Plant Stem Cells

STEM CELLS, WHICH have an ability of self—renewal and potency to differentiate multiple types of cells, exist in plants as well as in animals, and play essential roles for the growth and development of both plants and animals. The most characterized stem cells in plants reside in growing tips of shoots (above—ground part of plants) and roots. Stem cells in shoot tips differentiate into the tissues that compose leaves and stems. In some circumstances these cells change their identity to produce flowers. Stem cells in root tips differentiate into various root structures. In woody plants, stem cells named cambial cells produce additional tissue that forms a thick trunk for support of shoot tissues (i.e. secondary growth).

There are several unique features of stem cells in plants compared with those in animals. First, plant stem cells in shoot and root tips maintain their pluripotency throughout the plant lifecycle. They thus continuously produce organs during the plant lifetime. Sometimes they keep their pluripotency for thousands of years in long—lived species such as Sequoia sempervirens. In contrast, animal stem cells lose their pluripotency during embryogenesis. Second, stem cells in plants produce most of the organs post—embryonically (i.e. after germination), while those in animals produce most organs embry—onically. Third, plant cells are enclosed in thick cell walls and they are tighdy connected to each other. They cannot move like animal cells. Therefore, plant stem cells develop most plant tissues and organs by “stacking” cells. Finally, plant stem cells are often reprogrammed from differentiated cells, to produce lateral buds or roots, or to compensate the loss of original stem cells. Thus, they contribute to lateral growth in addition to vertical growth of plants.

In spite of these developmental differences and the evolutionary distance, stem cells in plants and animals are maintained by a similar system called a stem cell niche. A stem cell niche is a cellular microenvironment providing intercellular signals that maintain stem cells. It consists of stem cells and organizing cells. In principle, the identity of stem cells is maintained by physical interaction with organizing cells. After cell division of a stem cell, one daughter cell maintains physical interaction with organizing cells and continues to be a stem cell. The other daughter cell, which lacks these physical interactions with organizing cells, loses its identity as a stem cell and starts to differentiate. This is called asymmetric division. The structures of stem cell niches in plants are described below.