10.2 Options for differentiation

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Most cells start their lives as small, non-vacuolate entities. Usually, after several rounds of cell division and a phase of cell growth, they mature into one of the many plant cell types. This last phase of development is known as cell differentiation. In intact plants, simultaneous differentiation of several types is the norm, for example the epidermis, stomata, mesophyll and vascular cells that, in combination, become a functional leaf. Generation of organs is obviously an intricate process requiring precise control of positioning of cell types and directions of growth. Complexity, however, makes the regulatory processes very hard to study. Instead, much has been learned about differentiation from cells and tissues grown in isolation, usually in axenic cultures. Many plant tissues and organs are suitable sources of explants (meaning any small excised piece of tissue). An advantage of tissue culture is the ability to define almost every component of the cells’ physical and chemical environment. Successful tissue cultures can be grown almost indefinitely or can be manipulated to ensure regeneration of large numbers of genetically identical intact plants or clones. Many of the processes exploited are related to natural wound responses and vegetative propagation. In addition, specialised tissue culture conditions allow living cells to survive in the absence of a cell wall, as naked protoplasts (Dijak et al. 1986), or whole plants to be generated with haploid instead of diploid genomes.