Meristem


In cell biology, the meristem is a structure composed of specialized tissue found in plants, consisting of stem cells, known as meristematic cells, which are undifferentiated cells capable of continuous cellular division. These meristematic cells play a fundamental role in plant growth, regeneration, and acclimatization, as they serve as the source of all differentiated plant tissues and organs. They contribute to the formation of structures such as fruits, leaves, and seeds, as well as supportive tissues like stems and roots.
Meristematic cells are totipotent, meaning they have the ability to differentiate into any plant cell type. As they divide, they generate new cells, some of which remain meristematic cells while others differentiate into specialized cells that typically lose the ability to divide or produce new cell types. Due to their active division and undifferentiated nature, meristematic cells form the foundation for the formation of new plant organs and the continuous expansion of the plant body throughout the plant's life cycle.
Meristematic cells are small cells, with thin primary cell walls, and small or no vacuoles. Their protoplasm is dense, filling the entire cell, and they lack intercellular spaces. Instead of mature plastids such as chloroplasts or chromoplasts, they contain proplastids, which later develop into fully functional plastids.
Meristematic tissues are classified into three main types based on their location and function: apical meristems, found at the tips of roots and shoots; intercalary or basal meristems, located in the middle regions of stems or leaves, enabling regrowth; and lateral meristems or cambium, responsible for secondary growth in woody plants. At the summit of the meristem, a small group of slowly dividing cells, known as the central zone, acts as a reservoir of stem cells, essential for maintaining meristem activity. The growth and proliferation rates of cells vary within the meristem, with higher activity at the periphery compared to the central region.
The term meristem was first used in 1858 by Swiss botanist Carl Wilhelm von Nägeli in his book Beiträge zur Wissenschaftlichen Botanik. It is derived, in recognition of its inherent function.

Primary meristems

Apical meristems, also known as the primary meristem, give rise to the primary plant body and are responsible for primary growth, or an increase in length or height. Apical meristems may differentiate into three kinds of primary meristem:
After the primary growth, lateral meristems develop as secondary plant growth. This growth adds to the plant in diameter from the established stem but not all plants exhibit secondary growth. There are two types of secondary meristems: the vascular cambium and the cork cambium.
  • Vascular cambium, which produces secondary xylem and secondary phloem. This is a process that may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called arboraceous. This does not occur in plants that do not go through secondary growth, known as herbaceous plants.
  • Cork cambium, which gives rise to the periderm, which replaces the epidermis with bark and cork for example.

    Apical meristems

Apical meristems are the completely undifferentiated meristems of a plant. They give rise to primary growth, enabling the elongation of shoots and roots. Apical meristems give rise to three types of primary meristems, which later develop into secondary or lateral meristems, contributing to the plant's lateral expansion.
There are two main types of apical meristems: shoot apical meristem and root apical meristem. The SAM is located at the tips of shoots and produces leaves, stems, and flowers, while the RAM is found at the tips of roots and generates new root tissues. Both types consist of rapidly-dividing cells that remain indeterminate, meaning they continuously produce new cells without a predefined final state, similar to stem cells in animals, which have an analogous behavior and function.
Structurally, apical meristems are organized into distinct zones. The central zone serves as a reservoir of undifferentiated cells, while the peripheral zone generates new organs and tissues. The medullary meristem contributes to vascular development, forming the medullary tissue, which makes up the plant's central structure. The meristem layers also vary depending on the plant type. The outermost layer, called the tunica, determines the leaf edge and margin in monocots, whereas in dicots, the second layer of the corpus influences leaf characteristics.
Apical meristems are generally found at the tips of roots and stems, but in some arctic plants, they are located in the lower or middle parts of the plant. This adaptation is believed to provide advantages in extreme environmental conditions.

Shoot Apical Meristems

Shoot apical meristems are the source of all above-ground organs, such as leaves and flowers. Cells at the shoot apical meristem summit serve as stem cells to the surrounding peripheral region, where they proliferate rapidly and are incorporated into differentiating leaf or flower primordia.
The shoot apical meristem is the site of most of the embryogenesis in flowering plants. Primordia of leaves, sepals, petals, stamens, and ovaries are initiated here at the rate of one every time interval, called a plastochron. It is where the first indications that flower development has been evoked are manifested. One of these indications might be the loss of apical dominance and the release of otherwise dormant cells to develop as auxiliary shoot meristems, in some species in axils of primordia as close as two or three away from the apical dome.
The shoot apical meristem consists of four distinct cell groups:
These four distinct zones are maintained by a complex signalling pathway. In Arabidopsis thaliana, 3 interacting CLAVATA genes are required to regulate the size of the stem cell reservoir in the shoot apical meristem by controlling the rate of cell division. CLV1 and CLV2 are predicted to form a receptor complex to which CLV3 is a ligand. CLV3 shares some homology with the ESR proteins of maize, with a short 14 amino acid region being conserved between the proteins. Proteins that contain these conserved regions have been grouped into the CLE family of proteins.
CLV1 has been shown to interact with several cytoplasmic proteins that are most likely involved in downstream signalling. For example, the CLV complex has been found to be associated with Rho/Rac small GTPase-related proteins. These proteins may act as an intermediate between the CLV complex and a mitogen-activated protein kinase, which is often involved in signalling cascades. KAPP is a kinase-associated protein phosphatase that has been shown to interact with CLV1. KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it.
Another important gene in plant meristem maintenance is WUSCHEL, which is a target of CLV signaling in addition to positively regulating CLV, thus forming a feedback loop. WUS is expressed in the cells below the stem cells of the meristem and its presence prevents the differentiation of the stem cells. CLV1 acts to promote cellular differentiation by repressing WUS activity outside of the central zone containing the stem cells.
The function of WUS in the shoot apical meristem is linked to the phytohormone cytokinin. Cytokinin activates histidine kinases which then phosphorylate histidine phosphotransfer proteins. Subsequently, the phosphate groups are transferred onto two types of Arabidopsis response regulators : Type-B ARRS and Type-A ARRs. Type-B ARRs work as transcription factors to activate genes downstream of cytokinin, including A-ARRs. A-ARRs are similar to B-ARRs in structure; however, A-ARRs do not contain the DNA binding domains that B-ARRs have, and which are required to function as transcription factors. Therefore, A-ARRs do not contribute to the activation of transcription, and by competing for phosphates from phosphotransfer proteins, inhibit B-ARRs function. In the SAM, B-ARRs induce the expression of WUS which induces stem cell identity. WUS then suppresses A-ARRs. As a result, B-ARRs are no longer inhibited, causing sustained cytokinin signaling in the center of the shoot apical meristem. Altogether with CLAVATA signaling, this system works as a negative feedback loop. Cytokinin signaling is positively reinforced by WUS to prevent the inhibition of cytokinin signaling, while WUS promotes its own inhibitor in the form of CLV3, which ultimately keeps WUS and cytokinin signaling in check.

Root apical meristem

Unlike the shoot apical meristem, the root apical meristem produces cells in two dimensions. It harbors two pools of stem cells around an organizing center called the quiescent center cells and together produces most of the cells in an adult root. At its apex, the root meristem is covered by the root cap, which protects and guides its growth trajectory. Cells are continuously sloughed off the outer surface of the root cap. The QC cells are characterized by their low mitotic activity. Evidence suggests that the QC maintains the surrounding stem cells by preventing their differentiation, via signal that are yet to be discovered. This allows a constant supply of new cells in the meristem required for continuous root growth. Recent findings indicate that QC can also act as a reservoir of stem cells to replenish whatever is lost or damaged. Root apical meristem and tissue patterns become established in the embryo in the case of the primary root, and in the new lateral root primordium in the case of secondary roots.