Insights into Plant Embryology: Points to Remember for Botany Students
Explore key points in plant embryology including terminology, structure of anthers, pollen grain formation, pollen viability testing, and aspects of meiosis. Understand the significant contributions of Prof. Panchanan Maheshwari to Indian plant embryology.
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Plant Embryology Plant Embryology (Points to remember) (Points to remember) For B.Sc. Part II (Paper 1) & M.Sc. IV Sem.(Paper 1) For B.Sc. Part II (Paper 1) & M.Sc. IV Sem.(Paper 1) Dr. Sanjay Srivastava Botany Department H.C.P.G.College Varanasi
Father of Indian Plant Embryology Prof. Panchanan Maheshwari Embryological terms : Calyx and Corolla = Anthophylls Stamens and Carpels = Sporophylls (Microsporophylls and megasporophylls respectively) Pollen chamber = Microsporangium Ovule (without integuments) = Megasporangium Formation of microspore = Microsporogenesis (involves Meiosis) Formation of megaspore = Megasporogenesis (involves Meiosis) Microspore (after release from tetrad) = Pollengrain Pollen tube = male gametophyte or microgametophyte Embryosac = Female gametophyte or Megagametophyte Formation of male gamete = Microgametogenesis Formation of female gamete = Megagametogenesis Male gamete = sperm or microgamete Egg = Ovum or megagamete
Anther may be dithecous (bilobed anther or tetrasporangiate) or monothecous (single lobed anther or bisporangiate) Anther with single microsporangium Arceuthobium minutissimum Anther wall is four layered Epidermis , Endothecium, middle layers and Tapetum Tapetum is of two types 1. Amoeboid (invasive or periplasmodial type) ex. Alisma, Typha etc . 2. Secretory (parietal or glandular type) eg. Helloborus foetidus Microspore tetrad are of 5 types : Tetrahedral, Isobilateral, Decussate, T shaped and Linear All 5 types of tetrad are found in Aristolochia elegans Compound Pollengrains all 4 microspores remain united Ex. Drimys, Drosera, Typha etc. Polyspory more than 4 microspores in a tetrad eg. Hyphaene, Cuscuta reflexa ( 11 microspores in a tetrad)Pollinia found in Calotropis procera Wall of pollen grain 1.Exine- made of sporopollenin, a highly resistant material 2. Intine- made of pectocellulose Pollen kit found as an outer sticky layer on pollen grain in entomophyllous species. Pollenkit composed of lipid and carotenoids Palynology Study of pollen grains (Serjania has lethal pollengrains )
Synchronous Meiosis : All meiocytes in anther locule undergo meiosis simultaneously as they are interconnected by cytoplasmic channels. No such synchrony occurs during pollen mitosis. Nemec phenomenon : (pollen embryosac) found in petalloid anthers of Hyacinthus orientalis Sporophytes from pollen grains : By Guha and Maheshwari (1966) in Datura Pollen Viability Test : This test is conducted to determine the ability of individual pollen grains that whether they can deliver viable gametes in the embryosac. The test is performed through any of the following methods : 1. In vitro germination (in cavity slides) 2. Tetrazolium test 2,3,5-triphenyl tetrazolium chloride (viable pollen turn red in colour) 3. Fluorescein diacetate test ( depends on esterase enzyme in pollen) 4. NMR ( a non-destructive method in which pollen are not destroyed)
Ovule (Integumented megasporium) Tenuinucellate Ovule : Sporogenous cell is hypodermal in position ; Nucellus absent CrassinucellateOvule: Sporogenous cell is sub-hypodermal in position ; Massive Nucellus Perisperm : Remnant of nucellus eg. Nymphaea, Zingiber Megagametogenesis types : 1. Monosporic : Polygonum and Oenothera type 2. Bisporic : Allium and Endymion type 3. Tetrasporic : a)Without fusion (Adoxa, Penaea, Plumbago, Peperomia, Drusa types ) b)With fusion (Fritillaria and Plumbagella type ) Obturator : Tissue facilitating entry of pollen tube into the ovule Endothelium (integumentary tapetum) : Nutritive tissue ; develops in unitegmic tenuinucellate ovules. Aril : Fleshy, juicy, third integument in Litchi Caruncle : Integumentary outgrowth in Ricinus communis
Fertilization : First reported by Strassburger in Monotropa Stigmatic exudates include Sugar, gum, resins, lipids, phenols etc. These help in pollen germination. Gynoecium styles are of three types : Solid, hollow and semi-solid Entry of pollen tube : Porogamy ; chalazogamy ; mesogamy X-bodies : remains of synergid and vegetative cell nuclei Synergid : 2 in number (one degenerating and other persistent synergid) Double Fertilization : Discovered by S.G.Nawaschin (1898) in Fritillaria and Lilium . It involves two events a) Fusion of one male gamete with egg nucleus (syngamy or generative fertilization) b) Fusion of second male gamete with polar nuclei or secondary nucleus (Triple fusion or vegetative fertilization) Endosperm : It is the product of triple fusion and therefore triploid in nature. Endosperm is absent in families- Orchidaceae, Podostemaceae and Trapaceae. Based on mode of development endosperm are of three types : Nuclear, Cellular and Helobial Composite endosperm found in Loranthaceae (due to absence of true ovules) Ruminate endosperm found in Areca, Myristica etc. Chlorophyllous endosperm found in Crinum, Raphanus. Endospermous or albuminous seed eg. Cereal, castor bean, coconut Non-endospermous or ex-albuminous seed eg. Pea, beans
APOMIXIS Term was given by WINKLER (1908,1934) We know that the normal sexual life cycle consists of two essential components : Meiosis and Syngamy. It is called Amphimixis. Apomixis is the phenomenon in which an organism has a mode of reproduction devoid of these two components i.e., Meiosis and Syngamy. Apomixis is expressed in two ways : 1) Vegetative reproduction 2) Agamospermy 1) Vegetative reproduction : Seed is not retained as a means of reproduction eg. bulbil, bulb, runner. 2)Agamospermy : Seed has been retained but its in its formation meiosis and syngamy has been avoided.
Agamospermy can occur through any of three means : 1. Adventive Embryony 2. Diplospory 3. Apospory 1. Adventive Embryony : In this, any cell of the nucellus or integument directly forms the embryo. eg. In Citrus, Mango 2. Diplospory : First the megaspore mother cell (MMC) forms an unreduced embryosac i.e. a diploid embryosac. Then either the egg cell (2n) of this embryosac or any other cell of the embryosac (also 2n) directly forms the embryo. If the embryo is formed by unreduced egg then it is called Parthenogenesis. In the seond case it is called Apogamety. 3. Apospory : In this case, besides the normal embryosac (n), any other cell of the nucellus gives rise to an unreduced embryosac (2n). Any cell of the unreduced emryosac directly gives rise to the embryo, eg.Hieracium, Parthenium, Rubus.
Autonomous Apomicts : Apomictic plants in which even pollination stimulus is NOT required for apomixis to happen. eg. Asteraceae, Rubiaceae. Pseudogamy : Plants in which pollination stimulus is required for apomixis to happen. eg. Grasses, Citrus. Non-recurrent Apomixis : Here, the M.M.C. undergoes normal meiosis and a reduced i.e. haploid embryosac is formed. The egg cell (n) of this embryosac may directly (without fertilization) form embryo ( haploid Parthenogenesis). In other case any other cell of the embryosac may directly form embryo (haploid apogamy). Recurrent Apomixis : Embryosac may arise from a cell of archesporium (generative apospory) or from any other cell of the nucellus ( somatic apospory). But meiosis does not occur in the formation of embryosac and so an unreduced (2n) embryosac is formed. If the diploid egg of the embryosac directly (without fertilization) forms embryo it is called diploid parthenogenesis. If some other cell of the embryosac directly forms embryo it is called diploid apogamy. Causes of Apomixis : Apomixis is a result of either genetic (recessive genes) or environmental factors. Significance of Apomixis : Better plant varieties can be indefinitely multiplied.
PARTHENOCARPY : The term was given by Noll (1902). According to Nitsch (1965), it is the formation of fruits without fertilization. Parthenocarpy is very common in plants which have ovaries with large number of ovules. Ex. Banana, pine apple, fig, tomato, fig etc. Parthenocarpy can be of two types : a) Stimulative and b) Vegetative In stimulative type, pollination stimulus is required for parthenocarpy to occur. In Vegetative parthenocarpy pollination is NOT required for parthenocarpy to occur. According to Nitsch (1963) based on the reasons of occurrence parthenocarpy can be of three types :1) Genetic Parthenocarpy : It occurs due to certain genes which may originate either by Mutation (navel oranges which were result of mutation in axillary buds) or Hybridization (Cucumber- English forcing and Arilington white spine ). Other examples are Citrus, Cucurbita, Eugenia etc. 2) Environmental Parthenocarpy : Occurrence of fog, Frost or low temperature induces Parthenocarpy in some plants. 3) Chemically Induced Parthenocarpy : By application of Auxins or Gibberellins in conc. 10-7 to 10-6 M. In form of lanolin paste or as spray. Auxin is applied after anthesis and gibberellin as early as possible. Significance of parthenocarpy : 1.Consumption and processing becomes easy. 2. Edible part of fruit increases.
POLYEMBRYONY : It is the presence of more than one embryos in a single seed. Polyembryony is common in Gymnosperms (eg. Pinus ). It is rare among Angiosperms. Among Angiosperms it is common in Citrus and Mangifera. Polyembryony was discovered by Antony Von Leeuwenhook. Causes of Polyembryony : 1. Cleavage in zygote : eg. Gymnosperm, Orchids 2. Formation of embryo from cell other than egg : eg. Synergid (Argemone) ; Antipodals ( Ulmus) ; Endosperm (Balanophora ). 3. Presence of more than one embryosacs in a single ovule : eg. Citrus and Casuarina equisetifolia 4. Sporophytic cell activation (Adventive Embryony) : eg. Nucellar embryo in Citrus and Mangifera. Causes of Polyembryony : Haberlandt (1921) gave Necrohormone theory. According to it, degenerating cells of the nucellus act as a stimulant. Experimental Induction of Polyembryony : By application of 2,4-D, 2,4,5-T or NAA (0.1%)
