SEED GERMINATION

Types and Physiological Processes
Seed Germination
1

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

• 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

• 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
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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

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

 Water
 Gases
 Temperature
 Light
 Nitrates
Requirements for Germination.
Physiology of Seed Germination by Miller B. McDonald
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 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
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• 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.
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• 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
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• 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

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 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
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• 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
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• 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
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• 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
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• 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
21

• 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
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• Germination can be divided into three phases corresponding to the phases
of water uptake
 Phase I.
 Phase II.
 Phase III.
Pattern of seed germination
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• 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
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• 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

• 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
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 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

• 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

• 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

• 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

• Carbohydrates (starches),
• proteins
• lipids
Important stored resorces
35

• 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

• 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

• 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
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Physiology of Seed
germination
39

• Cereal grains consist of three parts:
• The embryo
• The endosperm
• The fused testa-pericarp
Structure of a cereal grain
40

• The embryo, which will grow into the new seedling, has a specialized
absorptive organ, the scutellum.
THE EMBRYO
41

• The triploid endosperm is composed of two tissues:
• starchy endosperm
• aleurone layer.
ENDOSPERM
42

• The nonliving starchy endosperm consists of thin-
walled cells filled with starch grains and it is
centrally located.
starchy endosperm
44

• 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

Environmental and Experimental Biology by Mohammad meransari.
46

• 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

• The gibberellin receptor GIBBERELLIN INSENSITIVE DWARF 1
(GID1) undergoes a conformational change when it binds to gibberellin,
which promotes the 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

Transcription
Factor
Transcription
Factor 51

• 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

• 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

 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

• 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

• 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

 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
• mcdonald.2@osu.edu
 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..0i8i30k1l2.3963
9.41517.0.41660.9.7.0.0.0.0.258.511.2-2.2.0....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
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SEED GERMINATION

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