Embryogenesis + Seed Development Flashcards
Embryogenesis and Seed development
- embryo (2n) development
- endosperm (2n) development
- seed development
embryo (2n) development
2n = diploid
- patterns and meristem regions
- endosperm (2n) development
3n = triploid
- storage materials and signals
- embryo development
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
Steps for embryo development
9 steps:
- elongation
- asymmetric cell division
- the octant stage
- dermatogen stage
- globular stage
- transition stage
- heart stage
- torpedo stage
- mature embryo
- elongation
- elongation of the fertilized egg
- asymmetric cell division
- 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
- the octant stage
- 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
- dermatogen stage
- 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
- globular stage
- the inner cells divide actively, endowing the embryo a recognizable axis
- more division occurs
- early globular –> late globular (lens-shaped cell)
- transition stage
- 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
- heart stage
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
- torpedo stage
- enlargement of cotyledons and hypocotyl
- vascular differentiation is visible
- suspensor cells undergo programmed cell death
- mature embryo
- 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
Embryo development
- pattern establishment: apical/basal and radial axes
- establishment of meristem regions: SAM and RAM
SAM
- 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
RAM
- 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.
Embryogenesis: endosperm development
- 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
Endosperms
- prominent in most monocots
- monocots are flowering plants whose seeds typically contain only one embryonic (cotyledon) leaf - wheat, cereals, rice, maize kernel
Morphological variation of endosperms
Most dicots: large cotyledons, small endosperms.
tobacco: has endosperm
arabidopsis thaliana: remnant endospore, one cell later
storage cotyledons: pisum sativum –> no endospore
Seed development
- 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
Nutrient composition in seeds
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
How to study embryogenesis & seed development?
- Direct observation of developing embryos
- Biochemical studies
- Molecular and genetic studies
- Dissection/laser ablation studies
Direct observation of developing embryos
- hand pollination
- fix the flowers at different time points
- section the fixed tissue to identify the developing embryo
Biochemical analysis of hormone profiles
- 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
Hormones and embryogenesis
and seed development
- 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.
hormones for early seed development
cytokinins: cell division
GA: cell elongation
hormones for mid-seed development
GA: cell elongation, auxin for cell growth, proper embryo development
IAA/ auxin: both cell division, enlargement and differentiation
hormones for late-seed development
ABA: inhibits seed germination and promotes seed desiccation and dormancy, prevents precocious seed germination
Molecular and genetic studies:
A role of auxin in embryogenesis & seed development
auxin biosynthesis:
- the yucca mutants: not auxin deficient mutant –> does not have embryo defect, not completely-auxin deficient mutant
Auxin transport
- 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
PIN1
- 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
Mutant analysis: the PIN1 family
- 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
PIN1
• 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.
Auxin signaling
- 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
Auxin affects apical-basal patterning during embryogenesis
auxin gradients:
- Up to globular stage: auxin levels are high in the apical
domain.
- Afterwards, auxin levels are high in the basal domain.
Auxin also affects radial patterning during embryogenesis
- 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
additional factors that control germination
- ethylene opposes ABA action and promotes germination
- sugars, depending on concentration, can promote or oppose germination
- seed coat inhibits germination
is auxin a morphogen?
- 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
Signaling between embryo and suspensor
If the embryo proper is dissected from the suspensor, the suspensor develops into an embryo.
- signaling occurs at top
Signaling between embryo and suspensor
- twn2 mutants
- -> -apical cell progeny die, and suspensor produces one or two fairly normal embryos, then seedlings
How long can seeds remain dormant?
- 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
Seed dispersal
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
