Abutilon mosaic virus

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By: Deveren Manley II

Introduction

Abutilon mosaic virus (AbMV) is a plant virus that infects evergreen upright shrubs, Abutilon striatum. The first recognized infection was in 1868, in which one shrub in a shipment of A. striatum brought to England had the signature bright yellow mottling symptom of the pathogen. Abutilon mosaic virus is a part of the Geminivirus/Geminiviridae family and Begomovirus genus. Viruses in this genus are transmitted via insect vector. The vector for this virus is the silverleaf whitefly, scientifically known as Bemisia tabaci. The prevalence of the whitefly across the world allows the virus to be just as widely prevalent. There are 3 strains of the AbMV: Abutilon mosaic A: West Indies virus, Abutilon mosaic B: Brazil virus, and Abutilon mosaic Hawaii virus. All strains are very appreciated by botanists and plant lovers because they add decorative color to their plant host with little negative effect to life cycle and health.

Structure

AbMV characteristics from the Geminivirus family

Abutilon mosaic virus is a plant viruses that infects weeds, crops, and ornamentals. They are mostly harmless to their host unlike others in the family that can cause serious damage to crop land. Abutilon mosaic virus has a circular, single-stranded DNA (ssDNA) structure that readily recombines and mutates. AbMV DNA and other geminiviruses are unable to replicate through methylation3, and take advantage of host cellular mechanisms to accomplish this task. Their ability to acquire novel genotypes allows them to adapt quickly to the vastly different hosts and environments they may encounter across the world.

AbMV characteristics from Begomovirus Genus

Abutilon mosaic virus has 321 peers in Begomoviruses Genus. The DNA of these species are packaged into a nucleocapsid4 that is 38 nanometers (nm) long and 15–22 nm in diameter. Each capsid is made of 22 capsomeres. The single stranded DNA inside the capsid are not enveloped, adopting icosahedral symmetry. Begomoviruses have a bipartite genome, in which both segments of DNA are required for successful infection of a host cell. The DNA in the first segment encodes for replication, transport, regulatory, and coat proteins. Specifically they control how genes are expressed, ability to evade or defeat host defenses, capsid formation, and transmission to vector. This segment of DNA are found in all geminiviruses. The second segment of DNA encodes for viral movement and symptom expression.

Transmission

AbMV life cycle: plant to whitefly

Bemisia tabaci, also known as the silverleaf whitefly, is the vector of Abutilon mosaic virus. The virus is transmitted to this specific whitefly through its digestive system. The virus enters the vector while it feeds on an infected plant. The longer the whitefly enjoys its plant meal the more likely it is to contract the virus. Once inside the vector it inhabits the salivary glands, the midgut, and the filter chamber. After fully establishing itself inside the vector the virus does not multiply inside it. The whiteflies will remain infected with the virus its entire life even throughout molts similar to Herpes simplex virus in humans. In contrast to Herpes, the virus cannot be passed from an infected white onto its offspring. Overtime, as the infected whitefly becomes older the virus loses its ability to further transmit itself.

AbMV life cycle: whitefly to plant

Figure 1. Abutilon striatum flower petals before it has fully bloomed.[1].

When whiteflies first contract AbMV there is a 12 hour window that must elapse before it can transmit the virus to other host. After this latency period, adult whiteflies that are vectors pass the pathogen on when it sucks on the next Abutilon species. From an evolutionary standpoint, the whitefly is a sound choice as a vector because it feeds on many types of plant species giving the virus the best opportunity to infect as many plants as possible. Once inside the plant the AbMV virus molecules congregate in the nuclei of bundle area cells. (_et al._) Post-transmission the virus will inhabit the plant likely for the rest of its life, just as the vector. Once in the plant, the virus in a purely natural environment remains to be transmitted by purely vector digestion11.

Non-organismal Modes of Transmission

AbMV can also be transmitted through grafting and gardening tool contamination. Grafting is the most prevalent method of transmission across Abutilon striatum populations. The aesthetic color patterns caused by the disease on host leaves generate consumer interest. Gardeners and botanists intentionally spread the virus to meet consumer demand. Outside of the spread of AbMV, gardeners also use grafting to:

• Get immature plants to produce fruit

• Pass on beneficial genes

• Repair damage to trees stripped of bark

• Conduct plant research

The purpose of the technique is to create contact between the vascular systems of two plants establishing a pathway of gene flow and nutrient-sharing. Grafting allows plants to share cell nuclei, DNA, and chloroplasts. Growth stimuli has also been shared across this way as well. When trying to directly transmit AbMV using this method, the best connection is between the rootstock of an infected plant and the scion of the non-infected plant. The genetic similarity between the two plants is important to the formation of a connection between them. The closer they are in species to one another the more likely they will grow together.

To graft two plants together, peel back the areas that are desired to be grafted together using a knife. The layers should be peeled so that the vascular layer is exposed. Bind the plants together where their vascular system is exposed using grafting tape or wax. These adhesives also reduce amount of water loss due to exposure. It is important that the two vascular areas remain compressed together with the tape or wax until the grafting process has been completed. It normally takes a few weeks for the plant to begin to form scar tissue on the grafted area which is a sign of their union18. Once successfully grafted the plants will grow together and the AbMV infection should be shared in both plants. Researchers and botanists are confident in this method beyond the use of passing on the aesthetic patterns of AbMV. They also are using this method to study the transmission of other viruses as well. They have created a Viral Index in which they graft a plant thought to be a carrier of a virus that may not be displaying symptoms to a plant that is very susceptible to the virus they are testing to see if present. If this susceptible plant becomes infected they know that the initial plant is a carrier of the virus.

Viral Vectors

Figure 2. Adult sweetpotato whitefly, Bemisia tabaci on the surface of a leaf. Note their yellow heads and white wings[2].

Bemisia tabaci was first identified in the United States 1896, not long after the first infection of AbMV in England. This species of whitefly are extremely small at about 1.5 mm long. They have a yellow torso with white wings. Their environmental preference tends to include the warm climate associated with the tropical, subtropical, and sometimes temperate habitats. This prevalence in warmer climates is not by chance as cold temperatures kill them.

Whiteflies suck the nutrients out of plants, which is why they are mostly found on the bottom of leaves. This method of nutrition is harmful to plants because it leads to poor growth, shedding, and lower crop yields. Outside of stealing nutrients from plants, it also corrupts plant life because it normally is a host for multiple diseases. Since its introduction to the United States, pathogen-carrying whiteflies have cost the agricultural industries in Texas and California over $100 million worth of crops. Researchers have not been able to figure out how it has been able to evade its natural predators which include lady beetles and several species of wasp. To improve crop yields, pesticides have been created to kill whiteflies, but these treatments have led to resistant strains.

Abutilon striatum are versatile plants that can be found in a number different areas. It naturally finds its home in South American countries such as Paraguay, Uruguay, Argentina, and Brazil, where there is a warm climate. The flower is mainly known for its colorful structure and is a popular ornamental house plant kept in pots anywhere in homes. There are no strict requirements for sun exposure but this does have a small effect on plant health in terms of vibrancy. They normally grow to be 5 meters tall and 2 meters with leaves that grow to be anywhere between 5-15 cm long. The petals of the shrub grow to be around 2-4 centimeters long. Their flowers are normally yellow or orange and have red veins. They begin to bloom in the spring during the month of April all the way through the summer until September. In countries with regular warm weather the bloom lasts longer such as where it naturally grows in South America. The shrubs do attract bees and other pollinators alike who the plants use to spread their pollen. Abutilon striatum can be eaten by humans. They are able to be cooked but do not have to be. They have a sweet taste to them.

AbMV Physiological Impacts on Abutilon striatum

Figure 3. AbMV infected plant showing the characteristic mosaic pattern resulting from the negative effects of carbohydrate buildup.[3].

Once a plant is infected with the virus it is likely infected for the rest of its life. Although the plant remains infected the amount of damage the disease causes is minimal. It is not a death sentence. Plants will still be able to bloom, growth, pollenate, etc. There may be some stunting of growth, dehydration, and decrease in the production of photosynthesis. The symptoms a plant experiences are not distinctly defined. Symptoms displayed often depend on the species of Abutilon infected and the strain of the virus encountered. The Hawaii strain of the virus causes crinkling and mottling in the leaves of the plants it infects. It does not cause the desired yellow mosaic pattern coveted by most botanist. Another key difference in the Hawaii strain from the other strains is that it is not transmissible through the whitefly vector.

Much of what is known about the Abutilon mosaic virus physiology has been obtained through research. There have been research done to make sure it has fulfilled all 4 of Robert Koch’s postulates. The second postulate of the ability to obtain in pure culture was hard to fulfill when first discovered because the disease is in low concentration in the vascular system of the plant. To satisfy the first two postulates the genome of AbMV was sequenced from a disease plant and then compared the amount of similarity in DNA between a host plant and uninfected plant. The process of DNA comparison for similarity is called a DNA hybridization test. Then researchers used an inoculation method in which they rubbed the disease onto the vascular system on a healthy plant which is called mechanical inoculation. After the disease was successfully spread then it was re-isolated and tested using the same methods used to satisfy the first and second postulate.

At a molecular level the virus only replicates in the phloem of the plant (Lohaus et al. 2008). The virus has the ability to take the red color out of the veins of Abutilon and with normally form the coveted color spots in between the veins. The effects of the disease are strongly linked to photosynthetic output of the plant. This means the more light the plant receives the more the plant will experience symptoms. The virus also causes carbohydrate accumulation in the leaves as well. Things such as starches, sucrose, and hexose are associated with infection.

Researchers have not developed a cure for this virus because it does little damage to the plant host. It is recommended that plants infected with the virus whose growth is severely stunted should be replaced. Before you completely uproot an infected plant you can also try to compensate for the constraints placed on the plants by the virus by taking great care of the plant. Making sure it receives high levels of sun, the right amount of water, and rooted in the correct soil. All these little things can improve the health of an ailing plant. There also has been some success in cutting off symptomatic leaves and allowing them to regrow. This method is not 100% guarantee to cure the virus but does provide the plant with an opportunity to get rid of the virus.

The carbohydrates or sugars buildup in the leaves as a product of photosynthesis. Under normal circumstances these carbohydrates a removed from the leaves by other mechanisms of the plant to distributed to the rest of the organism. Abutilon mosaic virus interferes with this process not allowing them to be transported. This interaction with sugar production is why the virus is most active after periods of high photosynthesis. The more sugars made the more it blocks. This buildup of sugar degrades the leaves photosynthetic systems which is why the leaves develop the mosaic pattern. The vibrant green color in leaves is a product of healthy chlorophyll conducting photosynthesis. An interruption in this process cause the chlorophyll to lose this pigment. High carbohydrate contents in the phloem20 alone.

When you consider, Abutilon mosaic virus’ attack on the phloem or vascular system of plants makes that the most common method of transmission is through grafting. The physical connection between the two plants’ vascular system gives the virus a direct pathway to travel to the uninfected plant’s photosynthetic organelles and cause the same carbohydrate buildup. This buildup of carbohydrates also explains why some plants experience a stunting of growth, withering, and crinkling. The carbohydrate buildup can be thought of as a blood clot in humans that causes swelling. If the clot goes untreated there is loss of function, color change due to being oxygen deprived among other symptoms. Plants experience the same thing. Crinkling, mosaic patterns, etc. are the plants form of clotting the area. The attenuation the virus experiences across time is one factor that explains why a range of symptoms is seen in infection. For example, passing on an old attenuated virus is a healthy plant in a shaded environment likely will not experience a large degree of symptoms.

Conclusions and Comparisons

While this virus is not life threatening for plants there are a lot of similarities between transmission, infection, and modes of infection relief to diseases that infect humans. The latency period that the virus goes through in insect vectors before it can infect other people can be seen in other disease such as tuberculosis even though the period is caused by different things. The lifetime infection and attenuation in the virus remind me a lot of Herpes virus where herpes remain dormant in the nerves of human cells Abutilon mosaic virus resides near the chloroplasts of plants. They both have a physical expression where AbMV is more pleasant to look at than a break out of herpes. It shares a similarity in the method of relief to cancer and diabetes in that success in ridding a vessel of the virus has been seen in ablation. Learning about these virus that are harmless to humans and animals can be tools to learn more about the structure, life cycle, and weakness of diseases that may cause more harm.

There are also more practical applications to studying this virus in plants other than the impact it can have for humans. Plant life is being destroyed through a number of avenues that include pests, viral infections pollutions etc. Studying how AbMV is able to interact with plant molecular systems without consistently impeding necessary functions for sustained life is amazing. This is especially interesting because AbMV interacts with photosynthetic systems of the plant. A number of questions could be raised about why the entire plant does not experience infection. Why infection is localized to spots and not the entire leaf.

It is strange that AbMV does not transmit as well to older plants or developing seeds. This means there are component of developing cells that the virus needs in order to replicate. Researchers are not yet sure what exactly they require of these cells. This is another similarity in the study of infection and viral diseases. Many other diseases and infections are related to age such as high blood pressure, heart disease, Alzheimer’s etc.

References

Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2018, Kenyon College.