Bacterial Wilt Of Tomato The Current Situation Biology Essay

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Introduction

Bacterial wilt is a major limiting factor in tomato production throughout the world. Furthermore, it can be directly related to economic hardship and hunger in Africa, Asia and South and Central America (Champoiseau 2009). Even more troubling, is the incredibly wide host range of Ralstonia solanacearum. Since the bacteria can be found on so many hosts, it is in turn a problem for countries importing propagules and other plant goods (EPPO 1997). With the increase in world trade and the incredible diversity of this pathogen, it is crucial that we understand it and work to prevent the damage it can cause. This research will provide insight into the biology of the bacterial wilt causal agent, current management tactics and advances in research that can potentially lessen the devastation bacterial wilt causes to tomato crops each year.

Economic Impact of Bacterial Wilt

Tomato is one of the most widely produced vegetables in the world. In 2004 4,397,873 hectares of tomatoes were planted worldwide. Only potato is produced in more areas than tomato. Many countries see extreme losses due to bacterial wilt. In Taiwan bacterial wilt infects anywhere from 15-55% of the tomato crop depending on environmental conditions. Infection rate in India is as high as 100% (AVRDC 2004). The losses incurred due to bacterial wilt add up to millions of dollars each year (Champoiseau 2009).

Causal Agent of Bacterial Wilt

Ralstonia solanacearum, the causal agent of bacterial wilt, is well adapted to survive in extreme conditions. R. solanacearum is capable of living as a saprophyte in the soil or water for many years. It can utilize almost anything as a food source. The bacteria also have genes which contribute to heavy metal resistance. This is similar to bacteria that have evolved to survive in polluted soil. Other genes in R. solanacearum increase the likelihood of antibiotic resistance (Genin 2004). Since the bacterium is so well adapted, the best method for managing the disease is to develop resistant varieties.

Classification of Ralstonia solanacearum

R. solanacearum is divided into phylotypes, races, and biovars. Phylotype is determined based on geographic origin of the R. solanacearum strain. Phylotype I strains originate in Asia, while Phylotype II strains isolated in the Americas. Strains from Africa and the surrounding islands compose Phylotype III and Phylotype IV strains originate in Indonesia (Prior 2005). The race of a particular strain is determined by the hosts which it infects. The biovar is then determined by examining how it uses and or oxidizes saccharides and hexose alcohols. Race 3 is typically associated with causing disease in potato and tomato (Wenneken 1999). Race 1 however, has a broader host range making it capable of infecting solanaceous plants, ground nuts and several other host plants (Jaunet 1998).

Host Range of Ralstonia Solanacearum

R. solanacearum has an incredibly broad host range which only adds to the challenge of controlling the pathogen. R. solanacearum can infect many important food crops besides tomato. Some of these include potato, eggplant and tobacco. R. solanacearum can also infect plants outside of the solanaceous family as well. Even more troubling, is the fact that R. solanacearum can infect weed hosts as well. Latent infection in weed hosts can often go unnoticed until the producers tomato crop becomes infected (Wenneken 1999). The table below lists the plants that strains of R. solanacearum can infect.

Geographic Range

Ralstonia solanacearum is present throughout many temperate, tropical and subtropical regions of the world. Countries in Africa, Asia, North America, South America and Europe have reported Ralstonia solanacearum infection. The maps below show countries where Ralstonia solanacearum (race 1 and race 3) is present.

Both Maps from EPPO (source listed separately for each map in works cited)

From these maps, it is obvious that R. solanacearum is becoming a world-wide problem. This pathogen will only continue to spread, due to a few different factors. First, the bacteria are incredibly diverse. Different races and biovars infect different hosts. This broad host range means most countries will handle at least one plant that the bacteria can infect. Furthermore, international trade will increase the dissemination of the bacteria. Any country that imports plants, cuttings, or soil harboring the bacteria risks bringing in new strains (Norman 2009). Countries such as the USA import many items that could increase the occurrence of new strains within the country. The ability of new strains to survive in more temperate climates also raises the risk of bacteria spreading to new regions of the world (EPPO 1997).

How Bacteria is Disseminated

R. solanacearum is disseminated a variety of ways, which increases the difficulty of managing bacterial wilt. Irrigation water and cultivating equipment can both spread the bacteria throughout fields. Bacteria can also be transferred by workers shoes and clothing, as well as pruning knives or shears (AVRDC 2004). Many factors also contribute to the rate of infection. Wounds from insects or cultivation leave more openings for bacteria to enter the plant. Hot, humid and wet environmental conditions are favorable to bacterial wilt development and lead to more severe symptoms (AVRDC 2004).

When and How Bacteria Infects Plant

Ralstonia solanacearum can infect a plant at any growth stage. Symptoms can appear in days or weeks depending on environmental conditions (Wang 2000). Ralstonia solanacearum is a soil borne bacteria. Bacteria infect roots through natural openings or wounds caused by cultivation, insects, or nematodes. Once inside the plant, R. solanacearum replicates and colonizes vascular tissue. As colonization in vascular tissue increases, water flow through the xylem is restricted. This restriction of water flow results in wilting (Grimault 1993). Wilting plants also put stress on the bacteria that infected them. Once the plant dies, the bacteria must adapt to living in soil or water again. Bacteria can also adapt to live in alternate host plants (Boshou 2005).

Adaptations for Increased Virulence

Ralstonia solanacearum is unfortunately well adapted to invade plants and cause devastating results once inside the xylem. The bacteria produce proteins that degrade cell walls allowing the bacteria to gain access to the xylem. Once inside the xylem, the bacteria synthesize proteins designed to inhibit the plant defense response. This allows for rapid colonization of bacteria in the early stages of infection (Genin 2004). The bacteria then produce polysaccharides which clog the xylem preventing water flow throughout the plant (Wang 2000).

Symptoms

As the common name implies, wilting is the most common symptom of Ralstonia solanacearum infection. In the early stages of infection, the wilting may only be present on one side of the plant or occur only in the afternoon heat. The plant will recover over night and appear to be fine the next day (AVRDC 2004). As bacteria begin to colonize more of the xylem, the plant will wilt irreversibly, resulting in death. Roots and pith inside the infected plants will initially appear water soaked, and then turn brown (Sikora 2004). When conditions are cooler and dryer, Ralstonia solanacearum infection typically results in stunted growth and yellowing of foliage before wilting (AVRDC 2004). Photos of symptoms are featured below (photos from Wang 2005).

B

Wilting caused by R. solanacearum infection Brown pith in infected plant

Death as a result of R. solanacearum bacteria colonizing xylem

Management Practices

Many management practices can help prevent the spread of the bacteria. Controlling insect pests and nematodes help prevent wounds which allow the bacteria to enter plants more easily. Sanitation measures, such as cleaning work boots and pruning tools help prevent spreading bacteria to healthy plants or uninfected areas of a field. Avoiding over irrigation also prevents standing water which is conducive to rapid bacterial growth. Application of antibiotics is not effective. Resistant tomato varieties provide protection from losses due to bacterial wilt, but tomatoes are often only resistant to a specific strain of the bacteria (Wang 2005).

Genetics and Physiology Behind Resistance

Since using resistant varieties is one of the most effective ways to manage bacterial wilt, it is important to understand the genetics and physiology of resistant plants. Resistance to R. solanacearum is a quantitative trait controlled by many genes. Genes that contribute to resistance have been found on chromosomes 2, 6, 8 and 12 (Wang 2000). Other markers associated with resistance to bacterial wilt were found on chromosomes 7 and 10 (Young 1996). Loci on chromosome 6 are connected with resistance to multiple strains of R. solanacearum. A locus found on chromosome 12 is associated with resistance to race 1 biovar 3 Pss4, which originated in Taiwan. The loci on chromosomes 2 and 8 only showed weak association with resistance to bacterial wilt (Wang 2000). Below is a map of the loci related to R. solanacearum resistance created using data from Wang’s study in 2000. The map depicts the chromosomes of F2 offspring of a cross between two resistant parents, Hawaii7996 and WVa700.

Chromosome map from (Wang 2000)

Physiology of resistant plants is also different than that of susceptible plants. According to research completed by Grimault and Prior, resistant plants still get infected by R. solanacearum. Unlike susceptible plants however, the resistant tomatoes are capable of eliciting a defense response to contain the bacteria. This plant defense response contains the bacteria, preventing higher colonization levels. Higher colonization levels of bacteria lead to clogged xylem and the restricted water flow throughout the plant (Grimault 1994). Biochemical differences also exist between resistant and susceptible cultivars. Researchers noted higher activity of phenylalanine ammonia lyase (PAL) and polyphenol oxidase (PPO). Increased PAL is linked to the pathway that produces compounds such as phenols which are related to the plant defense response. PPO is related to the oxidation of phenols to make quinines which are also related to the plant defense response to pathogens (Vanitha 2009).

How to Screen

Many techniques can be used to screen for resistance to bacterial wilt in tomatoes. Phenotypic observations can be used to screen for resistance to R. solanacearum. One of the earlier techniques utilized by Grimault and Prior was to measure percent colonization and bacteria colony density in vascular tissue of infected plants. Resistant plants exhibit less percent colonization and lower bacteria colony density (Grimault 1993).

Newer methods utilizing molecular markers are now in place to screen for resistance as well. Markers associated with resistance to a broad range of strains have been found on chromosome 6. Other molecular markers have been found on chromosomes 7, 10 and 12. These markers however, are related to resistance that is strain specific (Wang 2000). Biochemical markers have also been discovered as a possible way to screen for resistance to R. solanacearum. Researchers found increased activity of phenylalanine ammonia lyase and polyphenol oxidase in resistant cultivars in (Vanitha 2009).

Testing for Resistance

Testing for resistance requires manipulating both the plants and the environment. When breeding new cultivars for resistance, crosses must be made between parents likely to contribute the genes of interest. These crosses must be carefully recorded. Studies for resistance to R. solanacearum can be conducted in a field or greenhouse environment. Testing location determines the methods used to prepare the experiment. In greenhouse experiments growing media is sterilized, and then plants are seeded into pots. Once the plants are established (around 5 weeks after planting in Grimault and Prior experiment) they are inoculated. The benefit to this method is being able to select the strain of bacteria used as an inoculant. Environmental controls are also easier to maintain within a greenhouse (Grimault 1993).

In field testing, the soil is typically tested for the strain of R. solanacearum present as well as the density of bacteria. In this method, tomato plants are first seeded in the greenhouse, transplanted into the field, and then observations are made to determine resistance to R. solanacearum. This method is better able to represent how resistant varieties will behave in the field. With this method however, the researcher does lose a great deal of control over the environment. Once the inoculation is done, observations should be recorded every day due to the fact that wilting can occur rapidly in more susceptible varieties. The scale for scoring resistance is listed below.

Scoring Resistance:

1= No disease, healthy plant

2= only youngest leaves show signs of wilting

3=only one side of the plant is wilted, recovers when temperatures drop

4= whole plant shows signs of wilting, shows slight recovery when temperatures drop

5=complete wilting of plant, brown pith, death of plant

Scale devised using information from Plant Disease Notes (Sikora 2004).

This scale is a proposed visual scale for scoring disease resistance. Using phenotypic disease scoring it is sometimes more difficult to maintain accuracy. Phenotypic scoring however, does give a better picture of how the resistance appears in the field, rather than just finding markers associated with resistance found in a lab.

Difficulties in Developing Resistant Varieties

Developing tomato varieties resistant to R. solanacearum can be difficult since the strain of the bacteria vary widely between regions. With different strains in various regions of the world, multiple resistant varieties need to be developed to help prevent devastation from R. solanacearum infection. The genetics of bacterial wilt resistance also make breeding for resistant traits difficult. Bacterial wilt resistance is a quantitative trait. Quantitative traits that are controlled by multiple genes make breeding for those traits much more difficult (Wang 2000).

Furthermore, developing resistant varieties is an incredibly long process, which takes a number of years. This slow development means there is not always a short term solution for countries suffering from crop losses due to bacterial wilt. Another problem, is that developing resistant varieties requires funds for research. Many countries that suffer the most devastation from R. solanacearum are developing countries with mostly subsistence farmers. Fortunately, one short term solution is available to producers. Growers can graft susceptible varieties onto resistant rootstocks. This method is labor intensive, but affective. One study in India found 100 percent survival of grafted plants while none of the susceptible plants survived (Louws 2006).

Using Genetic Engineering to Develop Resistant Varieties

Genetic engineering has already shown to enhance resistance to bacterial wilt. One successful study was completed using tobacco. The gene isolated for this project was also supplied to tomato researchers. The gene of interest was isolated from Antheraea pernyi. This gene inhibits bacterial cell growth by producing peptides that create holes in the bacteria cell walls. Despite the success of genetic transformation, there is still one main problem with this approach. Many countries are resistant to the idea of growing and consuming tomatoes that contain transgenes (Boshou 2005).

Conclusion

Ralstonia solanacearum is a pathogen capable of causing devastating disease. Bacterial wilt is responsible for substantial losses in tomato crops throughout the world. This is problematic due to the fact tomatoes are an important source of lycopene, vitamin A and antioxidants necessary for proper nutrition. Furthermore, tomatoes are an important economic crop for many countries throughout the world. Devastation due to bacterial wilt is directly related to economic hardship and hunger in developing nations in Central and South America, Africa and Asia (Champoiseau 2009).

Other characteristics of Ralstonia solanacearum biology contribute to the devastation these bacteria cause. R. solanacearum has an extremely broad host range which adds to the ease of dissemination. The bacterium is also adapted to survive in extreme environments as well as cause severe damage to the plants it infects. These adaptations make R. solanacearum particularly difficult to manage.

Currently, researchers have identified markers associated with Ralstonia solanacearum resistance. Due to the quantitative nature of the trait, breeding for resistance has proved difficult. With modern science, screening for resistance has become faster and more accurate. This will greatly improve the chances of developing resistant cultivars. Genetic engineering has also proved effective in adding R. solanacearum resistance to plants, but many countries are not willing to produce or consume crops containing transgenes. It is important that breeding for resistant cultivars continue to be pursued since other tactics for managing bacterial wilt only provide limited effectiveness.

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