In the plant embryo, cells can divide and later differentiate into functional mature cells. However, as the plant grows, groups of undifferentiated cells remain in some regions of the plant body. In addition, groups of differentiated cells may become undifferentiated. Meristems are populations of undifferentiated cells present in the plant body with the ability to proliferate and differentiate. Meristems are responsible for the permanent growth of the plant since they are present during the plant's lifespan. Not all proliferating cells in the meristem end up as differentiated cells. Some cells, known as initial cells, remain undifferentiated after mitosis, leading to a permanent pool of undifferentiated cells that maintain the meristematic features as long as the plant is alive. The environment of the meristem has signaling molecules for both proliferating and retaining the undifferentiated features.
Meristems may be permanent and functional during the plant's lifespan, as it happens to the shoot and root apical meristems. Others are transient and active for a short period of time, such as those giving rise to the leaves, flowers, and petioles. Some meristems may appear quite late after germination, such as the pheloderm and vascular cambium.
Meristematic cells show the cytological features of undifferentiated cells: they are small, isodiametric, and show a very thin primary cell wall. The cytoplasm is rich in ribosomes, proplastids, and many small vacuoles but is poor in endoplasmic reticulum and inclusions. A well-developed Golgi complex synthesizes the components of the cell wall. A large nucleus contains condensed chromatin and is found centrally in the cell. Meristematic cells divide by mitosis and may be totipotent, i.e., are able to differentiate into the entire variety of cell types of an adult plant, or pluripotent when giving rise to some cell types. Plants grow by producing new cells and by growing these new cells, which is accomplished by the accumulation of water in their vacuoles. Normally, meristematic cells are densely packed with no empty intercellular space.
Almost all meristems contain a region called the niche, which contains the initial cells (stem cells), surrounded by progenitors cells. Progenitor cells divide many times and differentiate into functional cells. The final cell type in which a meristematic cell will differentiate depends on its position in the meristem and the molecular signals present in that place. Although the differentiation of meristematic cells is influenced by the cell lineage they belong to, the place of the meristem is a stronger signal and finally determines their differentiation pathway. The initial cells give rise to progenitor cells, which first form a proliferation zone through a high division rate. Then cells stop dividing and increase their size, generating an elongation zone. Finally, cells differentiate in a particular cell type of the organ.
The classification of meristems is based on their topographic position in the plant body and on the time they become active during plant development (Figure 1).
2. Primary meristems
Primary meristems are first observed in the embryo, and their activity results in primary growth, which is mainly involved in increasing the length of the plant. During this activity, cells undergo anticlinal cell divisions, i.e., the cell division plane is perpendicular to the surface of the plant body (see figure). Primary meristems are able to form new organs. Primary meristems are found in the tips of shoots (Figure 2) and roots, and hence they are called shoot (or caulinary) apical meristem and root apical meristem, respectively. All cells of the stem derive from the shoot apical meristem during primary growth, including leaves, flowers, and branches. The axillary buds, from which stem branches arise, are also derived from the shoot apical meristem. However, the lateral roots are generated from the endodermis instead of the root apical meristem. The shoot apical meristem is covered and protected by the youngest leaf primordia, whereas the root apical meristem is wrapped by the calyptra.
The number of cells constituting an apical meristem is relatively constant during plant life since there is a balance between the cells that leave the meristem and form tissues and the new cells that are generated in the meristem. In the shoot apical meristem, there is a central zone and a central organizer, which form the niche that sustains the initial cells, whereas in the root apical meristem, the niche is the quiescent center and initial cells are around it. Three lineages of cells are formed in the apical meristems. The outermost layer is the protoderm, which differentiates into the epidermis; then comes the procambium, which gives rise to the primary vascular tissues (primary xylem and primary phloem); and finally the ground meristem, the precursor of ground tissues (parenchyma, collenchyma, and sclerenchyma).
Plant cells do not move, and this makes it possible to predict what will be the fate of a cell by knowing the spatial location within the meristem. In the apical meristems, at the same time the central zone produces new cells, the meristem moves away, and these new cells fall under the influence of molecules that determine their differentiation process. This separation between the meristem and new cells is actually a consequence of the continuous mitotic activity of the meristematic cells, which push the meristem away.
The axillary buds are not associated with the leaves, although it appears otherwise. In fact, axillary buds develop in the axil of the second or third leaf from the apex. It means that they appear later than the leaves. Axillary buds arise as small cellular populations that detach from the apical meristem. However, their final differentiation is induced by the leaf, since the elimination of the leaf inhibits axillary bud formation.
3. Secondary meristems
Secondary, or lateral, meristems are present In those plants that grow in thickness, i.e., have secondary growth (Figure 1). These meristems arise later during development, and are responsible for producing wood and bark. They increase the diameter of shoots and roots, and their cells divide by periclinal division (see figure). Secondary meristems are typical of gymnosperms and dicots. They are not found in most pteridophytes and monocots, nor in specific organs such as leaves. Plants with secondary growth also perform primary growth. There are two types of lateral meristems: the vascular cambium, which differentiates from the procambium and produces secondary vascular tissues (secondary xylem and secondary phloem), and the cork cambium (phellogen), which arises from various parenchymatous tissues in the cortex and produces phelloderm inward and cork outward. Both meristems are arranged as a continuous cylinder, sometimes as an incomplete ring, along shoots and roots. The vascular cambium is found between the secondary xylem and the secondary phloem, whereas the cork cambium is located between the phelloderm and cork. The vascular cambium is formed from the procambium and from new meristematic cells differentiated from the interfascicular parenchyma. In the roots, the vascular cambium arises from the procambium and the pericycle.
4. Other meristems
In monocots, most of the growth in length of shoots depends less on the apical meristems and more on the intercalary meristems, which are meristematic tissue derived from the apical meristem. The intercalary meristem continues the proliferative activity at some distance from the place where it originated, i.e., it is inserted (intercalated) among tissues that are no longer meristematic. The best-known examples of intercalary meristems are those located in the stem internodes (mainly at the base of the internodal region) and in leaves, particularly in grasses. Close to the shoot apical meristem, the nodes are very close to one another. During development, the nodes are separated from each other by cell proliferation and elongation. In dicots, the separation of the nodes is mostly done by cell elongation.
There are two additional meristems involved in the development of the vascular system: provascular and preprocambium. The provascular tissue is an embryonic tissue located in the prospective vascular cylinder. Although this provascular meristem does not directly give xylem or phloem, it produces precursors of the procambium, which give rise to xylem and phloem. Preprocambium cells are found in leaves and are the precursors of procambium. They cannot be distinguished from the ground tissue cells. In the leaves, the procambial cells are recruited from a subepidermal population of the leaf primordia. These cells also differentiate into parenchyma cells. The meristematic cells of the leaf are formed under the influence of a high concentration of auxin. They arrange into cords that become the leave veins, which grow by the addition of new cells instead of division and elongation. The auxin canalization mechanisms lead to this behavior because auxin is conducted toward neighboring cells, which dedifferentiate into procambial cells.
Meristems may arise by dedifferentiation of parenchyma, colenchyma, and pericycle (roots). For instance, it was already mentioned that a part of the vascular cambium is formed from the interfascicular parenchyma in the stem. There are other examples of dedifferentiation, such as epidermal cells coming from parenchyma, or phloem, or the adventitious roots that emerge after dedifferentiation of stem functional tissues.
In monocots, the secondary growth was lost during evolution. It could have been the result of all cells of the procambium getting differentiated into vascular tissues, and no vascular meristem is left. In these plants, the growth in thickness is due to an increase in cell size. However, palms and other tree-like monocots (some asparagales) show very thick stems by enlarging the parenchyma tissue and increasing the number of vascular bundles in regions quite far from the stem tip. This type of growth is known as anomalous growth, and it is produced by the activity of the peripheral growing meristem. In palms, this meristem, referred to as the secondary growing meristem, is found more superficially than the vascular bundles. It gives parenchyma outward and vascular bundles and parenchyma cells inward. In some monocots, this peripheral meristem may extend to the roots as a cylinder, resembling a vascular cambium. The activity of this meristem produces tissue similar to the secondary tissues when that part of the stem stops growing in length. Curiously, it produces most of the tissue toward the inner region of the stem.
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Bibliography ↷
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Augstein F, Carlsbecker A. 2018. Getting to the Roots: A Developmental Genetic View of Root Anatomy and Function From Arabidopsis to Lycophytes. Frontiers in plant sciences. 9: 1410.
Miyashima S, Sebastian J, Lee J-Y, Helariutta Y. 2013.The EMBO journal. 32: 178-193.
Truskina J, Vernoux T. 2018. The growth of a stable stationary structure: coordinating cell behavior and patterning at the shoot apical meristem. Current opinion in plant biology. 41: 83-88.
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Introduction 
