Embryogenesis + Seed Development

Question 1

Embryogenesis and Seed development

Answer 1

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

Question 2

embryo (2n) development

Answer 2

2n = diploid

- patterns and meristem regions

Question 3

• endosperm (2n) development

Answer 3

3n = triploid

- storage materials and signals

Question 4

• embryo development

Answer 4

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

Question 5

Steps for embryo development

Answer 5

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

Question 6

1. elongation

Answer 6

• elongation of the fertilized egg

Question 7

2. asymmetric cell division

Answer 7

• 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

Question 8

3. the octant stage

Answer 8

• 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

Question 9

4. dermatogen stage

Answer 9

• 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

Question 10

5. globular stage

Answer 10

• the inner cells divide actively, endowing the embryo a recognizable axis
• more division occurs
• early globular –> late globular (lens-shaped cell)

Question 11

6. transition stage

Answer 11

• 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

Question 12

7. heart stage

Answer 12

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

Question 13

8. torpedo stage

Answer 13

• enlargement of cotyledons and hypocotyl
• vascular differentiation is visible
• suspensor cells undergo programmed cell death

Question 14

9. mature embryo

Answer 14

• 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

Question 15

Embryo development

Answer 15

• pattern establishment: apical/basal and radial axes

- establishment of meristem regions: SAM and RAM

Question 16

SAM

Answer 16

• 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

Question 17

RAM

Answer 17

• 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.

Question 18

Embryogenesis: endosperm development

Answer 18

• 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

Question 19

Endosperms

Answer 19

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

Question 20

Morphological variation of endosperms

Answer 20

Most dicots: large cotyledons, small endosperms.

tobacco: has endosperm

arabidopsis thaliana: remnant endospore, one cell later

storage cotyledons: pisum sativum –> no endospore

Question 21

Seed development

Answer 21

• 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

Question 22

Nutrient composition in seeds

Answer 22

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

Question 23

How to study embryogenesis & seed development?

Answer 23

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

Question 24

Direct observation of developing embryos

Answer 24

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

Question 25

Biochemical analysis of hormone profiles

Answer 25

- 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

Question 26

Hormones and embryogenesis | and seed development

Answer 26

- 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.

Question 27

hormones for early seed development

Answer 27

cytokinins: cell division GA: cell elongation

Question 28

hormones for mid-seed development

Answer 28

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

Question 29

hormones for late-seed development

Answer 29

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

Question 30

Molecular and genetic studies: | A role of auxin in embryogenesis & seed development

Answer 30

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

Question 31

Auxin transport

Answer 31

- 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

Question 32

PIN1

Answer 32

- 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

Question 33

Mutant analysis: the PIN1 family

Answer 33

- 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

Question 34

PIN1

Answer 34

• 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.

Question 35

Auxin signaling

Answer 35

- 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

Question 36

Auxin affects apical-basal patterning during embryogenesis

Answer 36

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

Question 37

Auxin also affects radial patterning during embryogenesis

Answer 37

``` - 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

Question 38

additional factors that control germination

Answer 38

- ethylene opposes ABA action and promotes germination - sugars, depending on concentration, can promote or oppose germination - seed coat inhibits germination

Question 39

is auxin a morphogen?

Answer 39

- 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

Question 40

Signaling between embryo and suspensor

Answer 40

If the embryo proper is dissected from the suspensor, the suspensor develops into an embryo. - signaling occurs at top

Question 41

Signaling between embryo and suspensor

Answer 41

- twn2 mutants | - -> -apical cell progeny die, and suspensor produces one or two fairly normal embryos, then seedlings

Question 42

How long can seeds remain dormant?

Answer 42

- 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

Question 43

Seed dispersal

Answer 43

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