The current awareness of the potential of stem cells in regenerative medicine has radically altered the way the general public views disease, biological injury and repair, medicine and cellular regeneration. Stem cells are discussed in almost every media around the globe, with much attention focused on the ethics of the therapeutic applications of embryonic stem cells. Thus, stem cells are now a deeply ingrained component of not just the scientific landscape, but popular culture as well. So what are stem cells? Stem cells in both animals and plants are defined by their remarkable ability to (1) replenish themselves through self-renewal as well as (2) the potential to generate differentiated(a) cells. Stem cells are the precursors of differentiated cells and are thus indispensable for growth and development in both plants and animals. The maintenance of stem cells in both plant and animal systems is dependent upon reciprocal signalling between stem cells and the specialized tissue microenvironment known as the niche that provides intercellular signals for stem cell regulation. The story really begins in the 19th century with plant physiologists but the animal researchers wish to claim their 19th century German scientists working with human bone marrow were the first to describe cells coming from other cells leading to the idea of "stem cells". During the 1960s Bun McCulloch and Jim Till did the landmark studies on bone marrow cells and developed the insights that particular cells that formed colonies on mouse spleens had the capacity to: (1) self-renew, and (2) differentiate. This was really the first easily recognisable animal stem cell work. Their work led to the realisation of external micro-environments and the effect of mutations on the various steps in the two characteristics of stem cells. These insights were all important in the study of leukaemia(b) and extra cellular growth factors. Later during the 1960s Leo Sachs demonstrated the growth of animal stem cells in culture. By 1998 the researchers led by James Thomson at University of Wisconsin developed pluripotent(c) embryonic stem cells from human blastocysts(d) and showed somatic differentiation(e) in vitro. Thinking about Plant Stem Cells Plants also have stem cells. Experiments in the late 1950s (Steward, Reinert) described the concept of totipotency in plants as the ability to develop an adult organism from a somatic or non-reproductive cell. However, the sequences of molecular events that switch a somatic to embryonic cell are still poorly understood. Plant stem cells naturally occur in their various meristems (region of actively dividing cells giving rise to new tissues) making them ideal for the study of differentiation. Using tissue explants to obtain somatic embryos, it was observed that the first response was a rapid replacement of the vacuole(f) with cytoplasm, followed by the first cell division. When explant(g) tissues from plants were used to generate in vitro plantlets, it appears that there is quite some variability in the first tissue that responds to hormone treatment. Using Daucus carota (carrot), researchers showed that following an increase in cytoplasmic content, cell division occurred in the pro-vascular cells and not in the cortical or epidermal cells. They recognised that the addition of a synthetic plant hormone auxin stimulated a small subpopulation of cells that they called competent cells, which were able to respond to hormone stimulus. Furthermore, they described a Somatic Embryogenesis Receptor-like Kinase(h) (SERK) as an early genetic marker of the somatic plant cells competent to form embryos. Many legumes are very difficult to regenerate, however, Ray Rose in the University of Newcastle succeeded in obtaining a highly regenerative mutant from the pasture legume Medicago truncatula CV Jemalong (Barrel Medic). At the ANU we have used the model legume Medicago truncatula (Barrel Medic) to examine the processes of de-differentiation, re-differentiation and cell amplification in plants. We have two systems in Medicago truncatula for studying the regeneration of plants from single cells in culture via somatic embryogenesis. Both of these systems provide a type of "stem cell" system for embryogenesis research. Each of these experimental approaches can be used to study the above processes plus the concepts of "totipotency" and "pluripotency". One form of totipotency occurs during the re-programming of leaf cells (mesophyll cells) into protoplasts and their subsequent culture to give rise to somatic embryos from individual cells. In this process of somatic embryogenesis, a somatic cell is switched into a zygotic pathway without the process of fertilisation. These processes of de-differentiation and differentiation are basic processes of all biology and are important during the life of any organism. When plant cells are grown in tissue culture they undergo sequential de-differentiation and re-differentiation before initiating cell division. By learning which regulatory networks of transcription factors and their target genes are involved in the shift of a cell to de-differentiate and become committed, we can influence and control particular pathways of cellular differentiation and plant architecture. In fact, this is the same approach being used in the studies of mammalian stem cells in culture. Proteomic analysis, which seeks to describe the protein content of a cell or tissue, is a valuable approach to the study of the protein changes in freshly isolated protoplasts and has revealed that multiple processes are occurring. This includes the up-regulation of key defence and related proteins and a general down-regulation of the other proteins that were required for previous functions in the leaf cell.By using a combination of plant hormones, cell histology, 2-dimensional electrophoresis, mass spectrometry, microarray analysis and bioinformatics we have focused on the very early events of the re-programming of the leaf cell protoplasts and of leaf explant cells to give rise to somatic embryos or root formation in culture. Currently, we have shown the expression of several early transcription factors, which are key to the commitment to the differentiation of roots. There are now many new molecular techniques, which will enable the dissection of these early events in the stages of commitment and differentiation of the plant. We can expect that over the next decade that there will be many basic advances in stem cell biology in both plant and animal systems. Furthermore, these advances will benefit the lives of humans, animals, plants, agriculture and the environment. - - - - - - - - - - - - (a)   Differentiated: Cells that have modified their structure and function. (b)   Leukaemia: A disease in which the number of white blood cells in the blood is        persistently and greatly increased; leading to anaemia and enlargement of        the spleen. (c)   Pluripotent: Cells capable of developing into several different cell types. (d)   Blastocyst: The hollow sphere of fertilised mammalian cells at the stage of        implantation in the uterus. The stage of embryonic development after the        morula, a solid group of cells the initial multiplication of a fertilised ovum. (e)   Somatic differentiation: The development from germ cells into body tissue cells. (f)   Vacuole: A large membrane bounded cavity within a cell. (g)   Explant: Referring to plant cells removed from the plant. (h)   Kinase: An enzyme [that transfers a phosphoryl group from ATP to a receptor] . *   *   *   *   *