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Comprehensive Study of Plant Shoot Apical Meristem and Its Molecular Regulation

Explores plant meristem types, shoot apical meristem structure, theories, molecular genetics, and its role in plant growth and organ formation.

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Plant Meristem & Shoot Apical Meristem (SAM) By- Manveer Kaur Chugga M. Sc. I semester
Meristem Meristems are regions of undifferentiated cells in plants capable of cell division, leading to the growth of new tissues and organs. In plants, these are the center of active mitotic cell division where plant growth occurs. Meristematic cells are often compared to stem cells in animals because their function and rapid division are similar. These tissues can be found in root tips, shoots, buds, and any place where new growth occurs.
Shoot Apical Meristem (SAM) The shoot apical meristem (SAM) generates above- ground aerial organs throughout the lifespan of higher plants. • Location- It is located at the terminal position of the shoot in contrast to sub terminal position of root apical meristem. It is dome shaped or somewhat flattened. • Function - The cells in this region have unlimited capacity of growth and divide continuously to form new tissues or organs. • Dynamic Structure: SAM can change depending on the plant’s growth cycle or environmental conditions, such as entering dormancy in winter or becoming more active in spring.
Theories related to SAM Apical Cell Theory- • Nageli proposed the theory in 1878. • It suggests the presence of single tetrahedral apical cell in the shoot apex and that cell is considered as the structural and functional unit of apical meristem. • The single apical cell divides to give rise to all the tissues of the shoot. • This theory is applicable only to the cryptogams not to the phanerogams because it is proved experimentally that shoot apical meristem of gymnosperms and angiosperms consist of a group of cells. • Hence, this theory was discarded.
This theory was postulated by Hanstein in 1868 stating that three different meristematic zones arise from the apical meristem. These layers are termed as histogens (layers). The three histogen are: Dermatogen:It is the outermost histogen and develop the epidermal tissue. Periblem:It is middle histogen layer and give rise to cortical tissue. Plerome: It is the innermost histogen and vascular cylinders are originated from it. HISTOGEN THEORY This theory was also discarded as these layers are not specific in their functions and these layers are not distinct in gymnosperms and angiosperms.
Tunica- Corpus Theory This theory was proposed by Schmidt based on the plane of cell division in the apex. According to this theory, there are two distinct layers in the shoot apex of angiosperms named tunica and corpus. • Tunica It is the outer zone and consist of one or more layers of cells that form outer region by anticlinal division. Usually tunica develop the epidermis. • Corpus It is the inner zone of shoot apex and form the central part of stem by irregular divisions in all planes. Therefore, shoot apex increases in thickness. It is covered by tunica from outside. Cells of corpus are larger as compare to tunica.
Cytohistological Zonation • Central Zone (CZ): Reservoir of undifferentiated stem cells that maintain the growth of the plant. • Peripheral Zone (PZ): Region where cells proliferate and initiate the formation of lateral organs (leaves, flowers). • Primordia Formation: New organs, such as leaves and flowers, develop from the PZ. • Rib Zone (RZ): Cells in this region elongate and contribute to the development of the stem. • Cell Differentiation: Cells in the rib zone differentiate into vascular tissues that form the central stem structure.
Structure of SAM • Shoot apex consists of apical meristem + leaf Primordia • It can be vegetative meristem or floral meristem. • SAM have 3 layers • L1 - outermost layer; cell division in this layer occur in anticlinal plane • L2- second layer; also divides in anticlinal plane • L3- third layer; divides in both anticlinal and periclinal planes L1 and L2 layers are the tunica and L3 represents the corpus.
Molecular Mechanism of SAM Maintenance Genes involved- • WUSCHEL (WUS) Gene: This gene is crucial for maintaining stem cell identity in the SAM. WUS is expressed in the organizing center and signals to the stem cells to prevent differentiation. • KNOX Genes: Maintain the undifferentiated state in meristem cells. • CLAVATA (CLV):Regulates the balance between stem cell proliferation and differentiation • SHOOT MERISTEMLESS (STM): Prevents cells in the SAM from differentiating prematurely.
Feedback Loops: The WUSCHEL- CLAVATA feedback loop is essential for maintaining a balance between stem cell maintenance and differentiation. WUS-CLAVATA Pathway: WUS promotes stem cell maintenance. CLAVATA proteins (CLV1, CLV3) restrict stem cell numbers by inhibiting WUS. Stem Cell Homeostasis: This pathway ensures that the meristem does not run out of stem cells while also generating differentiated cells for organ formation.
Feedback Loop The CLV genes negatively regulate WUS expression An increase in the number of stem cells promotes transcription of CLV3. CLV3, a small peptide, binds to CLV1 and suppresses the expression of WUS. As stem cell number decreased, the level of CLV3 is reduces, allowing the expression of WUS which causes and increase in stem cell number.
CLV3 CLV1 CLV2/CRN WUS SAM MAINTENANCE Peptide Receptor Targe t Function
Leaf Primordia: New leaves start as small bumps called leaf primordia at the SAM’s peripheral zone. Internode Formation: SAM also contributes to the formation of the stem, specifically the internodes (the segments between leaves). Phytomeres: The shoot grows in repeating units called phytomeres, consisting of a leaf, an internode, and an axillary bud. SAM Contributes to Leaf and Stem Formation
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