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PLANTBIO > Embryogenesis + Seed Development > Flashcards

Embryogenesis + Seed Development Flashcards

1
Q

Embryogenesis and Seed development

A
  • embryo (2n) development
  • endosperm (2n) development
  • seed development
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2
Q

embryo (2n) development

A

2n = diploid

- patterns and meristem regions

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3
Q
  • endosperm (2n) development
A

3n = triploid

- storage materials and signals

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4
Q
  • embryo development
A

single fertilized egg –> mature embryo –> embryonic growth (seedling) –> post embryonic growth (mature plant)

Seedling:

  • radial axis has epidermis, cortex/ ground tissue, vasculature
  • apical-basal axis has cotyledons, shoot meristem, hypocotyl, root and root meristem
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5
Q

Steps for embryo development

A

9 steps:

  1. elongation
  2. asymmetric cell division
  3. the octant stage
  4. dermatogen stage
  5. globular stage
  6. transition stage
  7. heart stage
  8. torpedo stage
  9. mature embryo
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6
Q
  1. elongation
A
  • elongation of the fertilized egg
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7
Q
  1. asymmetric cell division
A
  • the first cell division is asymmetric (uneven/unequal distribution)
  • ordered division

apical cell (top): gives rise to the majority of the structure of mature embryo, undergoes many cell divisions

basal cell (bottom): forms suspensor, connecting the embryo to maternal tissue and the root cap meristem

fertilized ovule (diploid zygote): divides to give two cells:

  • apical daughter cell
  • basal daughter cell
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8
Q
  1. the octant stage
A
  • the apical cell divides 3 times to create a sphere of 8 cells, an octant, and generates 2 domains
  • the basal cell divide transversely to create a file oc cells called suspensor
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9
Q
  1. dermatogen stage
A
  • 8 cells divide once to generate protodermal cells
    (tangential cell divisions occur, creating tissue layers)
  • outside cells become part of epidermal cells
  • hypophysis is formed, suspensor cells undergo programmed cell death
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10
Q
  1. globular stage
A
  • the inner cells divide actively, endowing the embryo a recognizable axis
  • more division occurs
  • early globular –> late globular (lens-shaped cell)
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11
Q
  1. transition stage
A
  • apical domain: begin to generate 2 symmetrically positioned cotyledon primordia
  • basal domain: form a radially patterned cylinder (from hypocotyl), hypophysis begin to develop into root apical meristem

–> cells begin to differentiate

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12
Q
  1. heart stage
A

heart shaped structure (2 cotyledons begin to form)

  • apical domain: cotyledon outgrowth, establishing shoot apical meristem (SAM)
  • basal domain: establishing shoot axis, establishing root apical meristem (RAM)
  • further establishing basic tissue types: cortex, provascular tissue, and protoderm
  • suspensor cells undergo programmed cell death
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13
Q
  1. torpedo stage
A
  • enlargement of cotyledons and hypocotyl
  • vascular differentiation is visible
  • suspensor cells undergo programmed cell death
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14
Q
  1. mature embryo
A
  • cotyledon is bent (for some plants)
  • cell layers are clearly visible, specifying tissue and organ types
  • embryo arrests and awaits desiccation and dormancy - protective mechanism
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15
Q

Embryo development

A
  • pattern establishment: apical/basal and radial axes

- establishment of meristem regions: SAM and RAM

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16
Q

SAM

A
  • shoot apical meristem
  • give rise to the shoot system of a plant
  • Initiated in the apical domain at the dermatogen stage (16-cell) and well established at the heart stage.
  • WUSCHEL (WUS) and CLAVATA genes play an
    essential role to maintain the integrity of SAM.
  • WUS and CLV as molecular markers
  • CLV genes are located above WUS genes throughout plant life cycle –> patterning of spatial distribution of gene expression
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17
Q

RAM

A
  • root apical meristem
  • Give rise to most of the root system of a plant
  • Initiated from the suspensor as hypophysis at the
    octant stage and established at the heart stage.
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18
Q

Embryogenesis: endosperm development

A
  • endosperm: part of the seed, transiently exist, triploid strcture
  • Provide nutrients/signals for embryo development and seed germination
  • Developmental stages are not well understood/defined - NO STRUCTURE
  • Morphologically different in different
    species
19
Q

Endosperms

A
  • prominent in most monocots
  • monocots are flowering plants whose seeds typically contain only one embryonic (cotyledon) leaf - wheat, cereals, rice, maize kernel
20
Q

Morphological variation of endosperms

A

Most dicots: large cotyledons, small endosperms.

tobacco: has endosperm

arabidopsis thaliana: remnant endospore, one cell later

storage cotyledons: pisum sativum –> no endospore

21
Q

Seed development

A
  • Cessation of embryo and endosperm growth
  • Seed desiccation : up to 95% of water, drying out, tolerance: ability to survive the removal of water
  • Seed dormancy: seeds are prevented from germinating even under favorable environmental conditions
22
Q

Nutrient composition in seeds

A

major storage organ (endosperm) - Cereals, including barley, maize, oats, wheat –> higher carb content

major storage organ (cotyledon) - Legumes, including broad bean, garden pea, peanut, soyben –> higher protein content

23
Q

How to study embryogenesis & seed development?

A
  • Direct observation of developing embryos
  • Biochemical studies
  • Molecular and genetic studies
  • Dissection/laser ablation studies
24
Q

Direct observation of developing embryos

A
  • hand pollination
  • fix the flowers at different time points
  • section the fixed tissue to identify the developing embryo
25
Q

Biochemical analysis of hormone profiles

A
  • take embryo out, grind it, measure chemical components
  • seed development:
  • -> CK: cell division
  • -> GA, IAA: cell elongation for GA, and cell division + cell elongation for IAA
  • -> ABA: promote desiccation and dormancy, inhibit germination
  • germination and seedling development:
  • -> GA: promote growth and germination
  • -> IAA
26
Q

Hormones and embryogenesis

and seed development

A
  • Cytokinins: high during early embryo development, coinciding with the high rate of cell division
  • IAA: contribute to both cell division, enlargement, and differentiation during embryogenesis
  • GA: regulate cell enlargement during embryogenesis
  • ABA: appear at the late stages of embryo development.
27
Q

hormones for early seed development

A

cytokinins: cell division
GA: cell elongation

28
Q

hormones for mid-seed development

A

GA: cell elongation, auxin for cell growth, proper embryo development
IAA/ auxin: both cell division, enlargement and differentiation

29
Q

hormones for late-seed development

A

ABA: inhibits seed germination and promotes seed desiccation and dormancy, prevents precocious seed germination

30
Q

Molecular and genetic studies:

A role of auxin in embryogenesis & seed development

A

auxin biosynthesis:

- the yucca mutants: not auxin deficient mutant –> does not have embryo defect, not completely-auxin deficient mutant

31
Q

Auxin transport

A
  • Early embryo of Brassica juncea were cultured in the presence of a chemical to inhibit auxin transport
    a. wild-type control: heart-shaped, auxin is required to form this structure
    b. wild-type with the presence of trans-cinnamic acid (inhibited auxin transport): produces a cup shape
32
Q

PIN1

A
  • PIN1 is an auxin transporter
  • pin1 mutant lacks lateral organs (severe defects)
  • pin1 can be rescued by a local auxin treatment.

–> pin1 genes act at different stages of embryo development and function to ensure proper auxin transport happens throughout the process

33
Q

Mutant analysis: the PIN1 family

A
  • 8 PIN1 like proteins in Arabidopsis.
    • single mutation in other members does not have an
    altered phenotype
    • dut double, triple, and quadruple do:

double mutations: pin4, pin7 result in fused cotyledons and shortened hypocotyl

triple mutations: pin1, pin3, pin4 result in fused cotyledons, shortened hypocotyl and root

quadruple mutations: pin1, pin3, pin4, pin7 result in fused cotyledons and/or shortened hypocotyl and root

34
Q

PIN1

A

• PIN1 is uniformly localized in 1-32 cell embryo, then becomes localized to the basal end of provascular cells, and also pumps auxin downward into the suspensor.

• PIN3 begins to accumulate late, at the heart stage, so
may not really play an important role in patterning
embryo.

• PIN4 accumulates in the progenies of hypophysis and
in the provascular cells, and also pumps auxin downward
into the suspensor.

• PIN7 transports auxin from the basal cell into the
apical cell of 2-cell embryo, but by 32-cell stage
switches polarity and pumps auxin downward, from the
hypophysis into the suspensor.

35
Q

Auxin signaling

A
  • monopteros (mp): no root, short hypocityl; auxin response transcription factor
  • bodenlos (bd): no root, aux/iaa protein

–> apical-basal patterning of an embryo is affected

36
Q

Auxin affects apical-basal patterning during embryogenesis

A

auxin gradients:
- Up to globular stage: auxin levels are high in the apical
domain.
- Afterwards, auxin levels are high in the basal domain.

37
Q

Auxin also affects radial patterning during embryogenesis

A 
- At the transition and heart
stages, auxin is synthesized
at the apex but is
transported away from the
central SAM region
  • auxin is also synthesized in
    basal domain and transported
    to cotyledon regions
38
Q

additional factors that control germination

A
  • ethylene opposes ABA action and promotes germination
  • sugars, depending on concentration, can promote or oppose germination
  • seed coat inhibits germination
39
Q

is auxin a morphogen?

A
  • concentration gradient across developing tissue/ organ
  • governs pattern formation in an organism

morphogen - can modify cell specification, pattern formation in a concentration-dependent manner
- different concentration leading to different outputs

40
Q

Signaling between embryo and suspensor

A

If the embryo proper is dissected from the suspensor, the suspensor develops into an embryo.

  • signaling occurs at top
41
Q

Signaling between embryo and suspensor

A
  • twn2 mutants

- -> -apical cell progeny die, and suspensor produces one or two fairly normal embryos, then seedlings

42
Q

How long can seeds remain dormant?

A
  • seeds have long storage time

1879: 20 bottles of seeds buried
- -dug up at 5-10 year intervals and tested
- -after 10 years, most still viable
- -by 2000, 2 species still alive

• lotus seeds carbon dated as 1500-3000 years old have
germinated

• ability to remain dormant:

  • -species specific
  • -can be very long
43
Q

Seed dispersal

A

Animal dispersal:
• seeds are inside fleshy fruits
• Seeds with barbs or other structures
• Seeds can be collected and buried by animals

Wind dispersal: some seeds have wings or other hairlike
or feather-like structures.

Water dispersal: aquatic plants and plants that live near water have seeds that can float.

other ways: “shoot” seeds out of pods

PLANTBIO (15 decks)

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