Plants: If I don’t catch the “mutation”, I will be the “lamb to be slaughtered”…

Produced by: Science Popularization China

Author: Jiri Hulcr (University of Florida), Dong Yiyi (University of Florida)

Producer: China Science Expo

In 2013, a large-scale die-off of sweetgum trees in a Shanghai nursery was discovered. An investigation revealed that the culprit was a new species, the sweetgum borer (dù). This incident well illustrates the fact that plants with a single genetic makeup are more susceptible to pests and diseases than plants with a more complex genetic makeup.

Liquidambar formosana

(Photo source: The Paper)

Genetic diversity: key to plant resilience to environmental change

Genetic diversity is the key to plant response to pests and diseases, and it is also the basis of biodiversity. Everyone knows that there are no two people in the world who are exactly the same. This is because the DNA that makes up each person is not exactly the same. Even identical twins cannot have exactly the same appearance (generally called phenotype in biology).

We can simply think of the differences in DNA as genetic diversity. Genetic diversity refers to the differences between individuals of the same species at the genetic level (differences between species are called biodiversity). This difference determines the performance of plant morphology, color, and adaptability to the environment, such as drought resistance, cold resistance, heat resistance, and resistance to pests and diseases.

High levels of genetic diversity allow different individuals to have different responses to the environment. Some individuals cannot survive the challenge, but some can, which ensures the survival and reproduction of the entire group. For example, in the same pine tree, some individuals may be better able to tolerate drought conditions than others, or have higher resistance to a certain disease. Similar phenomena are also common in humans, such as some people have innate resistance to certain diseases (such as malaria), while others are more susceptible to infection.

The genetic diversity of plants provides protection for the species as a whole against various external threats. Higher genetic diversity means that there may be individuals with different resistances in the population, thus showing stronger group survival ability when facing specific new pests and diseases. For example, when laurel wilt (a plant pathogen) broke out in the southern United States, although most laurel trees were killed by the disease, a few individuals survived. These surviving individuals may carry disease-resistant genes and become the genetic basis for future population reconstruction.

If there are multiple tree species in a forest, but the genetic composition of each tree species is exactly the same (for example, they all come from the same clone), then there will be almost no difference in the response or resistance of these tree species to pests and diseases. Once a certain pest and disease breaks out, the entire species may face a devastating blow. This is the key role of genetic diversity in a single tree species.

Even if there are multiple tree species in a forest, we must ensure that each tree species has sufficient genetic diversity. Only with sufficient genetic diversity can we retain individuals with resistance or adaptability when pests and diseases strike, and these individuals will become the key to future population recovery.

This is why, when afforestation is carried out, in addition to planting multi-species mixed forests to increase the diversity of forest communities, it is also necessary to ensure that the seeds of each tree come from different sources, avoiding the use of only clonal tree species or genetically monochromatic populations.

Factors inducing the occurrence of pests and diseases

At present, we are facing an increasingly severe challenge - the threat of foreign pests and diseases. At the same time, extreme climate change creates more opportunities for pests and diseases to occur. They are like "invisible killers" that silently threaten the health of plants. Take the fall armyworm as an example. This pest, which is native to North America, can feed on and harm a variety of crops in China, causing plant defoliation or even death.

Fall Armyworm

(Photo source: Sohu News)

According to incomplete statistics, rice blast caused corn production losses of about 14 million yuan in China in 2019. As one of the main fungal diseases of rice, rice blast has various transmission pathways, including air, insects, rain and soil. Between 1975 and 1990, global rice production fell by 30% due to rice blast, resulting in a loss of 15.7 million tons of grain; in China alone, about 3 million tons of grain are lost each year.

Pests induced by climate change, such as wheat aphids. It is one of the main pests of wheat. The rise in global temperatures in recent years has led to an earlier occurrence of wheat aphids. This means that the total time of wheat aphids occurrence increases in a year, which greatly increases their number and causes more damage.

The reduction in genetic diversity of plants themselves is another reason . In areas where large areas of the same crop with the same genetic composition are planted (large homogeneous plantations, artificially planted pure forests), the problem of pests and diseases is particularly prominent.

In many areas, for ease of management and the pursuit of higher economic benefits, organizers planted seedlings from a unified breeding in the same area, which made all the plants in the area have no variation in genetic composition. These plantations are like "lambs" waiting to be slaughtered.

Low genetic diversity means that they have a reduced ability to adapt to environmental changes. When faced with environmental pressures such as climate change and soil degradation, genetically homogeneous plants may gradually decline due to insufficient genetic variation to adapt to these changes. More importantly, due to genetic similarities, once a pest or disease can infect one plant, it is likely to infect other plants in the plantation, leading to the rapid spread of pests and diseases.

However, even wild tree species with high genetic diversity may have difficulty defending against certain new invasive pests. For example, despite their high genetic diversity, North American ash trees were unable to resist the invasive emerald ash borer. This pest is native to Asia, where ash trees have co-evolved with it and have developed defenses against it. However, North American ash trees have not experienced similar evolutionary selection pressures and therefore have no resistance to this invasive pest, resulting in the death of tens of millions of ash trees. This example shows that even if a species has high genetic diversity, it may still suffer severe losses when faced with a specific biological threat if it has not been exposed to it in its evolutionary history.

Shows North American sweet gum (Liquidambar spp.) being damaged by the sweet gum borer (Acanthotomicus suncei).

(Image credit: Jiri Hulcr)

How to increase genetic diversity in trees?

New pests and diseases may emerge at any time in the future. Although we cannot accurately predict every threat, we can improve the plant's ability to defend itself by increasing genetic diversity. Here are some effective strategies:

1. Genetic diversity is a priority: When growing plants, we are not completely guided by economic benefits and maximizing yields. We need to adjust our priorities from maximizing yields to maximizing genetic diversity and biodiversity.

2. Reasonable thinning strategy: When thinning forests, we should try to retain individuals with genetic diversity, rather than indiscriminately removing plants based on current needs (such as higher yields and growth rates). This not only retains different phenotypes, but also promotes the continuation of species genetic diversity.

3. Open pollination (pollination through natural mechanisms such as insects, wind, and birds): Reducing human intervention and adopting natural pollination mechanisms can increase natural genetic variation. This method can promote natural selection and increase diversity of genes, improve the survival rate of species. It can also enhance the adaptability and resistance of the entire population without relying on specific resistance genes.

4. Change the way of afforestation: In the past, afforestation only planted plants bred from a few lines, and the genetic basis of these plants was relatively simple. In the long run, planting such genetically simple clones may not really restore the health and resilience of the forest. It may be simpler and more economical to let the forest recover from seeds. We should be committed to protecting the naturally regenerated seedlings in the forest and provide necessary support when appropriate, rather than relying on genetically simple clones.

5. Planting mixed forests: For example, the continued high temperatures in Europe have caused serious damage to spruce plantations by the spruce beetle (Ips typographus). We need mixed forests of different ages and species, which can not only enhance the resistance and resilience of forests, but also ensure the long-term stability and sustainability of the forestry economy.

6. Emphasize forestry education: Driven by immediate profits, the number of schools focusing on forestry education is decreasing worldwide, which has led to a reduction in the provision of forestry-related degrees. This is particularly evident in the global South, where plantation timber production is dominant. We need to pay attention to ensure that forestry and forest protection-related expertise is not marginalized or forgotten over time.

7. Strengthen public awareness of genetic diversity: Relevant educational departments and institutions can use new media and other means to carry out popular science or special lectures on genetic diversity and biodiversity conservation, so as to enhance public awareness of the importance of genetic diversity.

References

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