Plant Tissues and Meristems (Angiosperms) PDF

Tissues and Meristems during Plant Development (for Angiosperms) Compiled by MEC Salvador Outline 1. Cell Fate and Totipotency 2. Embryogenesis 3. Germination 4. Morphogenesis 5. Primary Growth 6. Secondary Growth 7. Plant Tissue Culture 8. Asexual reproduction...

Tissues and Meristems during Plant Development (for Angiosperms) Compiled by MEC Salvador Outline 1. Cell Fate and Totipotency 2. Embryogenesis 3. Germination 4. Morphogenesis 5. Primary Growth 6. Secondary Growth 7. Plant Tissue Culture 8. Asexual reproduction in plants Cell Fate and Totipotency  Plant development during embryo stage follow patterns that establish the distribution of cells depending where the cells are positioned within the embryo.  Cells differentiates or specializes to take a certain role within the growing embryonic body, which in ontogeny and cell biology referred as cell fate.  Embryonic tissue regions, primarily concerned with formation of new cells are the meristems (from the Greek merismos, meaning division). Cell Fate and Totipotency  Meristems not only add new cells to the plant body, but also perpetuate themselves as some of the products of cell division in the meristems do not develop into adult cells but remain meristematic.  Initiating cells or meristematic initials (i.e., simply initials) - cells that maintain the meristem as a continuing source of new cells.  Derivatives - products of initial cells which after a variable number of cell divisions, give rise to the body cells.  Initials are known to possess cell potency, the cell's ability to differentiate into other cell types called totipotency.  Totipotent cells have the capacity to produce all cell types and develop into complete plants. Many living cells in a mature plant part remain developmentally totipotent. Embryogenesis  Sporophyte phase of the life cycle begins with the product of the gametic union  The unicellular zygote develops into an embryo  Initial stages of embryogenesis are the same in eudicots and monocots (A) Arabidopsis, a eudicot; (B) Zea mays, a monocot. (Zhao, et al., 2017. You can read more on cell fate determination during early embryogenesis from their paper at https://academic.oup.com/plphys/article/173/1/130/6115874) Embryogenesis  Embryogenesis establishes the body plan of the plant, consisting of two patterns: 1. Apical-basal pattern along the main axis 2. Radial pattern for concentrically arranged tissue systems  Patterns establishes the distribution of cells  Formation of the embryo begins with division of the zygote within the embryo sac of the ovule  First division: transverse and asymmetrical with regard to the long axis of the cell  This establishes polarity of the embryo  Upper pole consists of a small apical cell giving rise to most of the mature embryo  Lower pole consists of the larger basal cell which produces a stalk-like suspensor, anchoring the embryo to the micropyle NOTE: Not all plant embryos are derived from the zygote. Plant embryos could be derived from:  Zygotic embryogenesis (from the zygote, a product of the union of the gametic cells)  Somatic embryogenesis (from plant cells other than the zygote that may be induced to perform meristematic activity) Both embryos will have cells that will go through: 1. Differentiation – depending on the positioning of the derivative cells 2. De-differentiation 3. Re-differentiation Do all plants undergo zygotic and/or somatic embryogenesis?  Embryo differentiates into a nearly spherical structure  Embryo proper – a mass of relatively undifferentiated cells  Differential growth and vacuolation of cells initiate the organization of tissue systems in the embryo proper  Meristematic surface layer is the protoderm – precursor of epidermis  Beneath the surface layer distinguished by cell vacuolation (with large vacuoles) is the ground meristem – precursor of cortex  Centrally located, less vacuolated with apical-basal axis is the procambium – precursor of primary vascular system  Primary Meristems or primary meristematic tissues: 1. Protoderm 2. Ground meristem 3. Procambium  Continuous divisions of these tissues extend these into other regions of the embryo  As the embryo develops, there is gradual restriction of new cells to the opposite ends of the axis  Apical meristems are formed – future root and shoot Mature embryo consists of: 1. Stem-like axis bearing one or more leaf-like appendage(s), the cotyledon(s) 2. Stem-like axis below the cotyledon, the hypocotyl 3. Lower end of the hypocotyl (root pole) has the primordial root, the radicle 4. Upper end of the hypocotyl (epicotyl) may or may not have initiated development of shoot. If primordial shoot is present, it is called a plumule Dicot and Monocot Seed Germination  Mature embryo may become dormant (stops growth but remains viable) and continues to develop when dormancy was overcome (physical or physiological means) upon germination  Germination – resumption of growth by the embryo in a seed (term also used with beginning of growth of a spore, pollen grain, bud or other structure).  Elongation of radicle and breaking out from the seed coat – acceptable physical sign of seed germination  Morphogenesis: Shoot formation  Upon germination, apical meristem of the shoot forms in regular sequence leaves, nodes and internodes  Meristems in the axils of leaves produce axillary shoots (not all, e.g. axillary meristems remains inactive in palms)  Young plant or the seedling now bears a system of branches on the main stem Morphogenesis: Root formation  Apical meristem of the root located at the tip of the hypocotyl (radicle) forms the primary root  Primary root produces branch roots (secondary roots) from new apical meristems originating from the pericycle  When primary root was maintained and matures, it becomes the tap root (usually found in dicots)  Branch roots further form branch roots  Some roots arise from the stem like in monocots  Primary Growth  Plant organs originating from the apical meristems pass a period of expansion in length and width  Primary growth - initial growth of successively formed roots and shoots (increase in length); plant body resulting from this growth is the primary plant body which consists of primary tissues  Entire life of the sporophyte of seedless vascular plants and monocots is completed in a primary plant body Secondary Growth  Secondary growth – upon increase in width or thickness of stem and roots  Found in gymnosperms and most angiosperms (woody dicots and some monocots)  Tissues produced by the vascular cambium and phellogen referred as secondary tissues, plant body formed is known as secondary plant body  A secondary growth resulting from production of cells by a meristem called cambium Secondary Tissues  Vascular cambium forms secondary vascular tissues (secondary xylem and secondary phloem)  These tissues increase thickness of the stem and root axis  This growth is accompanied by the activity of the cork cambium or phellogen  Cork cambium develops in the peripheral region of the expanding axis and gives rise to the periderm (a secondary protective tissue system replacing the epidermis)  Why are the vascular and cork cambium known as lateral meristems? Primary Growth vs. Secondary Growth  All plants will undergo primary growth  All plants have primary meristems  All plants will have primary tissues: epidermis, vascular tissue, ground tissue  Not all plants will continue to secondary growth  Not all plants have lateral meristems or secondary meristems: vascular cambium and cork cambium  Lateral meristems allow formation of secondary tissues  Thus, not all plants will have secondary tissues.  Name examples of plants having only primary growth.  Name examples of plants showing secondary growth.  Plant tissue culture - harvest of any plant tissue beside the zygote to grow a new plant individual is an example of somatic embryogenesis - begins with an induction of plant cells to form meristematic undifferentiated cells (callus) to become embryos  Addition of synthetic or natural-sourced plant hormones in different combinations in the nutrient media (e.g., auxin and/or cytokinin) could “stimulate” induction of callus formation, root and/or shoot formation. Tobacco plant culture (left) and orchid culture (right)  But there are plants that needs no external application of chemical stimulants to propagate itself Asexual reproduction induced by drought, wounding or plant maturity Plant tissues stimulated to de-differentiate to become meristematic cells and re- differentiate to produce new roots and shoots Why plants needed to evolve asexual reproduction through these mechanisms? How important are meristems? Thank you!