Save the oranges!
Sad virus against the Green Dragon
Maxim Rousseau, Polit.<url>, based on Nature news: Geneticists enlist engineered virus and CRISPR to battle citrus disease
American orange growers hope to save their trees with the help of a new biological technology. Interestingly, the Citrus tristeza virus (CTV), which itself poses a serious danger to plantations, should act as a savior.
The name, meaning "sadness" in Spanish, was given to the virus during a massive outbreak of the disease in South American countries in the 1930s. The homeland of this virus is the south of China, where citrus fruits have acquired resistance to infection for a long time. But at the beginning of the XX century, it began to spread to other continents: in South Africa, in Indonesia, in the Mediterranean, in South America. In 1930, 90% of citrus plantations died in Argentina, in 1937 the virus penetrated Brazil and Uruguay, where it also massively destroyed trees. In the early 1950s, he infiltrated the United States. Now it is found in all regions of the world where citrus fruits are grown. The virus has killed at least 80 million citrus trees.
The aphid virus is transmitted, primarily by the species Toxoptera citricida. Other types of aphids are also capable of transmitting the virus: Aphis gossypii, Aphis spiraecola, Toxoptera aurantii. Aphids get the virus if they feed on an infected tree for at least half an hour or an hour, and remain able to transmit it further during the day. The virus spreads through the phloem – conducting tissue of plants, disrupting the transfer of photosynthetic products from leaves to other organs. In diseased trees, the veins of the leaves turn white and thicken, and then the leaves turn yellow and fall off. The growth of the plant is delayed. The structure of the wood is changing. Different strains of the virus cause damage of varying severity. They are distinguished by the reaction to the virus and different citrus fruits. Sour oranges, limes and grapefruits are the hardest to get sick. Some varieties of lemons and kumquats are asymptomatic, but these trees can still become a source of transmission of the virus to others.
The Chinese shrub poncirus trifoliata, or trifoliata (Citrus trifoliata or Poncirus trifoliata) turned out to be completely resistant to the virus. Due to this, it began to be used as a rootstock for grafting other citrus species, as well as creating hybrids based on it. Thus, citrange (a hybrid of trifoliate with orange), citranjdarin (with mandarin), citramon (with lemon), satsumanji (with Japanese mandarin), citrangquat (a hybrid of citrange with kumquat), and others were obtained. But the fruits of both the trifoliata itself and its hybrids have an unpleasant bitter-sour taste, so they are not suitable for fresh food. In the best case, fruit drinks are obtained from them after processing.
No effective methods of combating the virus have been found. Usually, plant protection specialists try to detect the defeat of a citrus plantation as early as possible and determine which specific strain of the virus infected the trees. If it is a "heavy" strain, all trees are destroyed to prevent the spread of the disease. Aphid control and control of its spread are also used as protective measures.
How can this dangerous virus be useful? It is proposed to use it as a tool to combat another, perhaps even more terrible disease of citrus fruits – greening (citrus greening disease). The birthplace of the disease is tropical Asia, in China it is called "green dragon disease" (huanglongbing). The disease is caused by a bacterium of the genus Liberibacter. The leaves of the affected trees turn yellow and fall prematurely. Then the branches and roots die off, and the tree dies. The fruits of diseased trees are small, irregularly shaped, with a thick, pale skin that remains green in the lower part. The taste of the fruit becomes bitter. The disease is transmitted by a small insect, only up to four millimeters long – the Asian citrus leaf block (Diaphorina citri).
The disease, which began its journey in China, has already spread to India, Sri Lanka, Malaysia, Indonesia, Myanmar, the Philippines, Pakistan, Thailand, the Ryukyu Islands, Nepal, Saudi Arabia and Afghanistan. Also, together with the leaf block, it penetrated to the islands of Reunion and Mauritius, to Brazil and Mexico. In 2005, the "green dragon disease" first appeared in Florida, and in March 2012 it was first noted in California and Texas. There is also a South African variety of the disease, caused by a related species of bacteria and also carried by one of the species of leafhoppers. Over the past decade, due to the greening of citrus fruits, the production of oranges in the United States has halved and, according to experts, the disease can completely destroy this branch of agriculture.
In February of this year, the agricultural company Southern Gardens Citrus from Cleveland in Florida applied to the US Department of Agriculture for permission to use a modified version of the CTV virus to combat the bacteria that causes citrus greening. The authors of this idea rightly suggested that since the virus affects citrus trees so effectively, it is possible to create a virus based on it that does something useful, for example, fights a bacterium. Last week, the statutory period of public discussion of the application ended, and now the Ministry intends to conduct research to assess the possible environmental consequences of the use of such a virus.
Field trials of the modified CTV virus are already underway. If the technique receives official approval for commercial use, it will be the first such example in the United States. Optimistic scientists hope that this will help to overcome the existing prejudice in society towards genetically modified species.
Over the past few years, Southern Gardens Citrus has already created genetically modified orange trees that have received a gene taken from spinach, which ensures the resistance of trees to infection with "green dragon disease". But the testing and verification procedure that is required for a genetically modified plant to be allowed to be used in the food industry will take many years. And there is a great risk that during this time the orange orchards will die completely.
That's why the developers remembered about the CTV virus. They changed the genome of one of the harmless strains of the virus, including the same gene from spinach. Then they intend to graft branches from trees infected with the modified virus onto other trees from which the harvest is being carried out. Since the virus does not affect the fruits, the authors of the work argue that such a method does not require all the checks that are prescribed for food genetically modified objects. In April, the Ministry of Agriculture announced that it would begin an environmental safety audit and it is expected to take about two years.
Other measures are also proposed to save oranges from the "green dragon". For example, plant pathology specialist Nian Wang from the University of Florida uses CRISPR-Cas9 genome editing technology to obtain infection-resistant orange trees. It expects to complete its work by 2019.
Other methods are aimed at combating the vector of the disease – the leaf block. In 2013, scientists discovered a type of rider that affects the Asian citrus leaf block. The insect was found in Pakistan and, after an extensive study of its environmental impact, was recognized as suitable for distribution in US gardens. In 2014, Anandasankar Ray, a professor of entomology, and his colleagues studied the olfactory system of the Asian citrus leaf block and isolated the odors that attract this insect. As a result, they were able to isolate a number of substances that they proposed to use in baits for leaf blocks. The test results of the baits were published in PLoS ONE, and the large-scale identification of the odors that the leaf block distinguishes is described in the journal Chemical Senses. The recipe proposed by scientists includes three natural substances: myrcene, ethylbutyrate and p-cymol. The attractant obtained from them was tested on citrus trees in an area in El Monte, California. It turned out that 230% more leaf blocks were caught in traps equipped with such bait than in control traps placed on the same trees. Professor Rai emphasizes that all three substances are easily accessible, harmless to humans and even useful in small quantities.
One can only hope that these methods will have time to spread, while citrus plantations have not yet died from the disease. According to Fred Gmitter, a scientist at the University of Florida, the disease is ravaging gardens and citrus growers needed effective protection a decade ago.
Portal "Eternal youth" http://vechnayamolodost.ru
17.05.2017