Introduction:
In recent years, the concept of stem cells has become widely acclaimed due to the potential use of these cells as a biomedical tool to rebuild and repair organisms. Stem cells are innately undifferentiated cells of multicellular organisms, with the potential to develop into many different cell types, particularly during the growth and regeneration of an organism.
Stem cells have certain characteristic features which allow them to be distinguished from other cell types. Stem cells undergo asymmetric cell division during which one daughter cell preserves the features of the undifferentiated mother cell, while the other obtains a particular cell fate, triggered by extrinsic and intrinsic signalling. In this way, stem cells are capable of creating any type of specialised cell, while preserving the number of undifferentiated cells (Miyashima et al., 2013). In certain areas of plants, such as the leaf apex, stem cells divide frequently in order to repair or replace damaged tissue. In contrast, in various other plant organs, such as in flowers in angiosperms, specific conditions are required for stem cells to divide (Sablowski, 2010). Therefore, these cells are pluripotent and facilitate plant growth and regeneration throughout a plants’ lifetime, as well as allowing for the production of new organs and tissues.
In plants, stem cells are found in the meristems, which are areas of plant tissue where growth can take place. Meristems can be categorised into three discrete kinds: the lateral meristem, which contributes to lateral growth, the intercalary meristem which controls plant growth in the mid-section and the apical meristem, which contributes to vertical growth, namedly plant height and root depth (Crawford et al., 2015). Plants are primarily multicellular, eukaryotic organisms, widely considered to be the foundation of all habitats. It is evident that human life on this planet couldn’t be sustainable without plants. From the food we eat, to the air we breathe, plants are undoubtedly crucial to our existence. In this decade, substantial research has been conducted on the topic of plant stem cells and their ability to make immense contributions to the fields of medicine and cosmetology, among others.
Origin and Function of Plant Stem Cell Structures
Unlike animals, plants are not required to scavenge or hunt for food, but instead harness energy from the sun through the process of photosynthesis. As a consequence, in contrast to animals, plants are immobile and are faced with competition, parasitic attacks, herbivory and radically changing environments without any means of escape. To survive, plants evolved with a post-embryotic form of development, which gives the organism the capacity to form and restore organs and tissues in response to stimuli (Greb and Lohmann, 2016). At the centre of this form of development are the active clusters of pluripotent stem cells, located in the specialised meristem tissues.
In addition, plants have evolved with an unparalleled plasticity in both growth and form. Plants contain a decentralised mechanical support system unlike the majority of animals which are supported by either endo- or exoskeletons (Sánchez Alvarado and Yamanaka, 2014). Accordingly, the mechanical integrity of plants heavily depends on cell walls that coat every cell in the organism. As a consequence, internal regulation of cell division, cell wall rigidity and cell form and size provide a decisive framework for plant morphology. In this way, cell migration and lineage-dependent cell fate are impeded by the physical boundaries set by the cell walls, resulting in cell fates that are continuously being synchronised to those of tethered neighbours, both by developmental and environmental signalling and sensing throughout the entire organism (Greb and Lohmann, 2016).
Many of the largest and oldest living organisms on earth are plants, demonstrating that the post-embryotic form of development and unrivalled plasticity employed by these organisms proves to be highly effective (De Rybel et al., 2016). Examples of such entities include Bristlecone Pines in California, which is over 4,840 years old, and the Giant Sequoia, also located in California, which weighs an immense 1,814 metric tons, while reaching over 85 meters in height (Shukla, 2015). These colossal scales are made possible by stem cells and the segmental organisation of plants that assists their survival if faced with extensive damage, by instigating the constant development of new tissues and structures, such as leaves and flowers (Hines, 2015).
Plant Meristem Morphology
Located at areas of growth in the plant body, meristems are the source of stem cells in plants. These areas of tissue are constantly producing cells whose fates are determined by their position and by environmental or developmental signals. As previously described, meristematic tissues can be classified into three categories: apical, which controls vertical growth, intercalary, which contributes to all growth in the mid-section of the plant and lateral, which leads to horizontal development (Crawford et al., 2015).
Apical Meristem Tissue: SAM and RAM
Apical meristem tissue can be subdivided into two distinct categories. The first, shoot apical meristem, often shortened to SAM, generates all organs that can be found above ground, such as flowers and leaves. The second, root apical meristem (RAM), produces cells required for root growth. The quantity of stem cells in both SAM and RAM are tightly preserved by pathways that regulate both stem cell division and stem cell differentiation (Miyashima et al., 2013).
The SAM has the ability to proliferate rapidly and is the location where most flower embryogenesis takes place. The first stages of development of many plant organs and tissues, including sepals, leaves, stamens, petals and ovaries originate here at a regular time interval, referred to as a plastochron, as long as the temperature is kept constant (Pierre-Jerome, Drapek and Benfey, 2018).
Four discrete zones are located in the SAM: the central zone, the peripheral zone, the medullary meristem and the medullary tissue and are preserved by an intricate signalling pathway, known as CLAVATA-WUSCHEL feedback signalling. This pathway regulates pluripotent stem cells and synchronises differentiation with cell proliferation and was first acknowledged in the model organism Arabidopsis thaliana (Somssich et al., 2016).
The CLAVATA gene stems from the last mutual ancestor of land plants. In A. thaliana, shown in Figure 1, three distinct CLAVATA genes interact to control the magnitude of the stem cell basin, by regulating both cell differentiation and cell division (Yadav et al., 2011). In this model organism, a receptor complex is comprised of CLAVATA 1 (CLV1) and CLAVATA 2 (CLV2), with CLAVATA 3 (CLV3) acting as a ligand. In A. thaliana, the CLV1, CLV2 and CORYNE (CRN) receptors sustain cell proliferation in the SAM by controlling expression of the WUSCHEL (WUS) transcription factor (Segonzac et al., 2012). WUS-CLV gene interactions carry out a vital role in the conservation of SAM stem cells and the development of adventitious somatic embryos and buds. Expression of WUS in the meristem inhibits the differentiation of stem cells. CLV1 has the ability to stimulate cellular differentiation by inhibiting WUS activity in the peripheral zone and the medullary meristem and tissue (Cao et al., 2015).