A patient battling a deadly disease has been placed in a heated tent for treatment. The heat is designed to kill bacteria that are attacking the victim. Experts hope it will buy the patient a few more years, but the sickness has no known cure.
The patient fighting for life may surprise you: It's a Florida orange tree.
"Plants get sick just like people do, with bacteria and viruses," says Erik Mirkov, a plant pathologist at Texas A&M University. The disease this tree has contracted, called citrus greening, is devastating the U.S. citrus industry. An estimated 80 percent of Florida's orange, grapefruit, and other citrus trees are already infected. Growers in other states are fighting to keep the disease from spreading through their orchards.
If citrus greening isn't stopped, growers and juice producers could go out of business. Tens of thousands of workers could lose their jobs, and that glass of OJ you have at breakfast could become an expensive luxury item. But researchers are pulling out all the stops to try to save the nation's citrus. Besides using methods such as heat treatment to save existing trees, they're using genetic engineering to develop trees that they hope will be able to resist the disease.
PARTNERS IN CRIME
Citrus greening--sometimes known by its Chinese name huanglongbing (HLB), which means "yellow dragon disease"--first appeared in Florida in 2005. Prior to its arrival in the U.S., it had already killed millions of citrus trees in other parts of the world. The disease is caused by a bacterium that multiplies in a tree's phloem--the vascular system that transports sugars and other materials throughout the plant. The bacteria clog this nutrient transport system. This stunts the tree's growth, eventually killing it. Meanwhile, the yellowing tree produces small, lopsided, bitter fruit that often don't ripen.
The bacteria spread to other trees with the help of an accomplice--a tiny insect called the citrus psyllid. As the insect feeds on trees, it picks up bacteria and carries them from tree to tree. "If you had a plant with the bacteria but you had no psyllids, the bacteria would never go anywhere," says Bill Dawson, a plant pathologist at the University of Florida in Lake Alfred.
The bacteria and psyllid likely hitched a ride to the U.S. on plants imported from other countries. Once they arrived, citrus greening quickly spread. Controlling the disease is tricky: To protect healthy trees, biologists recommend uprooting and burning infected ones. But a tree may be infected for two to four years before symptoms appear. "It looks OK, but psyllids are feeding on it, picking up the bacteria, and infecting other trees," says Mirkov.
To combat these insect pests, growers spray insecticides and release tiny wasps native to Pakistan that are known to prey on psyllids. Unfortunately, these strategies reduce psyllid numbers but don't eliminate them. "The disease is spread really efficiently, so even if you have just a few insects, it still spreads," says Dawson.
While growers battle the invaders in the fields, researchers in the lab are investigating other ways to save citrus. Some strategies--such as raising the trees' temperature in heated tents--are aimed at lengthening the lives of infected trees. This kills the bacteria so the phloem can transport sugars. But eventually, more bacteria move up from the roots and multiply again.
Other strategies involve developing citrus trees that are resistant to greening in the first place. In the past, breeders have created orange trees resistant to various diseases, so they're hopeful they can succeed against the current enemy. For instance, there's a virus called tristeza that attacks the roots of some species of orange trees. Breeders have protected orange trees from tristeza by grafting, or transplanting, part of a vulnerable plant onto the roots of a related species that is resistant to the virus. Then, if tristeza enters the roots, the tree that was once susceptible doesn't become sick.
Unfortunately, grafting won't work against citrus greening. Why not? No citrus species is resistant to the disease. Instead, researchers are turning to animals and unrelated plants--like beetles, scorpions, and spinach--for help. They're inserting genes, or units of hereditary material, directly from these resistant species into citrus trees. This process is called genetic engineering.
Mirkov is one of the scientists performing the research with spinach genes. When the genes are inserted into citrus trees, the trees produce molecules that kill bacteria and other disease-causing agents.
He has tested his genetically modified citrus trees in fields and greenhouses teeming with bacteria-carrying psyllids. The results have been very promising--and don't worry, the fruit from the altered trees won't taste like spinach. "The genes we've put in don't have anything to do with taste," says Mirkov. "They shouldn't have any effect on the fruit or juice quality."
LONG BATTLE AHEAD
Even if it turns out that genetically modified trees solve the problem, producing enough trees for widespread use will take time. Citrus trees take several years to mature and bear fruit.
Additionally, before a genetically modified tree can be used commercially, it must be approved by government regulatory agencies--a long process involving tests to ensure that it's safe for humans, animals, and the environment. Mirkov's genetically modified trees are further along in that process than others, but he estimates that it'll take another few years to gain approval if everything goes smoothly.
Meanwhile, the ongoing battle rages in the citrus orchards. The next time you pick up a glass of orange juice, enjoy it--and don't take it for granted.
Caption: DISEASED FRUIT: Oranges affected by citrus greening are small, misshapen, discolored, and bitter.
Caption: THE DEFENSE
These wasps from Pakistan prey on citrus psyllids and could help slow their spread.
A tiny Insect called the citrus psyllid spreads citrus greening disease.
Caption: FIGHTING BACK: A tent in Riverside, California, houses wasps that could help fight the disease.
Caption: SAVING TREES: Technicians apply heat treatment to citrus trees in Lake Alfred. Florida.
Caption: TEST PLOT: These genetically engineered orange trees in Clewiston, Florida, may resist citrus greening.
FOUR OTHER FOODS UNDER THREAT
Cavendish bananas--the variety most often sold in the U.S.--replaced a popular banana that was nearly wiped out by a fungus in the 1950s. Now Cavendish bananas are threatened by a similar fungus that's already invaded plantations in Asia, Australia, and Africa.
More than half of the world's chocolate is manufactured using cocoa grown in West Africa. A study by climate scientists predicts that rising temperatures due to climate change could make parts of the region too hot for cocoa crops by the year 2030.
Hailstorms and a late frost last spring destroyed hazel flowers in Turkey before the trees could produce their nuts, which are a main ingredient in Nutella. The price of hazelnuts skyrocketed as a result, but growers hope crop production will be back on track this year.
An ongoing drought in California, where 95 percent of U.S. avocados are grown, threatens crops of this water-loving fruit, which is crushed to make guacamole.
How might genetic engineering help protect citrus fruit from disease?
NATIONAL SCIENCE EDUCATION STANDARDS
Grades 5-8: Reproduction and heredity
Grades 9-12: Molecular basis of heredity
NEXT GENERATION SCIENCE STANDARDS
LS3.A: Inheritance of traits
COMMON CORE STATE STANDARDS
Writing Standards: 2. Write informative texts to convey complex ideas clearly and accurately.
Learn about a disease that threatens orange trees and how scientists are trying to protect them using genetic engineering.
1. Open the digital edition to page 14 and have students do the same in their magazines.
2. Use the highlighting tool to mark the words "genetic engineering" in the subheading of the article. Ask students what this term means. (altering the DNA, or genetic material, of an organism) Why would scientists alter an organism's DNA? (DNA controls an organism's characteristics, such as its size, color, and even whether it can resist certain diseases.)
3. Click on the skills sheets button on page 15 of the digital edition and print out the "Problem Solver" skills sheet. Have students form pairs and hand out the skills sheet. Have each pair read the article and identify problems facing orange trees and possible solutions.
4. When everyone has finished, have a representative from each pair type one of the possible solutions on a digital sticky note. As a class, discuss the pros and cons of each solution.
Ask the class if they have heard the term GMO. (A GMO is a genetically modified organism.) Explain that scientists can create genetically modified food crops that are more nutritious or resistant to drought, diseases, or pests than non-GMO crops. However, there is debate about whether GMOs should be planted or consumed. GMO foods are common in the U.S., but such crops are largely banned in the European Union. As a class, explore the scientific evidence behind the controversy and hold a classroom discussion about your findings.
Assessments are tailored to different science disciplines and the Common Core State Standards. You can find this entire assessment package by going online to www.scholastic.com/scienceworld and opening the digital edition. Simply click on the skills sheets button found on page 15.
BIOLOGY/COMMON CORE: CRITICAL THINKING
Students can use this graphic organizer to analyze the problem outlined in the article and evaluate the possible solutions discussed in the text.
CHEMISTRY: CHART READING
Have students complete this chart-reading activity to learn how the chemical composition of orange juice is affected by citrus greening and how those changes affect its taste.
PHYSICS: HANDS-ON ACTIVITY
Try this experiment with your students to learn how evaporation and capillary action help water move through trees and plants.
EARTH SCIENCE: READING COMPREHENSION
Students will learn why orange trees grow only in warm climates with this reading passage.
* VIDEO EXTRA: Watch a video about citrus greening at: www.scholastic.com/scienceworld
* Learn about many different types of citrus fruit here: http://mentalfloss.com/article/60137/know-your-citrus
* Learn more about the genetic engineering of plants and the debate surrounding the process here: http://science.kqed .org/quest/video/next-meal-engineering-food/
CHECK FOR UNDERSTANDING
DIRECTIONS: Match each item in the left-hand column below with its definition or description in the right-hand column.
-- 1. citrus greening
-- 2. citrus psyllid
-- 3. phloem
-- 4. tristeza
-- 5. graft
-- 6. genetic engineering
-- 7. spinach
-- 8. gene
-- 9. Cavendish
-- 10. drought
a. a threat to avocado crops in California
b. the process by which scientists insert new genes into an organism
c. a disease that causes citrus trees to grow small, bitter fruit
d. a source of genes that help fight bacterial infection and prevent citrus greening
e. unit of hereditary material
f. an insect that feeds on citrus trees, carrying with it the bacteria that cause citrus greening
g. a virus that attacks the roots of some types of orange trees
h. the most common variety of bananas sold in the U.S.
i. transplant part of a plant onto another plant
j. the system that transports sugars throughout a plant
1. c 2. f 3. j 4. g 5. i 6. b 7. d 8. e 9. h 10. a