The Many from the one
In a previous article on Mitosis and Meiosis, I described how the process of cell division works. In both cases, a single cell can give rise to a part of the plant structure or to an entire plant. Here I'm going to focus on the ability of normal cells in the plant body to reproduce and to generate an entire plant without sexual means. This ability is called totipotency, and it is a characteristic of specific types of tissue in plants, called meristematic tissue.
Totipotent cells serve the same role in plants that stem cells do in animals. They are found in shoot and root growing tips as meristems, and in the cambium layer (the layer of cells between the bark and the wood) of woody plants and trees. The activity of these meristematic regions gives rise to new roots, stems, leaves, and flowers or fruiting structures in plants, as well as increasing the diameter of woody plant trunks and branches. All of the structures found in a mature or growing plant are the result of cellular material produced by meristematic tissue. Knowing this helps in understanding how a cutting can root or how tissue culture labs can produce hundreds or thousands of young plants using just the meristematic cells.
Some plants have cells that are able to convert from a structured, or differentiated, state to a totipotent state. One example is the ZZ plant (Zamioculcas zamiifolia). This plant is so cellularly precocious that a mere piece of leaf or stem can be used to grow a new plant. Other plants and trees can produce spontaneous buds on the stem or trunk. These emerge from the activity of the meristematic tissue of the cambium layer, and since they are not buds that are normally produced in the axils of leaves, they are called adventitious buds. This can happen even when the tree in question is not the kind that will root from cuttings. The formation of adventitious buds begins with the production of a callus, which is essentially an undifferentiated lump of cells resembling a small tumor (see callus tissue in the thumbnail picture above, right). Through genetic and hormonal processes, cells differentiate into growing points which then produce new stems with leaves. A similar process can result in adventitious roots as well.
Advances in the understanding of totipotency and how cells can be stimulated to become totipotent has resulted in an entire industry, the business of micropropagation, or what is commonly referred to as "tissue culture". While micropropagation labs do culture tissue (meristematic tissue), this work is not the same as what is done in a scientific tissue culture laboratory. The work done in scientific labs is much more esoteric and amazing. I studied the science of tissue culture when I attended graduate school, and we did things like isolating single cells from a callus and removing the cell wall using enzymes to yield a naked plant cell called a protoplast. These are important in scientific research because genes are more easily inserted into plant cells when they don't have a cell wall around them. We grew pure callus cultures and cell lines that were, essentially, all totipotent cells. Using the right combinations of plant hormones, we could then cause the cells to turn into plant shoots with roots. These and other experiments done in scientific tissue culture labs are not the kinds of activities engaged in by micropropagation operations, but they do demonstrate vividly the power of totipotency in plant growth.
By contrast, commercial tissue culture is the business of producing many thousands of clones of particular plants using meristematic tissue. Different plants require different formulations of media, as the formula that is ideal for one plant will produce no results with some other plants. Conventional micropropagation requires a sterile or aseptic environment and the operating costs of such a facility are substantial. However, without totipotency, none of this would be possible at any cost.
Image credit: Wikimedia Commons