Solving the mystery of life: Why can plants regenerate?

Produced by: Science Popularization China

Author: Liu Xinchun, Xu Chongyi, Hu Yuxin (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.

A few days ago, I trimmed the branches of the ginseng fig bonsai at home. Today, I suddenly found that many fresh green leaves grew out of the cuts (Figure 1). Soon, the organs and tissues that grow these new leaves will develop into new branches. I was impressed by the tenacious vitality of plants and envied them. It would be great if humans could have the same ability as plants, and they could grow new "branches" even if they were seriously injured. In this way, Yang Guo in "The Return of the Condor Heroes" would not have to live a "clean life" for the rest of his life.

Figure 1 New leaves growing from the wound of Ficus ginseng

(Photo source: provided by the author)

Why can plant branches be "lost and recovered"? The secret is that plants have a strong regenerative ability. The process of self-repair or replacement of tissues or organs of organisms after damage or stress is called regeneration. Unlike animals that can escape when danger comes, plants are rooted in the soil and cannot move. They can only passively resist external damage. Therefore, the complex and changing living environment forces plants to evolve more outstanding regeneration capabilities.

The miraculous phenomenon of plant regeneration

Plant regeneration is ubiquitous in our daily lives. For example, if you leave a part of the rhizome of onion (Allium fistulosum) and garlic (Allium sativum L.) in the genus Allium of the Liliaceae family and insert it into the soil for cultivation, you will soon get new onions and garlic (Figure 2). Sweet potatoes (Ipomoea batatas) and potatoes (Solanum tuberosum) will also grow new shoots on their roots or tubers after being stored for a long time (Figure 3).

Figure 2: New sprouts growing from garlic cloves

(Photo source: provided by the author)

Figure 3 Sweet potato root tubers (left) and potato tubers (right) sprouting

(Photo source: provided by the author)

In production practice, people have invented techniques such as grafting, layering, cuttings and tissue culture by using the regeneration ability of plants. Grafting refers to grafting the branches or buds (scion) of one plant onto the stems or roots (stock) of another plant, which can integrate the excellent traits of different plants (Figure 4). The method of inserting part of the stem, leaf, root or bud of a plant into a suitable substrate (such as soil, sand, water, etc.) to take root and sprout, and eventually develop into a new plant is called cutting. Cuttings are therefore divided into stem cuttings, leaf cuttings, root cuttings, etc., such as rose and poplar stem cuttings, and leaf cuttings common in succulents (Figure 5). Through tissue culture, technicians can use tissues such as orchid stem tips and leaves for reproduction, and mass-produce orchid plants with excellent quality and consistent appearance to meet market demand; culturing the virus-free stem tip tissue of potatoes can breed healthy and non-toxic plants and improve the quality and yield of potatoes (Figure 6).

Figure 4 Fruit tree grafting

(Photo source: veer photo gallery)

Figure 5 Poplar stem cuttings (left) and Sansevieria leaf cuttings (right)

(Photo source: veer photo gallery)

Figure 6 Orchid tissue culture (left) and potato virus-free seedling tissue culture (right)

(Photo source: veer photo gallery)

Exploring the nature of plant regeneration

In order to clarify the principles behind these natural and practical phenomena, researchers have been exploring continuously. As early as more than a hundred years ago (1902), the famous German plant physiologist Haberlandt predicted that plant somatic cells have the ability to gradually develop into complete plants after in vitro culture, and first proposed the concept of "cell totipotency".

In the 1930s, auxin and its synthetic analogs were found to play an important role in root development. White and Nobécourt first observed the development of shoots and roots in plant tissue culture. In 1947, Levine found that culture medium without auxin could induce carrot root tissue to redifferentiate into roots, stems and leaves.

In 1957, Skoog and Miller, based on previous research, discovered that different ratios of kinetin/auxin (plant endogenous hormones or their analogues) can induce the occurrence of different plant organs: a high kinetin/auxin ratio induces bud formation; a low kinetin/auxin ratio promotes root formation.

It was not until 1958 that Steward et al. cultured cells from the phloem of carrot roots in vitro and found that these cells underwent dedifferentiation and redifferentiation, eventually forming embryos and developing into complete plants with organs such as roots, stems, and leaves (Figure 7), confirming the totipotency hypothesis of plant cells.

Figure 7 Schematic diagram of the experimental process of carrot root phloem cells developing into a complete plant

(Image source: drawn by the author based on reference 8)

Nowadays, researchers have built a more complete plant in vitro regeneration system using a variety of model plants, and have a deeper understanding of the principles of regeneration. Regeneration phenomena can be divided into three categories: tissue repair, de novo organ regeneration (root de novo regeneration and bud de novo regeneration) and somatic embryogenesis .

The repair of tree wounds and grafting in agricultural production mainly utilize the ability of plants to repair tissues. At the wound site of a plant, multiple plant hormones such as auxin, jasmonate and gibberellin work together to further activate key transcription factors to regulate the tissue repair process (Figure 8).

Figure 8 The vascular tissues of Arabidopsis and tobacco were reconnected after grafting, and the acid fuchsin absorbed by the roots was transported to the leaf veins.

(Image source: Reference 10)

The essence of cuttings is the process of root regeneration from scratch. The ex vivo tissue will sense wounds and environmental signals, regulate the synthesis, transport and accumulation of auxin, and promote the occurrence of adventitious roots. Adventitious roots refer to roots that do not originate from the normal root system of plants (such as taproots or lateral roots), but grow from other parts of the plant (such as stems, leaves or non-root tissues of old roots) (Figure 9). Auxin is the core hormone in root regeneration, which can promote cells to acquire pluripotency and transform to root cell fate.

Figure 9 Arabidopsis leaf explants grow adventitious roots

(Image source: Reference 11)

Regeneration in plant tissue culture can be divided into two main types: de novo organ regeneration and somatic embryogenesis. The plant in vitro regeneration system using the model plant Arabidopsis thaliana as research material has made outstanding contributions to revealing the cellular origin and regulatory mechanism of de novo plant organ regeneration.

Research has found that the process of plant organ regeneration can be divided into two main ways: direct and indirect. In the direct regeneration method, the explant (part of the living tissue or organ separated from the plant body) can directly form a pluripotent stem cell group - callus at the wound, and some cells in the callus can further develop into buds or roots. Indirect regeneration of plants requires high concentrations of auxin to induce the explant to transform into callus, and then add different ratios of cytokinin/auxin to induce pluripotent cells to transform into buds or roots (Figure 10).

Studies have shown that in vitro callus regeneration mainly originates from the pericycle or pericycle-like cells in the xylem of the explant. Indirect regeneration has important value in commercial breeding, production practice and basic research due to its wide adaptability, gene transformation and large-scale reproduction.

Figure 10 Indirect in vitro regeneration system of Arabidopsis thaliana (CIM, callus induction medium; SIM, shoot induction medium; RIM, root induction medium)

(Photo source: provided by the author)

Somatic embryogenesis can also be divided into two ways: direct and indirect. The mini-plants that can form on the edge of the leaf that takes root are direct somatic embryogenesis (Figure 11). Most studies on somatic embryogenesis are conducted in the indirect system (Figure 12).

Similar to the indirect organ regeneration process, somatic embryogenesis first induces the production of embryonic callus through treatment with auxin or auxin analogs, which is then transferred to a culture medium without auxin to initiate somatic embryogenesis and embryo morphology. The genetic transformation process of crops and forages such as wheat, rice, and alfalfa all depends on indirect somatic embryo regeneration.

Figure 11 Mini plants growing from the edges of the leaves

(Photo source: veer photo gallery)

Figure 12 Embryonic callus and adventitious buds of alfalfa

(Image source: provided by the author)

The future development vision of plant regeneration technology

Plant regeneration has made us realize the magic of nature, and human exploration of the cellular origin and regulatory mechanism of regeneration has never stopped. In 2005, the internationally renowned journal Science published 125 of the most challenging scientific problems, among which "How does a single somatic cell become a whole plant?" was listed as one of the 25 most important scientific problems (Figure 13). In September 2022, Science magazine once again released one of the world's 125 most cutting-edge scientific problems - "Why do only some cells become other cells?" The question of plant cell totipotency has always been a cutting-edge problem in world science.

From the little things around us to the application of technology in labor production, many interesting questions are waiting for us to think about and answer, such as why buds form on potato tubers? Can any two plants be grafted? If not, what factors limit the tissue repair between them? Why can't all cells develop into complete plants? Is it possible to break this limitation in the future?

Figure 13 Plant cell totipotency is listed as one of the 25 most important scientific issues by Science.

(Image source: Science magazine, issue 5731)

Plant regeneration technology has broad development prospects in the future. With the advancement of biotechnology, this field will usher in more innovations and applications.

First, the development of gene editing technology (such as CRISPR-Cas9) will greatly improve the efficiency and accuracy of plant gene function research. Using CRISPR technology, it may be possible to break the genetic limitations of plant regeneration, allowing certain species that are difficult to regenerate to be regenerated through somatic cells. This will be conducive to the combination of regeneration technology and precision agriculture, achieving precise control of the plant growth process, optimizing resource utilization, and improving agricultural production efficiency.

Secondly, molecular breeding is combined with artificial intelligence to find key genes that regulate regeneration ability and excellent traits through the analysis of high-throughput genomic data. As a key link in genetic transformation, efficient plant regeneration technology will accelerate the integration and promotion of excellent traits, making it possible to cultivate new varieties with higher yields, disease resistance and environmental adaptability on a large scale and in a customized manner, thus promoting the sustainable development of agricultural production.

Thirdly, the use of regeneration technology can produce a large number of drought-resistant and salt-tolerant plants in a short period of time for the restoration of desertified land. The reproduction of rare tree species and forest vegetation will help restore the damaged ecosystem and promote biodiversity.

In addition, regeneration technology can be used to propagate medicinal plants on a large scale to ensure a stable supply, while increasing the content of medicinal ingredients through genetic modification. Plant regeneration can also be used to produce important secondary metabolites, such as medicinal compounds, flavors and pigments, which are of great value to the pharmaceutical industry.

Finally, regenerative technologies may become a key tool in synthetic biology, used to design and synthesize new plant species to meet specific environmental or industrial needs.

In the future, plant regeneration technology is expected to play a more important role in ecological restoration, endangered plant protection, and medicinal plant resource development. By continuously optimizing the regeneration system and gene regulation, we will be able to better respond to challenges such as global food security, climate change, and biodiversity conservation, and promote the coordinated development of agriculture and the ecological environment.

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References

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