Reading Exercise: Plant Fragrances

Most people are familiar with the scents of flowers, and many know that floral odors help attract pollinators to plants. This common belief is generally true, although scientific evidence supporting it is surprisingly scarce. Not all flowers rely on biological agents for pollination; for instance, many grasses are wind-pollinated, yet their flowers may still emit volatile compounds. Plants continuously release organic molecules, although these may not be perceptible to humans. Flowers that attract moths and butterflies typically emit a 'sweet' fragrance, while those attracting certain flies often emit a 'putrid' scent to humans.

B

The emission of volatiles from plant parts other than flowers is well-known, although until recently, the biological functions of these chemicals were less understood and received less scientific attention. When the trunk of a pine tree is damaged, such as by a beetle burrowing into it, it produces a strongly scented resin. This resin is primarily composed of terpenes—hydrocarbons with carbon backbones of 10, 15, or 20 atoms, sometimes including oxygen atoms. The heavier C20 terpenes, known as diterpenes, have a viscous consistency and can immobilize insects by sealing their entry points. This defense mechanism is ancient and effective: Fossilized resin, or amber, often contains trapped insects. Many plants release volatiles when injured, and these signals can aid in plant defense. For example, (Z)-3-hexenyl acetate, known as a 'green leaf volatile,' is emitted by many plants upon injury, deterring female Heliothis virescens moths from laying eggs on injured tobacco plants. Interestingly, tobacco emits a different volatile profile at night compared to during the day, with the nocturnal blend, rich in several (Z)-3-hexen-1-olesters, proving most effective against night-active H. virescens moths.

C

Volatiles induced by herbivores often serve as indirect defenses. These defenses are found in various plant species, including corn, beans, and the model plant Arabidopsis thaliana. Plants release volatiles not only at the site where caterpillars, mites, aphids, or similar insects are feeding but also generally from undamaged parts of the plant. These signals attract various predatory insects that prey on plant-eaters. For example, some parasitic wasps can detect the volatile signature of a damaged plant and lay their eggs inside the offending caterpillar. Upon hatching, the wasp larvae feed on the caterpillar from the inside out, significantly impeding the caterpillar's growth to the benefit of the plant. Similarly, plants emit volatiles in response to herbivore egg-laying, attracting parasites that target the eggs and preventing them from hatching, thereby averting the threat of hungry herbivores. Plant volatiles also play a role in more indirect defensive strategies. In the rainforest understory tree Leonardoxa Africana, ants of the species Petalomyrmex phylax patrol young leaves and attack any herbivorous insects they encounter. The young leaves emit high levels of the volatile compound methyl salicylate, which the ants use as a pheromone or antiseptic in their nests. Methyl salicylate serves as both an attractant and a reward from the tree to incentivize ants to perform this valuable deterrent role.

D

The impact of floral fragrance on the economic success of many agricultural crops relying on insect pollination, such as bee-pollinated fruit trees like cherry, apple, apricot, and peach, as well as vegetables and tropical plants like papaya, is profound. Pollination not only affects crop yield but also the quality and efficiency of crop production. For optimal fruit size and shape, many crops require most, if not all, ovules to be fertilized. Reduced emission of fragrance diminishes flower attraction to pollinators, resulting in significant losses for growers, especially for introduced species lacking specialized native pollinators. Recent disease epidemics, which decimated honeybee populations—the primary insect pollinators in the United States—have exacerbated this issue.

E

One strategy employed by plant breeders to circumvent pollination challenges involves developing self-compatible or apomictic varieties that do not require fertilization. While effective, this approach has drawbacks, including genetic uniformity and increased susceptibility to pathogens. Some growers have attempted to augment honeybee foraging by applying scent compounds to orchard trees, but this method proved costly, required repeated applications, potentially had adverse effects on soil and local biota, and ultimately proved inefficient. The ineffectiveness of this strategy likely stems from inherent limitations of artificially applied compounds, which fail to accurately communicate flower location to bees. A more sophisticated approach is clearly warranted. Genetic engineering offers the potential to enhance existing floral fragrances, thereby enabling manipulation of insect pollinator types and visitation frequencies. Additionally, engineered fragrance metabolism could bolster crop defenses against pathogens and pests.

F

Genetic modification of fragrance also promises benefits to the floriculture industry. Ornamental plants, including cut flowers, foliage, and potted plants, play a vital aesthetic role in human life. Unfortunately, traditional breeding efforts often prioritize traits like vase life, shipping durability, color, and shape at the expense of desirable scents. The loss of fragrance in ornamentals, which represent a global market exceeding $30 billion, underscores the importance of genetic manipulation in enhancing flower fragrances. Initial work in this domain has begun, with several groups successfully introducing the linalool synthase gene from C. Breweri into petunia and carnation plants. However, these early experiments faced technical challenges, as the gene was expressed throughout the plant, resulting in linalool production levels undetectable by the human nose. Similar trials in tobacco using genes for other monoterpene synthases, such as limonene producers, yielded comparable outcomes.

G

Ongoing experiments in the next generation are exploring advanced strategies that target the expression of fragrance genes specifically in flowers or other organs, including specialized glands capable of storing antimicrobial or herbivore-repellent compounds.

Questions 1-4

The Reading Passage has seven paragraphs A-G.

Where can the information be found about which paragraph?

Write the correct letter A-G, in boxes 1-4 on your answer sheet.

1 Substance released to help plants themselves.

2 Scent helps plant’s pollination.

3 Practice on genetic experiments of fragrance.

4 Plant’s scent attracts the herbivore’s enemy for protection.

Questions 5-8

Do these statements agree with the information provided in Reading Passage 1?

In boxes 5-8 on your answer sheet, write

TRUE           if the statement is true

FALSE          if the statement is false

NOT GIVEN if the information is not given in the passage

5 We have little evidence to support the idea that scent attracts pollinators.

6 Heliothis virescens won’t eat those tobacco leaves on which they laid eggs.

7 Certain ants are attracted by volatiles to guard plants in rainforest.

8 Pollination only affects fruit trees’ production rather than other crop trees.

Questions 9-13

Choose the correct letter, A, B, C or D.

Write your answers in boxes 9-13 on your answer sheet.

9   How do wasps protect plants when they are attracted by scents according to the passage?

• A    plants induce wasps to prey herbivore.
• B    wasps lay eggs into caterpillars.
• C    wasps laid eggs on plants to expel herbivore.
• D    offending caterpillars and wasp eggs coexist well.

10   What reason caused a number of honeybees decline in the United States.

• A    pollination process
• B    spread illness
• C    crop trees are poisonous
• D    grower’s overlook

11   Which of the following drawbacks about artificial fragrance is NOT mentioned in the passage?

• A    it’s very expensive
• B    it can’t tell correct information to pollinators.
• C    it needs massive manual labour
• D    it poisons local environment

12   The number of $30 billion quoted in the passage is to illustrate the fact that:

• A    favorable perfumes are made from ornamental flowers.
• B    traditional floriculture industry needs reform.
• C    genetic operation on scent can make a vast profit.
• D    Scent plays a significant role in Ornamental industry.

13   What is weakness of genetic experiments on fragrance?

• A    Linalool level is too low to be smelt by nose
• B    no progress made in linalool emission.
• C    experiment on tobacco has a better result
• D    transgenic plants produce intense scent

Answers:

1. B (Đoạn B, “When the trunk of a pine tree is injured – for example, when a beetle tries to burrow into it – it exudes a very smelly resin.,”)

2. A (Đoạn A, “In fact, plants emit organic molecules all the time, although they may not be obvious to the human nose.’)

3. F (Đoạn F, “The loss of scent among ornamentals, which have a worldwide value of more than $30 billion, makes them important targets for the genetic manipulation of flower fragrance.”)

4. C (Đoạn C, “Herbivore induced volatiles often serve as indirect defenses.”)

5. TRUE (Đoạn A, “ This common notion is mostly correct, but it is surprising how little scientific proof of it exists.”)

6. NOT GIVEN (Không có thông tin)

7. TRUE (Đoạn C, “…ants use either as a pheromone or as an antiseptic in their nests.”)

8. FALSE (Đoạn D, “Floral scent has a strong impact on the economic success of many agricultural crops that rely on insect pollinators, including fruit trees such as the bee-pollinated cherry, apple, apricot and peach, as well as vegetables and tropical plants such as papaya.”)

9. B (Đoạn C, “…eventually, the wasp eggs hatch, and the emerging larvae feed on the caterpillar from the inside hatch, and the emerging larvae feed on the caterpillar from the inside out.”)

10. B (Đoạn D, “ This problem has been exacerbated by recent disease epidemics that have killed many honeybees,…”)

11. C (không được nhắc đến trong bài đọc)

12. D (Đoạn D, “ Ornamentals, including cut flowers, foliage and potted plants, play an important aesthetic role in human life.”)