Stem Cells in Plants and Animals

Stem cells are often in the news because of potential medical use and because of the ethical debate around human embryonic stem cells. From the point of view of biologists, however, stem cells have even wider significance as central players in the development of complex organisms, which include plants. There are surprising similarities between stem cells found in plants and in animals. How were they discovered, and what are their general implications for stem cell biology?

Stem cells function as a source of new cells to grow or replace specialised tissues. To perform this function, these cells must divide to renew themselves, while some of their descendants eventually differentiate to build up new tissues. By this definition, stem cells are particularly important for plant growth, because virtually all tissues of the plant descend from small groups of stem cells located in their growing apices, within structures called the apical meristems (Sablowski 2004; Scheres 2007) (Figures 1 and 2).

How do we know that just a few stem cells are the ultimate source of all new plant cells? By creating genetic differences between cells of the same plant, scientists are able to track how the descendants of a single marked cell make up organs and tissues as the plant grows. This method is called clonal analysis because it shows how clones (genetically identical groups of cells derived from the same progenitor) build up the plant. This type of experiment showed that most marked cells divide just a few times and make a small contribution to growth of the plant, while a few cells within the meristems divide many times and their descendants make up all new tissues and organs.

Clear evidence that the whole shoot descends from a small number of constantly dividing cells within the apical meristem came from clonal analyses done in the 1970s by Stewart and Dermen (Stewart & Dermen 1970). They looked at variegated plants, plants whose leaves and stems are not uniformly green, but have patches of white tissue (a feature often selected in ornamental plants). These white tissues descend from mutant cells that are unable to produce chloroplasts. Most of the time, the plants produce small colorless patches, but Stewart and Dermen also found plants with colorless sectors composing nearly a third to half of the whole shoot, and which were continuously produced by the meristem over long periods of time (Figure 3). These large, stable sectors of mutant cells could only be formed if all cells that make up the shoot descended from a small population of relatively stable, long-term progenitors — the shoot stem cells.

Comparable experiments showed that root tissues also descend from a small set of stem cells. For example, Dolan, Scheres and Kidner used a reporter gene (an artificially introduced gene that encodes an easily detectable product) to mark cell lineages in the roots of the model plant Arabidopsis (Dolan et al. 1994; Scheres et al. 1994, Kidner, C. et al. 2000). First the reporter gene was blocked by a transposon (a piece of DNA that can move around in the genome). Then the cells became genetically marked when the transposon moved, unblocking expression of the reporter in only a few cells of the root meristem. This allowed these cells and their descendants to express the reporter gene. Once again, a few large and stable sectors allowed the scientists to trace the progenitors of root tissues to just a few stem cells in the center of the root meristem.

In summary, by genetically marking stem cells, it was possible to show that small groups of stem cells function as a long-term source of new cells to build up the shoot and the root. However, the experiments described above also revealed that the large sectors originating from the stem cells were not maintained throughout the life of the plant. Eventually, although the meristems continued to function, they stopped producing the genetically marked cells. The marked cells could not revert to what they were before, so progenitor cells within the meristem that gave rise to marked sectors must have died or simply stopped functioning as stem cells. This observation is explained by another key feature of stem cells that is shared between plants and animals, as explained below.

The ability of stem cells to constantly generate new tissues has to be co-ordinated with the needs of the whole organism. If stem cells do not divide frequently enough, tissues cannot grow or be replaced (as happens during normal ageing); if stem cells proliferate out of control, they disrupt normal development (resulting for example in cancer) (Orford & Scadden 2008; He, Nakada, & Morrison 2009). So it is not surprising that the behavior of stem cells is strictly dependent on signals provided by other cells. Stem cells are normally maintained only in specific locations where these extracellular signals are available. As the cells proliferate, only the descendants that remain within reach of the maintenance signals continue to behave as stem cells, whereas those displaced away from the signals begin to differentiate. The locations where extracellular signals allow stem cells to exist are called stem cell niches (Spradling, Drummond-Barbosa, & Kai 2001). The maintenance of stem cells only within the niche explains the observation above that a genetically marked stem cell eventually stopped functioning as a stem cell. This can occur by chance if the marked cell is pushed away from the niche by the divisions of neighboring stem cells.

How do we know that signals from other cells are needed to maintain the plant stem cells? In the shoot, this was revealed by work on mutant Arabidopsis plants that are unable to maintain the apical meristem. Laux and colleagues (Laux et al. 1996) isolated Arabidopsis plants with mutations in the WUSCHEL (WUS) gene. In these plants, the shoot apical meristem starts to form, but then the meristem cells differentiate and the stem cells are lost. WUS turned out to encode a transcription factor (a protein that controls the activity of other genes) that is expressed in only a small number of cells just beneath the stem cells in the shoot meristem (Mayer et al. 1998) (Figure 1). Because the wus mutation affects the adjacent stem cells, where the gene is not expressed WUS must be required to produce a signal that functions between cells to maintain the shoot stem cells.

In the root, evidence that stem cells are maintained by intercellular signals came from experiments in which the cells making the signal were selectively killed. Van den Berg and colleagues (van den Berg, C. et al. 1997) used a laser beam to kill single cells within the root meristem. When they killed specific cells that flank the stem cells (called the quiescent center cells, or QC for short; Figure 2) the adjacent stem cells differentiated. This was not simply a result of the injury caused by the laser, because killing neighboring cells other than the QC did not induce differentiation. Therefore, the QC must be the source of a signal that prevents differentiation of the root stem cells.

Thus plant stem cells depend on the function of other, neighboring, cells to continue behaving as stem cells, and therefore some sort of signal must be passed between cells to maintain stem cell behavior. In animals, the signaling molecules used within stem cell niches are often the same used to organize growth and tissue patterning during embryogenesis, such as homologues of the Notch, Wingless and Hedgehog proteins from Drosophila (Orford & Scadden 2008; He, Nakada, & Morrison 2009). Plant genomes do not contain genes that encode proteins similar to these, so plants must use different signals, but a major gap in our knowledge is that we still do not know exactly what signaling molecules maintain plant stem cells.

We have seen above that there are intriguing similarities in the way stem cells function in both plants and animals to sustain growth and replace tissues. To perform their functions, stem cells are different from other cells in two ways. First, they are able to produce copies of themselves over long periods of time, at least until they become damaged or are separated from their niche. In contrast, other cells are programmed to divide at most during a relatively short period before all their progeny differentiate and stop dividing. The second difference is that stem cells are generally pluripotent, that is, they have the potential to produce different types of differentiated cells (for example, the marked stem cells that produced the white sectors in Figure 3 gave rise to all the different cell types that make up a leaf). Do similar mechanisms operate in plant and animal stem cells to maintain long-term division and pluripotency?

One gene that is conserved between plants and animals and has a central role in deciding whether a cell continues to divide or differentiates encodes the Retinoblastoma (Rb) protein. When activated, Rb represses genes required to replicate DNA, in addition to other less-well-characterised functions that lead to cell differentiation (Burkhart & Sage 2008). In Arabidopsis, if the gene encoding Rb is inactivated in the root meristem, the descendants of root stem cells cannot differentiate; conversely, if Rb is artificially activated in the stem cells, they stop dividing and differentiate (Wildwater, M. et al.). Similarly, Rb appears to promote exit from stem cell state in animals (He, Nakada, & Morrison 2009).

The property of pluripotency is believed to depend at least in part on the way the chromatin is organized, that is, how the DNA is packaged in the nucleus and how this affects the access of regulatory proteins to genes required for cell differentiation. Polycomb proteins play an important role in regulating the chromatin to repress differentiation genes and therefore maintain the pluripotency of animal stem cells (He, Nakada & Morrison 2009). In plants, Polycomb proteins also regulate the transition between pluripotent and differentiated states, but unlike in animals, they are required in the differentiating cells to repress genes that are normally expressed in the meristem. This is shown by Arabidopsis plants with mutated polycomb genes: in these plants, shoot meristem genes continue to be expressed in cells that are due to form leaves and consequently leaf development is abnormal (Katz et al. 2004; Schubert et al. 2006).

In conclusion, at least some of the genes that control the stem cell state in animals are also relevant for plant stem cells, but there may be variations in the way these genes are deployed. The Rb protein seems to function similarly in plants and animals to stop cell division and start differentiation in cells that leave the stem cell niche. Polycomb proteins are used to maintain a repressive chromatin state in both kingdoms but appear to function differently in the stem cells: they repress differentiation genes in animal stem cells, whereas in plants they are used to inhibit meristem genes in differentiated cells. It must be said, however, that we are far from understanding the molecular basis of pluripotency in any organism, so we cannot yet be sure whether pluripotency is controlled differently in plants and animals.

As described above, plant and animal stem cells have some surprising similarities in their developmental roles, in the ways they are organized within tissues, and to some extent in the molecular mechanisms controlling their behavior. This is surprising not simply because plants are so different from animals, but because plants and animals very likely evolved from unicellular to multicellular organisms separately (Meyerowitz 2002). Therefore, stem cells probably evolved independently in both kingdoms as an advantageous solution to the problem of balancing the need to grow with the need to produce specialized cells, which often cannot divide. In both plants and animals, stem cell niches likely evolved as devices to match the location and proliferation rate of stem cells to the needs of the whole organism. Molecular similarities, such as the role of Rb proteins, probably result from adopting mechanisms in the stem cells to control cell division and differentiation that already existed in unicellular organisms (Sablowski 2004; Scheres 2007). In conclusion, comparisons across large evolutionary distances, such as that between plants and animals, allow us to highlight the most fundamental principles of stem cell biology.

Stem cells function as the source of new cells to build tissues and organs and are central players in the development of complex organisms ranging from plants to humans. By genetically marking stem cells, it is possible to show that nearly all cells of a mature plant descend from small groups of stem cells located in their growing apices. Experiments with mutant plants and selective cell killing have shown that plant stem cells are maintained by signals from other, adjacent, cells. This feature is shared with animal stem cells and helps to adjust stem cell proliferation to the needs of the organism. The mechanisms that control whether a cell continues to function as a stem cell or starts to differentiate also show some similarities in plants and animals, such as the role of the Retinoblastoma protein in promoting differentiation. The functional similarities of stem cells in plants and animals probably have evolved independently as solutions to the problem of balancing the need to grow with the need to produce specialized cells, which often cannot divide.