Organization of shoot apical meristem (SAM): These are the small living cells, which retain the capacity of cell division. Shoot apical meristem has embryonic characteristics. The shoot apical meristem is the functional unit of the shoot apex, it consists of apical meristem along with primordia. During differentiation of leaf primordial, shoot axillary meristem develops from shoot apical meristem.
SAM normally consists of 800-1200 cells group, it is small and dome shaped. The size and shape of the shoot apical meristem vary a lot during the stages of development. It also shows contrast among different species.
Shoot apical meristem can be of two types:
- Vegetative meristem It is normally in between the development of shoot apical meristem. When plant is convinced to flower, the vegetative meristem can be directly converted to floral meristem.
- Floral meristem They produce floral parts of the plant like sepals, petals, carpels and stamen.
- Shoot apical meristem is formed very early in development during embryogenesis.
- A gene called shoot meristemless (STM) participates in organizing the embryo, is turned on in a specific set of few cells and this is the first sign of apical meristem.
- Meristems are small and self-renewing.
- Population of proliferating cells that produce all adult organs of flowering plants is from SAM.
- Shoot develops from shoot apical meristem as a series of modules called phytomers, defined by periodic initiation of lateral organ primordial that give rise to leaves and axillary buds.
- The shoot apical meristem of both monocots and dicots exhibits two types of organization: Tunica corpus organization or layer concept
The shoot apical meristem consists of three layers, namely
L1 is the outermost layer. It forms the epidermis layer of the apical meristem. During cell division, it can divide only in anticlinal plane i.e. division perpendicular to the plane. The second layer is the L2 layer. It can divide in both planes that are anticlinal as well as periclinal plane. Periclinal cell division means division parallel to the surface.
The L2 layer mainly divides in anticlinal plane but during organ formation it divides in periclinal plane. The outer layers together form tunica. That is the epidermis i.e. L1 layer and sub epidermis i.e. L2 layer compromise tunica. Under tunica is corpus and is essentially the center of meristem. The cell division in tunica is sideways whereas the cell of corpus divides randomly. Therefore, tunica layer is colonically distinct.
The L3 layer is the third layer from the outer surface. They divide in both planes, periclinal as well as anticlinal. They divide and provide cells for the organs of interior surface and stem. Undifferentiated cells that retain the capacity of cell division indefinitely are called to be stem cells.
Layered organization of shoot apical meristem is superimposed upon its division into three zones
- Central zone consists of group of cells located at the distal end of the apical meristem. It contains central mother cells which are large, vacuolated, slowly-dividing stem cells. The cell division in central zone is inconstant. They also have very easily seen nuclei.
- Peripheral zone contains rapidly proliferating apical initial cells which are small and cytoplasmically dense. They arise from cells of central zone and mainly from the lateral organs.
- Morphogenetic zone or rib zone surrounds meristem where differentiation occurs. It gives rise to internal tissues of the stem.
At flasks of meristem, morphogenetic zone compromises lateral organ primordial. Beneath the apical dome it compromises rib meristem in which files of cells are laid down to extend the stem as shoot grows. The central zone is at the top of the meristem. In the L1 and L2 layers, there are many undifferentiated cells with large vacuoles and these cells divide relatively slowly. Hence the cell division in the central zone pushes the cells outwards and downwards into the peripheral zone which surrounds the central zone at all sides. Here cells divide rapidly and become designated as founder cells and start to become recruited into primordial. The rapid cell division in the peripheral zone is eventually displaced outwards and downwards into the region where organ differentiation takes place. In the L3 at the base of the SAM is the rib zone, it contributes cells to the stem, including the vasculature and pith of the stem.
Genes involved in shoot apical meristem development:
From the studies of Arabidopsis thaliana embryogenesis, genes that control specific aspects of shoot apical meristem are identified. There are two classes of genes involved
- Shoot meristemless (STM) and WUSCHEL (WUS) Its mutation results in the seeds with no or reduced shoot apical meristem. The resultant of WUS and STM genes act to keep meristem cells in an undifferentiated state. WUS gene encodes a homodomain transcription factor. It is important for the maintenance of the shoot apical meristem by making neighboring cells to acquire stem cell fate and remain indeterminate. It is expressed in organizing center; it is a small region immediately under stem cells. The STM gene encodes a transcription factor. To avoid differentiation of stem cells it acts independently of WUS gene. It is expressed in the apical dome of vegetative meristem.
- Clavata (CLV) The mutation in these genes results into enlarged shoot apical meristem. These genes include CLV1, CLV2 and CLV3. CLV1 encodes a receptor ser-thr kinase. In the absence of the ligand the receptor is inactive. When ligand binds to the receptor it converts the protein into active kinase. The ligand of CLV1 is a small protein. This protein is encoded by CLV3 gene.