Do plants love forking paths? Decoding their branching wisdom and strategy

Produced by: Science Popularization China

Author: Gu Dan (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.

There are all kinds of plants in various shapes and sizes on the beautiful planet we live on. Some are tall and graceful, some are winding and creeping, some are lush and leafy, and some are unique. What dominates these shapes of plants?

For plants, leaves with various shapes and flowers with different colors and fragrances are the most attractive, and branches, as the carrier of beauty, are the key to forming the outline of the plant's appearance. The branches of a plant are the branches that extend outward from the main trunk of a plant. The branches of a tree are the branches that extend out from the big trees on the roadside.

Why do plants have branches?

Unlike animals, plants are usually rooted in the earth. When a bad environment occurs, they cannot escape the risk by running away. In order to cope with various adverse events, plants have evolved unique survival mechanisms over a long period of time. Branching is one of the important ways for plants to survive and protect themselves.

First, spreading branches and leaves helps the plant grow and reproduce. Even if some branches are damaged, the whole plant can still grow normally. Secondly, different branching methods of plants can adapt themselves to the environment to the greatest extent. A well-developed branching system allows plants to make the best use of resources such as water, nutrients, and light. Under certain conditions, plants will also independently choose the most suitable branching method according to environmental conditions to ensure that they can live well.

What are the ways in which plants branch?

Different plants have their own unique branching methods, which also create diverse plant forms.

Some woody plants have a very obvious main trunk, which is formed by the continuous upward growth of the terminal bud to form a main axis with obvious advantages, while the elongation and thickening of the branches are much weaker than the main trunk. For example, the poplar trees we see on the roadside are the most typical example. Most gymnosperms, such as larch and metasequoia, belong to this type. Some coniferous trees in the cold zone have obvious main stems and their branches are very close together, forming a pyramid-like structure, which helps prevent heat loss and can also prevent branches from breaking due to heavy snow.

Image source: veer gallery

Some woody plants have unclear trunks. The top buds grow slowly or die after a period of time. The lateral buds immediately below take over the mission and form strong lateral branches. After a period of growth, the growth advantage shifts to the next level of lateral branches. This is repeated. This trunk is composed of many lateral branches. This branching form is called sympodial branching, such as apple trees. It is conducive to occupying more space and bearing more fruits. In plants with opposite leaves, the branching method is slightly different. After the top bud stops growing, the pair of lateral buds below grow at the same time to form two forked lateral branches. The growth activity of the new branches is the same as that of the mother branch. For example, the lilac tree is a typical pseudo-binary branch, which is a special form of sympodial branching.

Image source: Veer Gallery

The branching phenomenon unique to grasses is called tillering. For example, rice and wheat will produce swollen and nutritious tiller nodes (i.e. the parts where tillers are attached) under or near the ground, and tillers with adventitious roots will be produced on these tiller nodes. Tillers that can produce ears and seeds are called effective tillers, and those that cannot are called ineffective tillers.

Rice

(Photo source: provided by Qin Huihui)

How are branches formed?

The formation of plant branches involves two key steps.

First, meristems containing clusters of stem cells, called axillary meristems, form at the base of the leaf (in the axil).

There are two different hypotheses about the formation of axillary meristem: one model holds that axillary meristem originates directly from the cells at the top growth point of the plant, just like a soldier who has laid an ambush in the large group early on, just silently waiting for the right time to become independent and form a new growth point.

Another model holds that axillary meristems are derived from differentiated leaf cells, and are independent of the apical growth point from birth, and are born in the axils of leaves. Once these axillary meristems receive instructions, they become active and divide faster, producing axillary buds that we can see with the naked eye; then, the axillary buds continue to grow and form branches under suitable conditions.

When the external environment is not suitable, the axillary buds will choose to stop growing and become dormant buds. Therefore, the activity of axillary stem cells and axillary buds is the key to determining the formation of plant branches.

Axillary meristem (a small protrusion)

Image source: Current Plant Biology (2015) 13–19

Factors affecting branch formation and development

If the growth of branches is not restricted, it will also have a negative impact on the plant's shape (i.e. the shape of the plant), reproduction and survival. Therefore, plants have evolved sophisticated control mechanisms to regulate the development of branches. Generally speaking, the number of branches is affected by genetic factors, hormones and growth environment.

Genes have the greatest influence on the branching morphology of plants. After all, "you reap what you sow." However, where there is heredity, there is variation. The domestication and breeding process of corn is the most typical example. The closest ancestor of corn is the Mexican teosinte, which has multiple tillers. In the process of corn domestication, we tend to choose fewer tillers or no tillers.

Studies have found that there is a key gene TB1 in corn that regulates tillering. This gene is a core commander that can control the growth of lateral organs and promote the occurrence of female inflorescences.

In modern corn, a sequence was inserted into the regulatory region of this gene, so it was like taking stimulants, and its activity was increased by two times, greatly enhancing the apical dominance of corn and reducing tillers. Isn’t it amazing that a small increase in the activity of a single gene can turn corn with many tillers into the corn with few tillers that we have been growing now?

Teosinte and modern corn

Image source: Nature Communication (2019) 10: 5608

Plant hormones have an important influence on the formation of plant branches. Plant hormones are compounds produced in extremely small amounts in plants and can regulate many physiological processes. Among them, auxin has a great influence on the formation of branches.

Auxin is first formed in the plant's apical bud, which allows the apical bud to maintain its growth advantage while inhibiting the growth of lateral buds. Defects in the synthesis and transport of auxin will affect the formation of plant branches. Existing studies have reported that once the key gene for transporting auxin loses its function, the plant will form a "one-needle" structure and may not produce any branches.

(Photo source: provided by Guo Zhiai)

In addition, the growth environment also has a great impact on the formation of branches. People who like to grow flowers and plants will know that when the planting density is too high, the water and fertilizer are improper, or the light is insufficient, the plants will grow too long, which will show that the plants are thin and tall, not strong, and the number of branches will be significantly reduced. This is an important way for plants to adapt to the environment.

In order to survive, the plant needs to keep its main stem growing upward as much as possible to compete with surrounding plants for more light resources, which will cause the plant to elongate its stem faster on the one hand, and bloom early on the other hand to complete reproductive reproduction as soon as possible.

Learn about plant branches and cope with daily life

Now that we know what factors affect plant branching, we can make good use of this to manage the planting of flowers, plants, fruits and vegetables. According to different needs, different plants need to choose the appropriate number of branches that maximizes their own interests.

For most crops, proper branching/tillering helps plants compete with weeds and increase the number of fruit clusters, but too many branches/tillers will lead to poor ventilation, making it easy for crops to become diseased, causing them to grow too tall and reducing crop yields.

According to the characteristics of fruit formation and our planting purposes, the branching of different crops requires different management modes . For example, ordinary corn needs to improve the aboveground structure by reducing branches/tillering to achieve eugenics and improve its economic benefits.

However, if the goal is to produce silage corn, the entire plant will be harvested and processed for livestock feeding, which requires an appropriate increase in the number of branches/tillers to obtain more biomass; each branch/tiller of crops such as soybeans, rice, and wheat is the attachment site of the grain, and it is difficult to maximize the yield of a single plant through a single ear/pod, so a moderate number of tillers is required.

The ideal rice plant type proposed by Academician Li Jiayang

(Image source: State Key Laboratory of Plant Genomics)

For forage grasses such as centipede grass and Medicago truncatula, it is necessary to increase effective branches to improve seed yield, grass yield and bare land coverage; vegetable varieties such as tomatoes, eggplants and peppers need to be pruned regularly to avoid excessive branches affecting ventilation; fruit trees need to be pruned regularly to remove weak and diseased branches and retain a certain number of strong fruiting branches, which can significantly increase yields.