Fancy propagation, controlled dormancy... seeds are very smart in order to reproduce!

Produced by: Science Popularization China

Author: Li Yu, Zhao Jun (Institute of Botany, Chinese Academy of Sciences)

Reviewer: Liu Yongxiu (Institute of Botany, Chinese Academy of Sciences)

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

On our blue planet, there are plants of all shapes and sizes. Spring orchids, summer lotus, autumn chrysanthemums, winter plums... They embellish the four seasons and enrich our lives.

Seeds are a manifestation of plant wisdom. They carry the hope of the population and are spread to suitable places for growth in various ways. They germinate at the right time, thus achieving the continuation of the plant population. The seed propagation and germination mechanism demonstrates the wonderful wisdom of plants in nature in adapting to the environment, and is also an important topic that the scientific community has long paid attention to.

There are many ways to spread seeds

Seed plants have evolved various ways that are conducive to seed dispersal, using various forces in nature to spread offspring and allow seeds to reach an environment and space suitable for germination, such as wind dispersal, water flow dispersal, catapult dispersal, and animal dispersal.

Some plants make "little wings" for their seeds, so that the wind can carry them far away; some seeds are natural "good swimmers" that can float on the water and drift to a new environment to take root and sprout, such as coconut and lotus pods.

Wind propagation and water propagation

(Photo source: Veer Gallery)

Animal dispersal can be seen as a transaction between plants and animals. Animals eat the fruits of plants and then excrete the seeds elsewhere. In this way, the animals get food and the seeds get "tickets". For example, squirrels collect and store pine nuts, which are either eaten, forgotten or lost. Among them, the pine nuts that "escape" from the mouths of mice may take root and sprout in new places. In addition to these, the cockleburs that forcibly hitchhike are also used for animal dispersal.

Animal transmission

(Image source: Reference 1)

In addition, some plants have fruits that burst open when ripe, ejecting seeds in all directions. Spray melons, peas, and rapeseed seeds are spread in this way.

Catapult propagation (Video source: Smithsonian Channel image conversion)

During the long evolutionary process, in order to adapt to the unstable environment of the coastal intertidal zone with ebb and flow, mangrove plants have evolved a unique "viviparous" reproduction strategy. This strategy allows the seeds to germinate inside the fruit and develop into a rod-shaped hypocotyl. When the hypocotyl develops to a certain extent, it will break away from the mother tree and fall into the mud on the beach under the action of gravity, quickly taking root and growing new individuals. This viviparous phenomenon is an important adaptation of mangrove plants to coastal habitats, which ensures that plants can effectively reproduce and survive in high-salt, oxygen-deficient swamp areas.

Mangrove "viviparous" plants

(Photo source: Veer Gallery)

The formation and release of seed dormancy is full of wisdom!

Not only do animals hibernate (such as the dormant characteristics of fertilized eggs of fish and shrimp), but seeds in the plant kingdom also hibernate.

Seed dormancy is a biological property that prevents viable seeds from germinating even under suitable environmental conditions. This property is a self-protection mechanism formed by plants in the long-term evolution process to resist adverse natural environments. Seed dormancy helps to extend the life of seeds and increase their propagation distance, thereby expanding the spatial distribution of plant populations. In addition, the dormant state can prevent seeds from germinating in large numbers in a short period of time, reduce intraspecific competition, and improve the ability of populations to adapt to environmental changes.

In the same species, seed dormancy in different ecotypes also varies greatly. Taking the model plant Arabidopsis thaliana as an example, the seed dormancy period of the Col-0 ecotype is shorter, only about 2 weeks, while the seeds of the Cvi ecotype have a longer dormancy period, which can last up to half a year. This difference may be related to the climatic conditions of the origin of the Cvi ecotype. It originally grew in the tropical desert areas of Africa, where the environmental conditions are relatively harsh. Seed dormancy is a key biological strategy that ensures that seeds can germinate at the right time and environment, thereby increasing the success rate of survival and reproduction. It is an ingenious self-protection mechanism for plants to adapt to the environment.

Different ecotypes lead to differences in seed dormancy

(Photo source: provided by Yang Yue)

In the late stage of seed development, seed dormancy gradually forms as the seeds dehydrate. Studies have found that the ambient temperature during the seed formation process has a significant effect on the degree of seed dormancy. For example, seeds formed at lower temperatures in the model plant Arabidopsis have a high dormancy level; conversely, if the temperature is high, the seed dormancy level is low. This is related to the accumulation of the low-temperature-induced dormancy factor DOG1, which promotes the deepening of seed dormancy.

In addition, plant hormones also have an important influence on the formation of seed dormancy. They are extremely trace compounds produced in plants and can regulate many physiological processes. Among them, abscisic acid plays a key role in the formation of seed dormancy, while gibberellins are very important in promoting seed germination. The relative balance between the two determines seed dormancy and germination. It is worth noting that gibberellins cannot promote the germination of all plant seeds. For example, the seeds of parasitic plants have evolved a unique mechanism. They rely on sensing strigolactone (SL) secreted by the host to initiate germination. Strigolactone is a class of sesquiterpenoid plant hormones derived from carotenoids. It has the function of regulating plant branching and promoting the symbiosis between plants and arbuscular mycorrhizal fungi. It is named because it can promote the germination of the seeds of the parasitic plant strigolactone.

Seed dormancy and germination

(Image source: drawn by the author based on literature)

Seeds carry the memory of the growth and development of their parent plants, and their special structure is like a password, which, to a certain extent, hinders contact with water, gas and other necessary conditions for germination, thus forming a dormant state. To simplify understanding, we can divide seed dormancy into two main categories: one is physical dormancy of seeds, which is caused by the structural characteristics of the seed coat or pericarp, which affect the water absorption and air permeability of seeds, or limit the growth of embryos; the other is physiological dormancy of seeds, which is caused by the seed embryo itself, including incomplete embryo development, physiological incomplete after-ripening, and the presence of substances that inhibit germination.

In nature, the process of seed dormancy is also full of wisdom. Seeds will make full use of natural environmental changes such as the changing seasons, wind, frost, rain, snow, light and temperature cycles, animal feeding and microbial action to effectively break physical and physiological dormancy and wait for the right time to germinate.

Gradually ripening ginkgo seeds

(Image source: Reference 2)

The story of the dodo and the olive tree is a classic example of how animals can help plant seeds germinate. The dodo, a flightless bird that once lived on the island of Mauritius, was hunted and killed by Western colonists, eventually becoming extinct, which in turn led to the deterioration of the island's ecological environment. The olive tree is a tree species on the island of Mauritius, and its seeds need the help of the dodo's digestive system to weaken the seed (fruit) shell in order to germinate smoothly. The extinction of the dodo resulted in the inability to properly process the olive tree's seeds, which in turn led to the gradual death of the olive tree forest.

It was not until 1981 that scientists discovered that by imitating the digestion process of the dodo, they could promote the germination of the seeds of the big-skulled olive tree. This discovery brought hope to the conservation of the big-skulled olive tree and made people realize that every link in the ecosystem is very important and the loss of a certain animal or plant may cause an irreversible chain reaction. Currently, the only remaining dodo soft tissue specimen in the world is preserved in the Oxford University Museum of Natural History, which also uses the cartoon image of the dodo as its logo to commemorate this species and remind people to pay attention to the protection of biodiversity.

Dodo soft tissue specimen and the Oxford University Museum of Natural History logo

(Image source: Museum of Nature History)

Seeds regulate their life processes by sensing light and temperature

The seeds that have settled down are waiting for dormancy to end and start the next stage of life. Water and suitable temperature are necessary conditions for seeds to germinate. Temperature not only provides seeds with information about the season and local microenvironment, but also affects the enzymatic reaction during the germination process. Seeds sense changes in temperature and continuously adjust their life processes accordingly.

In the study of the model plant Arabidopsis, scientists discovered that the photosensitive pigment phyB is not only a receptor for plants to perceive red light, acting as the "eyes" of plants, but also a receptor for sensing temperature. When phyB loses its function, seed germination becomes more sensitive to high temperature stress and causes a sharp drop in seed germination rate. The reason is that high temperature gradually reduces the content of active phyB and causes an increase in the content of ABA, a germination inhibitor in seeds.

Temperature sensitivity of seed germination

(Image source: Reference 3)

Wildfires cannot extinguish them, spring breezes bring new life: the molecular wisdom of seeds

After a forest fire, vegetation will gradually repair itself without human intervention. When plants are burned, their bodies turn into ashes, but these ashes hide the secret of the birth of new life. The burning plants will produce a small molecule compound called Karrikin. With the first heavy rain after the fire, Karrikin will seep into the soil and strongly promote the germination of seeds in the soil.

Forest recovery after fire

(Image source: Reference 4)

Kalijin in forest ash promotes seed germination

(Image source: Reference 5)

The growth after seed germination is also full of ingenuity

After the seeds germinate, the seedlings need to adapt to the dark environment and overcome the mechanical pressure of the soil before they can break out of the soil. When it is completely in the soil, the seedlings rely on the remaining nutrients in the "fuel tank" to "bury their heads" and sprint upward. The "buried head" state, in which the cotyledons are closed and bent downward, reduces the soil pressure encountered when growing in the back, and also protects the fragile apical meristem. When breaking out of the soil, the "buried head" state is also conducive to pushing away the heavy objects above the seedlings, further protecting the seedlings. Finally, the seedlings open their cotyledons and begin their life outside the soil. "If it does not fall into fertile soil but into rubble, a living seed will never be pessimistic and sigh. It believes that only with resistance can there be tempering." Such great wisdom is hidden in the tiny seeds!

Seedlings breaking through the soil

(Image source: Reference 6)

Conclusion

After millions of years of evolution, stable and suitable ecosystems have gradually formed. In these ecosystems, each life form has evolved its own survival philosophy for its own survival and reproduction, in order to avoid or resist adverse environments, adapt to the environment, and maintain harmony with the environment.

Seed plants, as a highly evolved group in the plant kingdom, constitute the majority of green vegetation on the earth's surface. They reflect the ingenuity of nature in terms of seed formation, structure, shape, propagation, seed dormancy and germination mechanisms.

This is just the tip of the iceberg of nature's rich biological resources. There is still more "wisdom" in nature waiting for us to discover, learn and study. There is still a long way to go in the future, and pathfinders are still moving forward.

References:

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3. Piskurewicz, U., Sentandreu, M., Iwasaki, M., Glauser, G., and Lopez-Molina, L. (2023). The Arabidopsis endosperm is a temperature-sensing tissue that implements seed thermoinhibition through phyB. Nature Communications 14, 1202.

4. Abella, SR, and Fornwalt, PJ (2015). Ten years of vegetation assembly after a North American mega fire. Global Change Biology 21, 789-802.

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