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Plant Development Plant Development Plant development is an umbrella term for a broad spectrum of processes that include: the formation of a complete embryo from a zygote ; seed germination; the elaboration of a mature vegetative plant from the embryo; the formation of flowers, fruits, and seeds; and many of the plant's responses to its environment. Cell Divisions Cell division is process by which a parent cell divides into two daughter cells . Opinion that microbes and worms are generated spontaneously from dust & would multiply by breaking apart during collisions. First cell division was observed by Lazarro Spallanzani – a catholic priest – & another of the Lord’s men for the sciences. S. spent months trying to divide drops with hairs so that he had only one microbe under the microscope. It divided by fission. S.was first to recognize fertilization as unification of sperm and egg cell; proved by first artificial insemination in female dogs. (1) For simple unicellular organisms such as bacteria, amoeba etc. cell division is equivalent to reproduction – a new organism is created. cell division in prokaryotes is known as binary fission. (2) Cell division in multicellular eukaryotes is part of mitosis and cell cycle resulting in daughter cells that belong to the same, growing organism . Mitotic cell division can create progeny, such as plants grow from cuttings. (3) A novel type of cell division developed only in eukaryotes: meiosis. A cell is permanently transformed into a gamete and cannot divide again until fertilization. This is the ontogenetic reflection of their prokaryotic origin. Cell Division in animals vs plants Mitosis was discovered by German botanist Eduard Strasburger in 1875 in onion cells and later by German zoologist Walther Flemming in animal gill cells in 1879. Flemming named it ‘Mitosis’ in 1882. Cell Growth For a multicellular organism to grow, it takes more than cell division, which does not increase cell volume It needs cell ell expansion. expansion vacuole Cell expansion in animals is slow since based on the synthesis of more cytoplasm (s –phase) . Cell expansion in plants is quick since it involves only water uptake into the vacuole (novel phase) . This not only enlarges the organism quickly but is also the basis for plant movements (tropisms ). Animal movement is based on musles = contractile actin/myosin protein chains. Development Cell division and expansion are particularly simple & fast in plants Most embryos start with a fused cell called zygote- a fertilized ovule. Most higher plants start their life as an embryo contained in a seed or a vegetative prop unit. Most higher plants start their adult life as germinating seedlings Plants reach sexual maturity by flowering & subsequent seed production E. Haeckel’s biogenetic rule” “ Ontogenesis repeats phylogenesis” plant seedlings look very much alike in the monocots or dicots! Meristems – the future leaders Meristems contain dividing cells that produce cyclins in one cell to re- divide (eternal youth) &less of cyclins in the other one that will turn into an organ. Meristems have to be complete to generate complete plant s Meristems can change, especially the Shoot Apical Meristem (SAM): it ages from juvenile to mature generative to reproductive (i.e. flowering ) Root & Shoot Ap. Meristems establish bipolar growth pattern of plants Embryo Size - Dicots Gingko biloba Phaseolus vulgaris Avocado Persea amer. Pea Pisum sativum Red Pepper Capsicum Onion Alium Polarity – a basic feature of plant cells in stem & roots Polarity of plants is well established during embryogenesis where it is set with the first division of the zygote and apparent in the torpedo stage. Polarity of plants is established by the opposite growth of shoot and root apical meristems. It reflected in the polarity of its stem and root cells as well as their buds. At the level of the individual cell polarity is organized by asymmetric distribution of organelles & and Golgi network of transport proteins, especially for the polar transport of auxin. Polar cells exist also in animals at tissue level. Hermann von Vochting 1878 : established term “Polarity “ for plants. Went 1974 environmental factors regulate polar auxin flows to induce physiol. responses like reorientation, leaf abscission, new apical leaders 1974 Rubery & Sheldrake chemisosmotic model: PM with auxin uptake carriers and an efflux carrier confined to the lower side of cells 1999 PIN proteins are PM proteins acting as auxin efflux carriers. Their localization in the cell determines direction of auxin flow in the tissue!! REF: E. Ferraru & Jiri Friml: PIN polar targeting, Plant Physiol 147, 1553-1559 (2008) Meristems – the future leaders Meristems contain dividing cells that produce cyclins in one cell to re- divide (eternal youth) &less of cyclins in the other one that will turn into an organ. Meristems have to be complete to generate complete plant s Primary meristems (root, shoot apical M.) maintain axis & polar structure of plant, they are often indeterminate. Apical shoot meristems survive winter as terminal buds. Secondary meristems like axillary M. of dicots (in axils of leaves) cause branching. Vascular cambium growth in girth woody thickening of perennial stems Cork cambium protective outer layer = bark, Pericycle cells lateral root growth In monocots intercalary meristems expand shoot axis of grasses Various meristems of shoot & root and their resp. functions Meristems – the future leaders Meristems are self-perpetuating populations of small isodiametric cellsthat retain their embryonic character (at least one daughter cell retains embryonic character & eternal youth medicine calls stem cell Shoot apical meristem plus leaf primordia is a slightly mounded region 0.1 to 0.5 mm wide pinhead in dicots. We differentiate three layers: L1, L2 (anticlinal divisions) L3 periclinal division L1 epidermis L2, L3 internal layers Apical Meristems determines the growth habit of plants Meristem are the zones of the plant where growth takes place. The term “meristem” was first used by Karl Wilhelm von Nägeli (in his book “Beiträge zur Wissenschaftlichen Botanik” in 1858 . The primary mersitem is like the head of Where is the meristem? an animal, its location decides survival! Why can you mow the grass but not the roses? beet Radish Carrot kohlrabi The Grass Plant and its growth Monocots grow in length not at the tip of the shoot but stretch their internde regions due to the activitivity of the intercalary meristems situated at the nodes. The Grass Plant and its meristem … and all flesh, it is like grass. You can trample it and it will stand up again…. Grass – a symbol of passive resistance! Meristem location is crucial to survive lawn mowers Dicot plants with apical meristems are very sensitive to decapitation - very similar to brain-centered vertebrates. Monocot leaves - however- do not proudly carry their meristem center at the plant apex - but smartly moved it to the root-shoot interface. But what about dandelions Taraxacum or carrot Daucus carota? They are not monocots, are they? Answer:? Dandelion have their apical shoot meristem at the transition area between taproot and shoot consisting mainly of leaves arising from an acaulescent (or hidden ultra- short) stem. The Bud – an embryo for shoot development A bud is an embryonic shoot and occurs in the axil of a leaf or at the tip of the stem. A bud may remain dormant or form a shoot immediately. Bud is protected by scales. When the bud develops, the scales drop off, leaving on the surface a bud scar. Since each year's growth ends in the formation of a bud, one can determine the age of any young branch, Buds are classified according to: location, status, morphology, function. Buds – the reserve meristems ready for emergencies The dome of the apical meristem Dormant axillary buds are generates secondary meristems in activated in emergency situations . They grow into a branch (1) the leaf primordia ; and simulating & eventually replacing (2) the axillary bud primordia the defunct main shoot Polarity of secondary meristems is fixed as well Like magnets also plants have a clearly defined polar structure with the apical end producing leafy shoots and the basal end making roots. The buds sense the incompleteness & also remember their pole / polarity. Polarity does not change when gravity vector is inverted. Only plant cells that are temporarily without fixed polarity are the zygote & callus or tumor cells. Vernalization of seeds & buds! Vernalization means to turn a somebody into spring mood. In plants this is done by exposing them to cold in a simulation of winter. As soon as they leave the refrigerator they are turned on to grow. Important to find both the right temperature & right time. Shoot meristem undergoes phases of development That the shoot apical meristem ages is obvious in most seedlings that start with seed leaves and them switch to other leaf types and also into flowers. Some perennial plants - however – show more than this. From (1) fleshy cotyledons (2) to simple primary leaves (3) to compound secondary leaves & (4) flower petals beans show 4 leaf types during life. As the shoot meristem ages it makes different leaves Apples, beech, ivy, holly, sassafras, Thuja , Acacia heterophylla and many other trees show a changing leaf shape with increasing age of the plant. Every plant undergoes 3 stages or phases: 1. Juvenile phase 2. Adult vegetative 3. Adult reproductive phase (flowering) These changes can sometimes be seen in an altered leaf shape. As the shoot meristem ages it makes different leaves Apples, beech, ivy, holly, sassafras, Thuja, Acacia heterophylla and many other trees take various times before they reach sexual maturity, some considerably longer than humans. Phaseolus vulgaris 7 days As the shoot meristem ages it makes different leaves Acacia heterophylla – a popular park tree in Seattle – with sudden death & extinction in January 2011. Acacia heterophylla , the Highland tamarin , is a tree (or shrub in its higher places) endemic to Réunion island (France) near lower branches upper Madagascar & Mauritius.
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