Under the curled and yellowed leaves, a "war" is taking place

Produced by: Science Popularization China

Author: Gao Kaixing (Institute of Microbiology, Chinese Academy of Sciences)

Producer: China Science Expo

When walking on the road, have you ever noticed the yellowing and wrinkled leaves on the treetops? Have you ever imagined what is happening inside the leaves? In addition to the yellowing and falling caused by the changing seasons, the rampant invasion of pests and diseases is also one of the important reasons for the yellowing of leaves.

Peach leaves infected with disease

(Photo source: veer)

Throughout history, mankind has never stopped fighting against plant diseases and pests, especially crop diseases. As early as the Han Dynasty, "Five Elements Records" recorded a locust plague that broke out in the first year of Kaiping in the Later Liang Dynasty (907 AD): "Locusts broke out in the five states of Xu, Ru, Cai, Chen, and Ying. Flocks of wild birds flew across the sky and ate all the food." Wherever they went, no grass grew.

"Youshan Miscellaneous Stories" also recorded a rice disease that had broken out: "In May and June, the rice seedlings grow vigorously, but in July, they begin to shrivel and turn yellow, especially in areas with too much manure." This caused a great famine that year.

Ancient people surrounded locusts

(Image source: Reference 1)

These ancient records show that humans have been fighting against diseases and pests since ancient times. The rampant spread of diseases and pests has caused humans to suffer from famine. If we want to solve the threat of diseases and pests in the long run, we must understand how they break through the plant's defenses, enter cells and cause harm.

"The Tyrant of Diseases" - Plant Fungi

It is not difficult for us to understand that insects can rely on their unique conditions - the stylet to pierce plant cells/tissues and absorb juice and nutrients. Then, what ability do tiny plant pathogens have to "steal" plant nutrients?

Generally speaking, plant pathogens can be divided into four categories: fungi, bacteria, viruses and nematodes. Each category has its own unique knack for invading plant cells to absorb nutrients and continue its offspring.

First, let's look at how fungi, the "tyrants of the disease world," infiltrate plant cells and steal nutrients.

Plant fungi are a large family that accounts for 80% of plant pathogens. When they reproduce, they produce a variety of small spores. These clever spores rely on the power of nature (wind, rain, soil, etc.) or agricultural operations to attach to suitable plants.

Spore morphology under the microscope

(Photo credit: Photo taken by the author)

These spores will germinate and produce hyphae, which will "metamorphose" like tadpoles turning into frogs, specializing in a variety of structures that can invade plants. They can attach to cells, fix bacteria, and even prey on nematodes to absorb nutrients and give birth to offspring.

Fungal spores attached to plant surfaces

(Image source: sciencephotolibrary)

Plant bacteria: the king of leaks

Unlike fungi, bacteria are a type of prokaryotic organisms and do not have the natural skills of fungi. When they encounter plants, they cannot use their hyphae structure to help them invade plant cells, but they are the most observant hunters.

Perhaps because they know they cannot actively cause harm, bacteria are naturally good at finding loopholes. Whether it is the plant's own stomata, water pores, nectar glands, traces left by insect bites, or wounds accidentally scratched by humans, they can all be discovered and exploited by bacteria.

Relying on these favorable conditions created by nature, they can successfully sneak into the "food", easily complete their infection, and spread in the vascular tissue inside the plant that is used to transport water, nutrients and other substances.

Many bacteria also rely on insects as transmission media, and the two sides live in a mutually beneficial symbiosis, allowing them to spread over a large area among plants.

The big family of viruses: plant viruses

At a time when the novel coronavirus is raging, I believe everyone must be familiar with the word virus. As a large family in the virus world, plant viruses also pose a huge hidden danger to food security. As early as 2004-2006, the rice stripe virus that broke out in Jiangsu and other places in my country caused large-scale damage to rice in my country and even a total crop failure, posing a huge threat to national food security.

So, can plant viruses also be transmitted through air and other channels like the new coronavirus? The answer is no.

Because plants cannot move autonomously like humans, plant viruses have evolved differently from animal viruses. Most viruses choose to use insects as important vectors to help them spread and invade plant cells, just like some bacteria.

When insects suck the sap of diseased plants, they will also inhale the virus particles and temporarily store them in their bodies. When they attack the next healthy plant, the cunning virus will enter the plant cells along with the saliva released by the insect's bite, and reproduce with the help of the host's energy to complete the subsequent pathogenic process.

I believe that after reading this, everyone has questions: Are insects good guys? Do they help spread viruses for free without asking for anything in return?

The answer is also no.

A large number of studies have shown that when faced with a virus-infected plant and a non-infected plant of the same species, insects tend to prefer to eat the former. This is because the intervention of the virus will prevent the plant from releasing insect repellent chemicals, reducing the insect repellent effect, thus achieving a mutually beneficial symbiotic relationship between insects and viruses, which is conducive to the joint infection of plants by both. It really makes people sigh at the great wisdom of these small lives!

Plant parasites: plant nematodes

Seeing plant nematodes, many readers may ask: Aren’t plant nematodes insects? How are they related to pathogens?

In fact, nematodes belong to the phylum Nematoda and insects belong to the phylum Arthropoda. The two are not in the same category. But compared with pathogens, nematodes and insects are indeed more closely related.

Plant nematodes under the microscope

(Photo source: veer)

Why are plant nematodes classified as plant pathogens by plant protection scientists? There are three reasons:

1. The symptoms they cause are similar to those of plant pathogens;

2. They are small and require the use of a microscope to observe;

3. The occurrence, research and control of plant nematodes are similar to those of plant pathogens.

Nematodes harm plants in a similar way to insects, relying on their mouth needles to obtain nutrients, but they have different forms. Some only pierce the plant tissue with their heads to feed, while others enter the tissue with their entire body to feed. They can all cause great damage to plants.

Conclusion

When placing the yellowed and curled leaves under a microscope, we may be able to see that the plant cells are fighting against pathogens. Both sides are working hard to survive better. The wonders of nature are fully reflected in the tiny cells!

Editor: Wang Tingting

References:

[1] Zhou Yao. History of Early Entomology Research in China. Science Press: Beijing, 1957, 125-132;

[2] Wei TY, Yang GJ, Liao FL et al. Genetic diversity and population structure of rice stripe virus in China. Journal of General Virology. 2009, 90: 1025-1034.