MESISTEM

A meristem is the tissue in most plants consisting of undifferentiated cells (meristematic cells ), found in zones of the plant where growth can take place. The meristematic cells give rise to various organs of the plant, and keep the plant growing. The Shoot Apical Meristem (SAM) gives rise to organs like the leaves and flowers. The cells of the apical meristems - SAM and RAM (Root Apical Meristem) - divide rapidly and are considered to be indeterminate, in that they do not possess any defined end fate. In that sense, the meristematic cells are frequently compared to the stem cells in animals, that have an analogous behavior and function. The term meristem was first used in 1858 by Karl Wilhelm von Nägeli (1817–1891) in his book Beiträge zur Wissenschaftlichen Botanik. [1] It is derived from the Greek word merizein (μερίζειν), meaning to divide, in recognition of its inherent function. In general, differentiated plant cells cannot divide or produce cells of a different type. Therefore, cell division in the meristem is required to provide new cells for expansion and differentiation of tissues and initiation of new organs, providing the basic structure of the plant body. Meristematic cells are incompletely or not at all differentiated , and are capable of continued cellular division (youthful). Furthermore, the cells are small and protoplasm fills the cell completely. The vacuoles are extremely small. The cytoplasm does not contain differentiated plastids (chloroplasts or chromoplasts), although they are present in rudimentary form (proplastids). Meristematic cells are packed closely together without intercellular cavities. The cell wall is a very thin primary cell wall. Maintenance of the cells requires a balance between two antagonistic processes: organ initiation and stem cell population renewal. Meristematic zones Apical meristems are the completely undifferentiated (indeterminate) meristems in a plant. These differentiate into three kinds of primary meristems. The primary meristems in turn produce the two secondary meristem types. These secondary meristems are also known as lateral meristems because they are involved in lateral growth. At the meristem summit, there is a small group of slowly dividing cells, which is commonly called the central zone. Cells of this zone have a stem cell function and are essential for meristem maintenance. The proliferation and growth rates at the meristem summit usually differ considerably from those at the periphery. Meristems also are induced in the roots of legumes such as soybean , Lotus japonicus, pea, and Medicago truncatula after infection with soil bacteria commonly called Rhizobium. Cells of the inner or outer cortex in the so-called "window of nodulation" just behind the developing root tip are induced to divide. The critical signal substance is the lipo-oligosaccharide Nod- factor, decorated with side groups to allow specificity of interaction. The Nod factor receptor proteins NFR1 and NFR5 were cloned from several legumes including Lotus japonicus, Medicago truncatula and soybean (Glycine max). Regulation of nodule meristems utilizes long distance regulation commonly called "Autoregulation of Nodulation" (AON). This process involves a leaf- vascular tissue located LRR receptor kinases (LjHAR1, GmNARK and MtSUNN), CLE peptide signalling , and KAPP interaction, similar to that seen in the CLV1,2,3 system. LjKLAVIER also exhibits a nodule regulation phenotype though it is not yet known how this relates to the other AON receptor kinases Apical meristems Organisation of an apical meristem (growing tip) 1 - Central zone 2 - Peripheral zone 3 - Medullary (i.e. central) meristem 4 - Medullary tissue The apical meristem , or growing tip, is a completely undifferentiated meristematic tissue found in the buds and growing tips of roots in plants. Its main function is to begin growth of new cells in young seedlings at the tips of roots and shoots (forming buds, among other things). Specifically, an active apical meristem lays down a growing root or shoot behind itself, pushing itself forward. Apical meristems are very small, compared to the cylinder-shaped lateral meristems (see 'Secondary Meristems' below). Apical meristems are composed of several layers. The number of layers varies according to plant type. In general the outermost layer is called the tunica while the innermost layers are the corpus . In monocots, the tunica determine the physical characteristics of the leaf edge and margin. In dicots, layer two of the corpus determine the characteristics of the edge of the leaf. The corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems. Apical meristems are found in two locations: the root and the stem. Some Arctic plants have an apical meristem in the lower/middle parts of the plant. It is thought that this kind of meristem evolved because it is advantageous in Arctic conditions[citation needed]. Shoot apical meristems The source of all above- ground organs. 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 axillary 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 4 distinct cell groups: -. Stem cells The immediate daughter cells of the stem cells A subjacent organising centre Founder cells for organ initiation in surrounding regions The four distinct zones mentioned above 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 .[2] CLV1 and CLV2 are predicted to form a receptor complex (of the LRR receptor like kinase family) to which CLV3 is a ligand.[3][4][5] CLV3 shares some homology with the ESR proteins of maize, with a short 14 amino acid region being conserved between the proteins. [6][7] Proteins that contain these conserved regions have been grouped into the CLE family of proteins.[6][7] 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.[2] These proteins may act as an intermediate between the CLV complex and a mitogen- activated protein kinase (MAPK), which is often involved in signalling cascades. [8] KAPP is a kinase-associated protein phosphatase that has been shown to interact with CLV1.[9] KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it. [9] Another important gene in plant meristem maintenance is WUSCHEL (shortened to WUS), which is a target of CLV signalling.[10]WUS is expressed in the cells below the stem cells of the meristem and its presence prevents the differentiation of the stem cells.[10] CLV1 acts to promote cellular differentiation by repressing WUS activity outside of the central zone containing the stem cells. [10]STM also acts to prevent the differentiation of stem cells by repressing the expression of Myb genes that are involved in cellular differentiation.