Plant embryo development

seed germination in plants

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2. 2 Embryo Development Begins once the egg cell is fertilized -The growing pollen tube enters angiosperm embryo sac and releases two sperm cells -One sperm fertilizes central cell and initiates endosperm development (nutrients for embryo) -Other sperm fertilizes the egg to produce a zygote -Cell division soon follows, creating the embryo

3. 3 Plants differ from most animals in that embryogenesis does not directly generate the tissues and organs of the adult. For example, angiosperm embryogenesis forms a rudimentary plant body, typically consisting of an embryonic axis and two cotyledons.

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9. 9 Plant embryogenesis begins with an asymmetric cell division , resulting in a smaller apical (terminal) cell and a larger basal cell. Periclinally/anticlinally= 4 cell stage

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14. 14 the apical-basal pattern   (organization of organs along the apical-basal axis) and the radial pattern (organization of the three basic tissue systems - dermal , ground , and vascular ). The shoot apical meristem the root apical meristem one or two cotyledons are attached just below the shoot apical meristem the hypocotyl, followed by the root, the root apical meristem, and the root cap

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16. 16 An outermost layer of epidermal cells (the epidermis) covers a cylinder of cortical tissue (the cortex), which in turn overlies the vascular cylinder (the endodermis, pericycle, phloem, and xylem) The protoderm is the meristem that gives rise to the epidermis, the ground meristem produces the future cortex and endodermis, and the procambium is the meristem that gives rise to the primary vascular tissue and vascular cambium

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19. 19 Genes involved in embryo development mutants lacking body segments along the apical-basal axis. This class includes gurke (gk), fackel (fk), monopterous (mp), and GNOM (gn) showed a loss or distortion of the root, hypocotyl or cotyledon regions mutants with disturbed radial symmetry - alterations of the radial pattern of tissue layers. This class includes knolle (kn) and keule (keu) mutants with disrupted organogenesis - these mutants have grossly abnormal overall shapes, but have all of the pattern elements along the apical-basal and radial axes. This class includes fass (fs), knopf (knf), and mickey (mic).

20. 20 GNOM gene Seedlings homozygous for mutations in the GNOM gene lack both roots and cotyledons. In the most extreme mutants, gnom embryos are spherical and lack axial polarity entirely. We can conclude that GNOM gene expression is required for the establishment of axial polarity The MONOPTEROS gene: Primary root and vascular tissue. Mutations in the MONOPTEROS (MP) gene result in seedlings that lack both a hypocotyl and a root, although they do produce an apical region. Thus the MP gene is required for the formation of the embryonic primary root, but not for root formation in the adult plant. The MP gene is important for the formation of vascular tissue in postembryonic development

21. 21 Mutations in the KNOLLE ( KN ) gene affect the rate and plane of cell divisions as well as cell morphology, resulting in mutant seedlings with a disturbed radial organization of tissue layers. KN protein has similarity to syntaxins, a protein family involved in vesicular trafficking SHORT ROOT (SHR) and SCARECROW ( SCR), are necessary for tissue differentiation and cell differentiation not only in the embryo, but also in both primary and secondary roots and in the hypocotyl. FASS gene is required for morphogenesis, i.e., oriented cell divisions and position-dependent cell shape changes generating body shape, but not for cell polarity which seems essential for pattern formation.

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24. 24 -In many angiosperms, development of the embryo is arrested soon after meristems and cotyledons differentiate -The integuments develop into a relatively impermeable seed coat -Encloses the seed with its dormant embryo and stored food Seeds

25. 25 Pistils = One or more Carpels Carpels = Stigma, Style & Ovary The ovary wall is termed the pericarp -Has three layers : exocarp, mesocarp and endocarp -One, some, or all of these layers develop to recognized fruit Fruits can be: -Dry or fleshy -Simple (single carpel), -Aggregate (multiple carpels), -Multiple (multiple flowers) Fruits = Pistils...During Seed Formation Flower Develops into Fruit

26. 26 Germination Germination is defined as the emergence of the radicle (first root) from the seed coat Germination begins when a seed absorbs water & oxygen is available for metabolism -Often requires an additional environmental signal such as specific wavelength of light

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28. 28 The first step in barley seed germination is imbibition . In this process, water penetrates the seed coat and begins to soften the hard, dry tissues inside. The water uptake causes the grain to swell up. The seed/fruit coat usually splits open allowing water to enter even faster. The water begins to activate the biochemistry of the dormant embryo.

29. 29 The water coming into the seed and embryo dissolves a chemical made inside the embryo. This chemical is called Gibberellic Acid (GA). It is a plant hormone, not too different from steroids. The dissolved GA is transported with the water through the rest of the seed tissues until it arrives at the aleurone layer.

30. 30 The Gibberelic Acid which is transported by the water arrives at the aleurone layer. The GA crosses into the cytoplasm of the aleurone cells and turns on certain genes in the nuclear DNA . DNA is, of course, the hereditary molecule and contains the instructions for making every protein needed for the survival of a barley plant. The precise mechanism of how GA turns on the DNA is unknown at present. It is clear however that the mode of action is to turn on just certain genes in the DNA. The genes that are turned on are transcribed. The information archived in the DNA is precious, so the aleurone cells make a disposable RNA copy of the gene that is turned on. This disposable copy of the information, a kind of blueprint, is often called messenger RNA. The process of making this RNA copy is called transcript

31. 31 he RNA that was made in the transcription process is transported into the cytoplasm of the aleurone cells. In the cytoplasm, the messenger RNA joins up with a ribosome to begin the process of making a protein. This process is often called protein synthesis or translation . In this process, the ribosome examines the information held in the sequence of bases in the RNA. Transfer RNAs charged with particular amino acids are moved into the position specified by the instructions in the messenger RNA, and the amino acids are joined in a proper sequence by the ribosome. The sequence of amino acids determines the properties of the protein being assembled. In this case, the critical protein made with the information held in the RNA is amylase . This protein turns out to be an enzyme of great importance. The process of information held in the genes of DNA being transcribed into RNA and then translated into protein constitutes the central dogma of genetics. You will want to remember later, that it is the signal of Gibberellic acid that initiates this whole chain of events.

32. 32 A question one might ask right away is: from where do the amino acid building blocks for the amylase come? The answer is: from some other biochemistry in the aleurone cells. This biochemistry causes the storage proteins in the aleurone cells to be digested by hydrolytic enzymes. The hydrolysis is accelerated by enzymes known as proteases. These enzymes increase the rate at which the storage protein is cut into individual amino acids . The amino acids released by the hydrolysis are then free to be reassembled by the ribosomes into the structure of amylase. The same thing happens in people. You are not what you eat! You do not slowly become a steer by eating beef. The protein in your hamburger is digested into amino acids. Those amino acids are then reassembled into human proteins. Since the instructions for reassembling the amino acids come from your human DNA, the proteins produced are human, not bovine!

33. 33 The amylase is secreted (transported out) from the aleurone cells into the endosperm. the cotyledon (or seed leaf). Since there is only one, barley is a monocot. the epicotyl (becomes the shoot) the radicle (becomes the root)

34. 34 Releasing Sugars From Cotyledon...So the Embryo Can Grow

35. 35 Embryo produces gibberellic acid -This hormone signals the aleurone (outer endosperm layer) to produce a -amylase -Breaks down the endosperm’s starch into sugars that are passed to embryo Releasing Sugars From Cotyledon...So the Embryo Can Grow

36. 36 Germination New growth comes from delicate meristems As the sporophyte pushes through the seed coat , it orients with the environment such that the root grows down & shoot grows up -Usually, the root emerges before the shoot -The shoot becomes photosynthetic , and the postembryonic phase is under way Cotyledons may be held above or below the ground -May become photosynthetic or shrivel

37. 37 Germination