SEED GERMINATION

Types and Physiological Processes

1. Types and Physiological Processes 1

2. Contents introduction Seed germination ; Definition, processes, Seedling Establishment and Post germination process Requirement for seed germination: Gases, Temperature, Water Types of Seed germination: Epigeal germination, Hypogeal germination. Trigger and germination agents. Phases of seed germination: Maintenance Phase , Phase I , Phase II, Phase III; Details Important Reserved resources and their moment, Enzymes and degradation of starch and protein Physiology of seed germination; structure of grain, Endosperm, aleurone layer Gibberellin and germination, Enhancement of mRNA and signal transduction Positive and negative regulation. Seed Germination types and Physiological process 2

3. In the process, the seed play role in that Reproductive unit it is: Thread of life Assures Survival of Plant species. Key to Modern Agriculture. Essential for Crop Production INTRODUCTION 3 Plant Physiology and Development by Taiz and Zeiger

4. Definition: “ The process that begins with the water uptake by the dry seed and ends with the emergence of the embryonic axis, usually the radicle, from its surrounding tissue”. SEED GERMINATION 4 Plant Physiology and Development by Taiz and Zeiger

5. SEED GERMINATION Activation Of Embryo Seed germination is a mechanism, in which morphological and physiological alterations result in activation of the embryo Elongation: Before germination, seed absorbs water, resulting in the expansion and elongation of seed embryo. Emergence of Radicle . When the radicle has grown out of the covering seed layers, the process of seed germination is completed (Hermann et al., 2007) 5

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7. Strictly speaking, germination does not include seedling growth after radicle emergence, which is referred to as seedling establishment. Similarly, the rapid mobilization of stored food reserves that fuels the initial growth of the seedling is considered a post germination process. Seedling Establishment and Post Germination Process 7 Plant Physiology and Development by Taiz and Zeiger

8. Water Gases Temperature Light Nitrates Requirements for Germination. Physiology of Seed Germination by Miller B. McDonald 8 Plant Physiology and Development by Taiz and Zeiger

9. Air is composed of about 20% oxygen, 0.03% carbon dioxide, and about 80% nitrogen gas oxygen is required for germination of most species. Carbon dioxide concentrations higher than 0.03% retard germination. while nitrogen gas has no influence. Gases Physiology of Seed Germination by Miller B. McDonald 9

10. Seed germination is a complex process involving many individual reactions and phases, each of which is affected by temperature. The optimum temperature for most seeds is between 15 and 30o C. The maximum temperature for most species is between 30 and 40o C. The response to temperature depends on a number of factors, including the species, variety, growing region, quality of the seed, and duration of time from harvest. Temperature . Physiology of Seed Germination by Miller B. McDonald 10

11. water is the most essential factor. The water content of mature, air-dried seeds is in the range of 5 to 15%, well below the threshold required for fully active metabolism. In addition, water uptake is needed to generate the turgor pressure that powers cell expansion, the basis of vegetative growth and development. water 11 Plant Physiology and Development by Taiz and Zeiger

12. For example Incubating tomato seeds at high ambient water potential ('I‘ = 0 MPa) allows 100% germination, whereas incubation at low water potential ('I'= -1.0 MPa), which nullifies the gradient in water potential, completely suppresses germination Example 12 Plant Physiology and Development by Taiz and Zeiger

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14. Based on the fate of the cotyledons, two kinds of seed germination occur, and neither appears to be related to seed structure. These two types are illustrated by the germination of bean and pea seeds. Although these seeds are similar in structure and are in the same taxonomic family, their germination patterns are quite different TYPES OF SEED GERMINATION Physiology of Seed Germination by Miller B. McDonald 14

15. Hypogeal germination is characteristic of pea seeds, all grasses such as corn, and many other species. During germination, the cotyledons or comparable storage organs remain beneath the soil while the plumule pushes upward and emerges above the ground. Hypogeal Germination Epigeal germination is characteristic of bean and pine seeds and is considered evolutionarily more primitive than hypogeal germination. During germination, the cotyledons are raised above the ground where they continue to provide nutritive support to the growing points. Epigeal Germination TYPES Physiology of Seed Germination by Miller B. McDonald 15

16. In hypogeal germination, the epicotyl is the rapidly elongating structure. Regardless of their above-ground or below-ground locations, the cotyledons or comparable storage organs continue to provide nutritive support to the growing points throughout germination During root establishment, the hypocotyls begins to elongate in an arch that breaks through the soil, pulling the cotyledon and the enclosed plumule though the ground and projecting them into the air. Afterwards, the cotyledons open, plumule growth continues and the cotyledons wither and fall to the ground. Types Physiology of Seed Germination by Miller B. McDonald 16

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19. cotyledon hypocotyl Roots 19

20. At some point, the seed becomes sensitive to the presence of “trigger” agents. A “trigger” agent can be defined as a factor that elicits germination but whose continued presence is not required throughout germination. A “trigger” agent such as light or temperature alterations shift the balance of inhibitors to favor promoters such as gibberellins. Trigger and germination Agents Physiology of Seed Germination by Miller B. McDonald 20

21. In contrast, a “germination” agent is a factor that must be present throughout the germination process. An example is gibberellic acid. The major sequence of events leading to germination is imbibition, enzyme activation, initiation of embryo growth, rupture of the seed coat and emergence of the seedling. Trigger and germination Agents Physiology of Seed Germination by Miller B. McDonald 21

22. Most seeds undergo a specific sequence of events during germination. Prior to germination , seeds are in a “maintenance” phase. It is often characterized as dormancy being imposed by ABA, metabolic blocks or some other agent hindering the transition to germination. Seed dormancy: is a mechanism by which seeds can inhibit their germination in order to wait for more favorable conditions (secondary dormancy) (Finkelstein et al., 2008). However, primary dormancy is caused by the effects of abscisic acid during seed development. Pattern of seed germination Maintenance Phase Physiology of Seed Germination by Miller B. McDonald 22

23. Germination can be divided into three phases corresponding to the phases of water uptake Phase I. Phase II. Phase III. Pattern of seed germination 23 Plant Physiology and Development by Taiz and Zeiger

24. The dry seed takes up water rapidly by the process of imbibition. It is the first key event that moves the seed from a dry, dormant organism to the resumption of embryo growth. The extent to which water imbibition occurs is dependent on three factors: (1) composition of the seed (2) seed coat permeability (3) water availability PHASE I : IMBIBATION 24 Plant Physiology and Development by Taiz and Zeiger

25. Water uptake by imbibition declines and metabolic processes, including transcription and translation, are reinitiated. The embryo expands, and the radicle emerges from the seed coat. PHASE II: LAG PHASE 25 Plant Physiology and Development by Taiz and Zeiger

26. Water uptake resumes due to a decrease in ljl as the seedling grows, and the stored food reserves of the seed are fully mobilized. PHASE III: MOBILIZATION OF RESERVE FOOD 26 Plant Physiology and Development by Taiz and Zeiger

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29. IMBIBATION 29

30. The initial rapid uptake of water by the dry seed during Phase I is referred to as imbibition, to distinguish it from water uptake during Phase III. Although the water potential gradient drives water uptake in both cases, the causes of the gradients are different. In the dry seed, the matric potential ( lflm ) component of the water potential equation lowers the 1f1and creates the gradient. The matric potential arises from the binding of water to solid surfaces, such as the micro capillaries of cell walls and the surfaces of proteins and other macromolecules There hydration of cellular macromolecules activates basal metabolic processes, including respiration, transcription, and translation.  Imbibition ceases when all the potential binding sites for water become saturated, and lflm becomes less negative. IMBIBATION 30 Plant Physiology and Development by Taiz and Zeiger

31. LAG PHASE : 31

32. The rate of water uptake slows down until the water potential gradient is reestablished. Phase II can thus be thought of as the lag phase preceding growth, during which the solute potential (If!.) of the embryo gradually becomes more negative due to the breakdown of stored food reserves and the liberation of osmotically active solutes.   The seed volume may increase as a result, rupturing the seed coat. At the same time, additional metabolic functions come online, such as the re-formation of the cytoskeleton and the activation of DNA repair mechanisms.   PHASE II:LAG PHASE 32 Plant Physiology and Development by Taiz and Zeiger

33. The emergence of the radicle through the seed coat in Phase II marks the end of the process of germination. Radicle emergence can be either a one-step process in which the radicle emerges immediately after the seed coat ( testa ) is ruptured, or it may involve two steps in which the endosperm must first undergo weakening before the radicle can emerge. LAG PHASE 33 Plant Physiology and Development by Taiz and Zeiger

34. During Phase III the rate of water uptake increases rapidly due to the onset of cell wall loosening and cell expansion. Thus, the water potential gradient in Phase III embryos is maintained by both cell wall relaxation and   PHASE III 34 Plant Physiology and Development by Taiz and Zeiger

35. Carbohydrates (starches), proteins lipids Important stored resorces 35 Plant Physiology and Development by Taiz and Zeiger

36. The major food reserves of angiosperm seeds are typically stored in the cotyledons or in the endosperm. The massive mobilization of reserves that occurs after germination provides nutrients to the growing seedling until it becomes autotrophic. Mobilization of Stored Reserves 36 Plant Physiology and Development by Taiz and Zeiger

37. At the subcellular level, starch is stored in amyloplasts in the endosperm of cereals. Two enzymes responsible for initiating starch degradation are a- and ~-amylase. a-Amylase (of which there are several isoforms) hydrolyzes starch chains internally to produce oligosaccharides consisting of a(1,4)-linked glucose residues. ~ -Amylase degrades these oligosaccharides from the ends to produce maltose, a disaccharide. Maltase then converts maltose to glucose. The hormonal regulation of these enzymes is described in more detail in the next section. ENZYMES AND STARCH DEGRADATION: 37 Plant Physiology and Development by Taiz and Zeiger

38. Protein storage vacuoles are the primary source of amino acids for new protein synthesis in the seedling. In addition, protein storage vacuoles contain phytin , the K+, Mg2+, and Ca2+ salt of phytic acid a ( myo -inositol hexaphosphate ), a major storage form of phosphate in seeds.  During food mobilization, the enzyme phytase hydrolyzes phytin to release phosphate and the other ions for use by the growing seedling. ENZYME AND PROTEIN 38 Plant Physiology and Development by Taiz and Zeiger

39. Physiology of Seed germination 39

40. Cereal grains consist of three parts: The embryo The endosperm The fused testa -pericarp Structure of a cereal grain 40 Plant Physiology and Development by Taiz and Zeiger

41. The embryo, which will grow into the new seedling, has a specialized absorptive organ, the scutellum . THE EMBRYO 41 Plant Physiology and Development by Taiz and Zeiger

42. The triploid endosperm is composed of two tissues : starchy endosperm aleurone layer. ENDOSPERM 42 Plant Physiology and Development by Taiz and Zeiger

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44. The nonliving starchy endosperm consists of thin-walled cells filled with starch grains and it is centrally located. starchy endosperm 44 Plant Physiology and Development by Taiz and Zeiger

45. Living cells of the aleurone layer, which surrounds the endosperm, synthesize and release hydrolytic enzymes into the endosperm during germination. As a consequence, the stored food reserves of the endosperm are broken down, and the solubilized sugars, amino acids, and other products are transported to the growing embryo via the scutellum . The isolated aleurone layer, consisting of a homogeneous population of cells responsive to gibberellin, has been widely used to study the gibberellin signal transduction pathway in the absence of non responding cell types. Aleuron layer 45 Plant Physiology and Development by Taiz and Zeiger

46. Environmental and Experimental Biology by Mohammad meransari . 46

47. Even before molecular biological approaches were developed, there was already physiological and biochemical evidence that gibberellin enhanced a-amylase production at the level of gene transcription . The two main lines of evidence were: • GA3-stimulated a-amylase production was shown to be blocked by inhibitors of transcription and translation.  • Isotope-labeling studies demonstrated that the stimulation of a-amylase activity by bioactive gibberellin involved de novo synthesis of the enzyme from amino acids, rather than activation of preexisting enzyme. Gibberellins enhance the transcription of a-amylase mRNA 47 Plant Physiology and Development by Taiz and Zeiger

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50. T he gibberellin receptor GIBBERELLIN INSENSITIVE DWARF 1 (GID1) und ergoes a conformational change when it binds to gibberellin , wh ich promotes th e binding of DELLA repressor proteins. The DELLA protein also undergoes a conformational change , facilitating interaction with the E3 ubiquitin ligase SCfSLYl . As a result, the binding of gibberellin receptor GID1 to the DELLA repressor proteins triggers ubiquitination and subsequent degradation by the 26S proteasome, which allows the gibberellin response to proceed The gibberellin receptor, GID1 , promotes the degradation of negative regulators of the gibberellin response 50 Plant Physiology and Development by Taiz and Zeiger

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52. Within the aleurone cells there are both Ca2+-independent and Ca2+ -dependent gibberellin signaling pathways. The former leads to the production of a-amylase, while the latter regulates its secretion. Ca + dependent and Independent Environmental and Experimental Biology by Mohammad meransari . 52

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55. The sequence of the gibberellic acid response element (GARE) in the a-amylase gene promoter (TAACAAA) is similar to the DNA sequence to which MYB proteins bind . MYB proteins are a class of transcription factors in all eukaryotes including plants. In barley, rice, and Arabidopsis, a subset of MYBs have been implicated in GA signaling. Evidence that GA-MYB activates a-amylase gene expression (i.e., that GA-MYB is a positive regulator of a -amylase) includes the following: GA-MYB is a positive regulator of a-amylase transcription 55 Plant Physiology and Development by Taiz and Zeiger

56. Synthesis of GA-MYB mRNA begins to increase as early as 1 h after gibberellin treatment, preceding the increase in a -amylase mRNA by several hours Time course for the induction of GA-MYB and a-amylase mRNA by GA3 The production of GA-MYB mRNA precedes that of a-amylase mRNA by about 3 h. These and other results indicate that GA-MYB is an early gibberell in response gene that regulates transcription of the a-amylase gene. In the absence of gibberellin, the levels of both GA-MYB and a-amylase mRNAs are negligible.(After Gubler et al. 1995.) A mutation in the GARE that prevents MYB binding also prevents a -amylase expression. In the absence of gibberellin , constitutive expression of GA-MYB can induce the same responses that gibberellin induces in aleurone cells, showing that GAMYB is necessary and sufficient for the enhancement of a-amylase expression. 56 Plant Physiology and Development by Taiz and Zeiger

57. Cycloheximide , an inhibitor of translation, has no effect on the production of GA-MYB mRNA, indicating that protein synthesis is not required for GA-MYB expression.GA-MYB can therefore be defined as a primary or early response gene. In contrast, similar experiments show that the a -amylase gene is a secondary or late response gene. DELLA repressor proteins are rapidly degraded Drawing together our information for the cereal aleurone 57 Plant Physiology and Development by Taiz and Zeiger

58. we can hypothesize that the binding of bioactive gibberellin to GID1 leads to degradation of the DELLA protein. As a consequence of DELLA degradation , and via some intermediary steps that have not yet been defined, the expression of GA-MYB is up-regulated. Finally, the GA-MYB protein binds to a highly conserved GARE in the promoter of the gene for a-amylase, activating its transcription . a -Amylase is secreted from aleurone cells by a pathway that requires Ca2+ accumulation. Starch breakdown occurs in cells of the starchy endosperm by the action of a-amylase and other hydrolases, and the resultant sugars are exported to the growing embryo. Some of the genes encoding other hydrolytic enzymes whose synthesis is promoted by gibberellin also have GAMYB-binding motifs in their promoters, indicating that this is a common pathway for gibberellin responses in aleurone layers. 58 Plant Physiology and Development by Taiz and Zeiger

59. Environmental and Experimental Biology by Mohammad meransari . Mehrabad Rudehen , Imam Ali Boulevard, Mahtab Alley, #55, Postal Number: 3978147395, Tehran, Iran b AbtinBerkeh Limited Co., Imam Boulevard, Shariati Boulevard, #107, Postal Number: 3973173831, Rudehen , Tehran, Iran c Plant Science Department, McGill University, 21111 Lakeshore Road, Ste. Anne de Bellevue, Quebec, Canada H9X 3V9 Physiology of Seed Germination by Miller B. McDonald Seed Biology Program Department of Horticulture and Crop Science The Ohio State University Columbus, OH 43210-1086 [email protected] Plant Physiology and Development by Taiz and Zeiger Images From Google and Articles. https://www.google.com/search?q=seed+germination+phases+hd&client=opera&hs=Oof&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj-uYmatp7RAhUEuRoKHYQcAhIQ_AUICCgB&biw=1304&bih=698 https://www.google.com/ search?client = opera&hs = Tof&biw =1304&bih=698&tbm= isch&sa =1&q= seed+germination+structure&oq = seed+germination+structure&gs_l =img.3..0i8i30k1l2.39639.41517.0.41660.9.7.0.0.0.0.258.511.2-2.2.0....0...1c.1.64.img..7.2.511...0j0i30k1j0i5i30k1.5PMVbLv7tN0 Videos Youtube.com https://www.youtube.com/watch?v=E__rbDzNOZI httphttps://www.youtube.com/watch?v=oFOpDT3hXv4 s://www.youtube.com/watch?v=G2RuVxdr0mA References 59

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