1. Plant cell totipotency
The totipotency of plant cells is the theoretical basis of tissue culture. A living plant cell, as long as it has a complete membrane system and nucleus, it will have a whole set of genetic basis that develops into a complete plant. Under an appropriate condition, a complete plant can be generated by division and differentiation. This is called Cell totipotency. But in the natural state, due to the cells
Due to the different positions and physiological conditions in the plant, its differentiation is regulated by various aspects, so that the genetic information it possesses cannot be fully expressed, so it can only form a special cell and constitute a tissue of the plant. Or part of an organ. This shows that the conditions are very important, or critical, as long as the conditions are appropriate, the cell's potential genetic ability will be shown. Plant tissue and cell culture technology is based on the theory of cell totipotency, using artificial methods to create an ideal condition suitable for growth, so that the cell totipotency can be exerted.
In theory, as long as it is a living cell, it has the potential to regenerate a complete plant, but the actual situation is not so simple. As far as is known, the regeneration potential of cells is inversely related to the degree of differentiation, that is, the higher the degree of cell differentiation, the lower the regeneration ability. The mechanism of cell differentiation. Although the reason has not been completely revealed in theory, the fact is that the older the cell, the more strictly its gene expression is restricted, or the more genes that lose or fail to function, so you should try to choose younger ones. Plant tissue is used as experimental material for cultivation. At the same time, it should also be considered that the regenerative capacity of a certain genotype or explant is not static. Under different culture conditions, the performance of the same genotype or explant is different. As long as the conditions are suitable for highly differentiated cells or tissues, There is also the possibility of producing regenerated plants. Whether this possibility will become a reality remains to be continued.
2. Cell differentiation, dedifferentiation and redifferentiation
(cell differentiation, didifferentiation and ridifferentiation)
A mature seed contains a small embryo, which can also be called an embryo. Almost all the cells that make up the embryo maintain an undifferentiated state and a strong ability to divide cells. Their cytoplasm is dense, the nucleus is large, and there is no big difference between cells. These cells can be called embryonic cells, or Meristematic cells or undifferentiated cells. Under suitable conditions, as the seeds germinate, all the cells that make up the embryo begin to divide and increase the number of cells. Over time, the fate of the cells changes, and the morphology and function also change. Some cells form leaves, some cells form roots, some cells form stems, and some still maintain the ability to divide. The cells gradually lose their ability to divide, and the process of a permanent (irreversible) moderate change in the morphological structure and function of a cell is called differentiation. Differentiation is mainly determined by genes within the cell, which means that differentiation is the result of poor expression of genes in both time and space. The result of differentiation leads to the loss of cell division ability, accompanied by cell differentiation, maturation and tissue formation, the formation of plant roots, stems, leaves, flowers, fruits and seeds, and the emergence of a mature plant body. Under normal natural conditions, these differentiated cells will no longer restore their ability to divide and restart cell division until the plant dies.
When a cell that has lost its ability to divide and is in a state of differentiation and maturation and quiescence is placed on a specific proliferation medium, the first change it takes is to revert to a meristematic state, which includes the activity of lysosomes The degraded cytoplasmic components are degraded and new cytoplasmic components are produced (ie, destruction and reconstruction of organelles). At the same time, the types and activities of enzymes in the cell are changed, and protein synthesis and cell metabolism are also changed, resulting in genes. Changes in expression have reversed the nature and state of the cell, which can be said to be rejuvenation. The phenomenon (or process) in which cells that have lost their ability to divide revert to a meristematic state and divide to form an undifferentiated cell mass, called callus, is called "dedifferentiation". If the dedifferentiated cells are in proper conditions, they can maintain a vigorous division for a long time without differentiation. The transformation of cells from undifferentiated callus into cell masses and tissues with certain structures and performing certain physiological functions, forming a complete plant body or plant organ (or process), called "redifferentiation" . If a differentiated cell is to express its totipotency, it must go through the process of dedifferentiation and redifferentiation, which is what plant tissue and cell culture should achieve. The main principle of designing medium and creating suitable culture conditions is how to promote the dedifferentiation and redifferentiation of plant tissues and cells. The main work of culture is to design and screen the medium, discuss and establish suitable culture conditions.
Plant hormones play a major role in regulating cell dedifferentiation and redifferentiation. Plants are very sensitive to hormones. The types, relative proportions and absolute amounts of auxins and cytokinins in the medium can directly affect the process of cell dedifferentiation and redifferentiation. Hormones are often used in tissue culture Kind, concentration and relative proportion to achieve the purpose of regulating dedifferentiation and redifferentiation.
3. Organogenesis and embryogenesis
During the process of dedifferentiation and redifferentiation of cultured plant tissues and cells to regenerate a new plant body, especially during redifferentiation, there are two ways. One is that some cells of callus differentiate first to produce buds (or roots ), And then produce roots (or buds) on another medium to form a complete plant. Because buds and roots are both organs of the plant, this process is called organogenesis. The other is that some structures similar to the embryos in the seeds are produced in the callus, that is, a bipolar structure with shoot ends and root ends is formed at the same time, and then developed into rooted shoots on another medium at the same time, because This process is similar to the process of seed mesoembryo formation and the formation of seedlings during seed germination, so it is called embryoid body or asexual embryogenesis. Under tissue culture conditions, whether plant regeneration takes the path of organogenesis or embryoid body, which varies from plant to plant and from medium to medium, and sometimes even the same plant under the same conditions, that is, somatic embryogenesis There are also pathways for organogenesis. This is a natural phenomenon and is normal, except that the frequency of the two development pathways varies significantly with different genotypes and changes in culture conditions.
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