to late blight is determined by single dominant R genes inducing an hypersensitivity response (HR) response upon infection with specific races of P.Infestans leads to disease resistance of the host plant to the incompatible pathogens (Zhen et al., 2000). A total of 11 resistance genes that originated from wild type S.demissum have been used into diverse potato cultivars which confer race-specific hypersensitive resistance (Song et al., 2003). S.demissium became the sources of resistance in breeding programme by using classical breeding method (Halterman et al., 2008). However, all of these genes are easily defeated by potato blight in most regions, although there is evidence that combination with other sources of resistance will be useful (Stewart et al., 2003). In contrast, disadvantages of Rgene-mediated resistance are, it is often short-lived, quickly overcome by new races of the late blight pathogen because it is only compatible with the appropriate avirulence gene of P.infestans when they secrete effector protein into the host cytoplasm or apoplast (Bhaskar et al., 2008). For an example, no detection of strains in central Mexico by the Solanum berthaultii R gene located on chromosome 10, which at the same time can detect the same strains of P. infestans in the USA (Ewing et al., 2000).
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Durable R genes have been identified in wild species Solanum bulbocastanum. Several years has been used to test and to confirm that it has the potential in identifying all genotypes of P. infestans (Song et al., 2003). Potentially by somatic hybridization it is impossible to introduce valuable R genes from sexually incompatible wild species (S.bulbocastanum) into cultivated plants (S.tuberosum), since both differ in ploidy. S.tuberosum is tetraploid while S.bulbocastanum is diploid. Two alleles located on the same locus on chromosome 8 of S.tuberosum is the RB (further refer as Rpi-blb1), whereas on chromosome 6 is localized by Rbi-blb2 (Song et al., 2003, van der Vossen et al., 2003). Rpi-blb2 and Rpi-blb1 both belong to the NBS-LRR class of R-genes (van der Vossen et al., 2005). Discoveries of both genes give hopes that they might be more “durable” than the classic race-specific R genes formerly used in potato breeding. The other factor that stimulates the RB resistance in potato blight is SGT1. In one study focusing on which genes is responsible for the RB mediated broad spectrum resistance to potato blight by silencing either Rar1andSgt1genes using RNA interference method. Both of these genes are believed to have function in resistance of broad mediated spectrum. SGT1 is more essential compared to RAR1 because the lines containing (Bhaskar et al., 2008). On the other hand, there is a correlation between the abundance of the RB Gene and the level of the RB-Mediated in late blight resistance in potato shown by Kramer et al. RB gene that has been cloned subsequently belongs to the largest class of R genes and encode proteins with a nucleotide binding site and leucine rich repeats (NB-LRR). By making a comparison between RB genes and R genes derived from the S.demissum, RB confer a degree of resistance rather than immunity (R genes).That why, from our observation, R genes will quickly produce necrotic lesion resulting of hypersensitive response (Vleeshouwers et al., 2000). Hence, RB gene is more interesting and reliable in defending the host from the blight.
Gene codes region of RB with about 5kb of upstream regulatory sequences was integrated into S.tuberosum mediated by a transform Agrobacterium by PCR producing transgenic plant that are highly resistance to the blight although being treated with multiple pathogens under optimal blight conditions (Colton et al., 2006). An experiment was conducted to observe how four famous potato cultivars in USA (Katahdin, Superior, Dark Red Norland, and Russet Burbank) perform with integrated RB gene based on the foliage and tuber resistance. The result shown that all transgenic lines of S.tuberosum with RB have strong resistance in foliar, whereas tubers did not exhibit an increase in resistance although the experiment has been done again in two years period to confirm the findings (Halterman et al., 2008). The reason there is no resistance in tubers is not because of tissue-specific transcription of the RB transgene since tubers of RB-transgenic plants have RB mRNAs. So, instability of the RB protein is the reason for the loss of the resistance (Halterman et al., 2008). In another study, Song et al. (2003) reported that of 14 RB transformed lines, 5 were highly resistant and 9 were reasonably resistant, with less than 10% and 11% to 25% infections, correspondingly compared to non-transformed lines.
In addition, with susceptible potato (S.tuberosum) a variety of Kathdin which has different amount of RB gene, Kramer at el have found that there is a correlation of abundance of RB gene and level of RB mediated late blight resistance in potato. Transgenic lines with multiple numbers of RB genes constantly have a high level of resistance compared with lines which only have a copy of RB gene that relates to the basal amount of RB transcription. High resistance is measured by the amount of RB transcript being induced right after the pathogens infection, therefore if there is an increase in the RB transcript and a constant level of its production indicate a high level of resistance (Kramer et al., 2009).
The Rpi-blb2 gene was originally mapped in a number of ABPT (ellipsis of four Solanum species involved: S. acaule, S. bulbocastanum, S. phureja and S. tuberosum) -derived from tetraploid backcross populations. It has the same region as the Mi-1 gene from tomato which located on chromosome 6. Rpi-blb2 gene is capable to complement the susceptible phenotype in both cultivated potato and tomato (van der Vossen et al., 2005). Similar to the function of Rpi-blb1, Rpi-blb2 also has the ability to perform broad spectrum resistance to P.Infestans. In Netherland, annual screening of potato clones harboring the Rpi-blb2 genes showed no sporulation lesions at the end of the growing season (van der Vossen et al., 2005).
Future of Transgenic Potato.
In recent years, German chemical & biotechnology firm BASF - one of the world's leading companies in plant biotechnology working on developing genes which focusing on yield and quality traits. BASF has introduced potatoes that genetically modified .They are working on a promising fungus-resistant potato which contains two genes from Mexican wild species potatoes as describe before which are Rpi-blb1-Rpi-blb2. Several countries remain opposed to the cultivation of GM potatoes, raised few reasons such as biodiversity reduction in certain regions and natural resistance to pests and disease. Similar to traditional breeding method, the BASF scientists searched the potato strain that have a high level of resistance to the P.infestans. Molecular biology techniques were used to locate the specific genes into the cultivar species. In order to avoid any possibility of resistance that the pathogens may initiate, they offer "double" resistance by these two genes. Since 2006, a number of field trials have been done in Netherland, Britain, Sweden, Germany and Ireland to test the resistance in greenhouse environment and all of the field trials showed a success (GMO, 2009). The next plan is to commercialize the GM potatoes on the market that can be access by the community. Proposal made by the BASF Plant Science indicate that genetically modified potatoes that resistance to Phytophthora infestans are not expected to exert any toxic or harmful effects on both human health and the environment (BASF, 2006). That is because Rpi-blb1 and Rpi-blb2 belong to the NBS-LRR class and no member of the NBS-LRR protein class has been identified to confer any toxic or allergenic properties.
Due to the aggressive genetic exchange of the P. infestans populations and the rapid nature of disease development, late blight can cause a total loss of the crop (Fry 2008). In United States alone, the late blight cost for growers were estimated to be $288 million yearly (Guenthner et al. 2001). This definitely brings a big problem to the governments that need to invest huge amount of money on potato disease. Until now, massive effort has been done to manipulate the potato plant resistance genes in wild species by various experiments. Since late blight remains one of the most serious diseases of cultivated potato, the findings need to be applied as soon as possible to reduce a huge loss since potatoes are among the four major crops worldwide.
The increasing pool of genes confer resistance to late blight raises few question of how to incorporate such resistance into cultivated lines with more than one source of resistance presents therefore increased challenges for the potato breeder. Plant transformation offers an efficient method to transfer genes from divergent organisms directly into the plant genome. Resistance genes are potentially useful control measures for potato late blight. Both Rpi-blb1 and Rpi-blb2 have broad-spectrum resistance and conferred resistance to a range of isolates of P. Infestans.
The awareness of global pathogen migration and how it can colonize in a short time has raised a few questions such as are there any more aggressive genotype of oomycete that soon might be migrated to new locations or will sexual reproduction which produce oospores that have a strong resistance to fungicides and does mating type influence the pathogen fitness. Therefore, plant pathologists need be alert about new pathogens and becoming more aware of any signals of changes in populations of old pathogens. There is still possibilities that the Great Irish Famine might happens again in the near future, hence plant pathologist have a huge responsibilities to find the cure for the disease and at the same time to draw consumers' confidence to accept the new findings and then can be commercialized so that potatoes are still exist to fulfill nutrition of the world populations.
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