Early Development of the Plant Body

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Learning Objectives

1. Understand the role of pattern formation in embryogenesis.
2. Describe the steps of embryogenesis.
3. Identify the embryonic organs.
4. Describe the steps of seed maturation.
5. Compare and contrast quiescence and dormancy.
6. Compare and contrast multiple types of monocot and eudicot seedling development.

Solutions

1. Plants are modular, unlike animals. The patterns of the three organs (roots, shoots, and leaves) are repeated over and over as the plant grows. These patterns are established during embryogenesis, and including apical-basal and radial patterns.
2. Embryogenesis can be broken down into a few steps. Single cell → apical and basal cells → globular stage (4 cells, development of protoderm, nucleus, endosperm) → heart stage (cotyledons appear, procambium, root tip, SAM/RAM clearly present) → torpedo stage (cotyledons bend, primary meristems obvious, protoderm procambium and ground meristem, suspensor disappears)
3. Roots, shoots, and leaves originate as the radicle, cotyl, and cotyledons. Ground, dermal, and vascular tissue originate as the ground meristem, protoderm, and procambium.
4. Seed formation has 3 main steps. First, it stores food. Secondly is dessication, where metabolism and water content is reduced. Third is that the seed coat hardens.
5. During dormancy, seeds won't sprout even with favorable conditions, defined as the presense of O₂, water, and warmth. During quiescence, seeds will sprout only when all three are present.
6. In eudicots, hypogeous → hypocotyl sprouts first, and epigeous → epicotyl sprouts first. In monocots, hypogeous → coleoptile sprouts first, and epigeous → bent cotyledon sprouts first.

Notes

Embryogenesis

• Pattern formation
• Plants are modular
• Patterns have to be established because patterns are repeated over and over
• Unlike animals; repetitious and continuous development of the same 3 organs (root, stems, and leaves)
• Established during embryogenesis
• 2 patterns
• Apical-basal: establishes up versus down (roots versus shoots); 2 apical meristems
• Radial: establishes inward verses outward; inside pattern that very early shows which layer is which, like what will be epidermis etc

Meristems Produce New Cells

• Establishment of apical-basal axis is a critical event in plant embryogenesis, evident from the earliest stages
• Polarity evident in embryo sac, egg cell zygote, and embryo-suspensor complex
• Apical-basal pattern elements are, rfom top to bottom, shoot meristem, one or two cotyledons (embryonic leaves), hypocotyl (embryonic stem), radicle (embryonic root), and root meristem

Radial Patterning

• Vascular tissue is arranged in a different way in roots, stems, and leaves, and/or as monocot and eudicot, but has the same general pattern
• Epidermis: same as mature plant
• Vascular tissue: one "tube" at the center of plant, not small or big
• Ground tissue: all the tissue between the epidermis and the vascular tissue

Apical-Basal Polarity

• Right after fertilization, we have a single cell
• Even this early, the cell has polarity; specific proteins that arelocalized to one end of that cell, so that when the single cell divides, it divides asymmetrically
• Apical cell is smaller than the basal cell (top versus bottoms = polarity)

Suspensor Formation

• Apical-basal cell will continue to divide and eventually give rise to the embryo proper
• Basal cell will divide transversely several times, and that group of cells will compose what we call the suspensor
• The suspensor functions as an umbilical cord:
• Pushes the embryo proper up into the main space in the ovule; will see the embryo inside an ovule. "Remember egg down by micropyle opening when it gets fertilized by the sperm cell is still in that very tiny space not enough room to grow" (I don't know what he meant to say; this is word-for-word what was written.)
• Suspensor pushes it up into larger space; also anchors embryo to materal tissue, allowing nutrients and hormones in from the maternal tissue
• It doesn't last forever; suspensor goes away during development

Radial Patterning & Tissue Formation

• As embyro continues to divide, we begin to see differentiation of inner and outer
• Falsely colored yellow (single outer layer of cells) becomes protoderm that will develop into the epidermis (dermal tissue)
• Soon will see radial patterning of tissue
• 1^st: established top and bottom (immediate)
• 2^nd: established in and out (need more cell to grow to the side before radial patterning can mathematically happen)

Eudicot Embryogenesis

• "Embryo in context"
• 2 cell embryo has a larger basal cell and smaller apical cell
• Embryo is crowded into a small space
• 4 cells: suspensor pushes embryo proper upwards
• Additional divisions take place and we move towards the globular stage
• Globular stage: from 4 cells to the development of the protoderm
• Nucleus is a tissue in teh developing oule of a plant that supports the development of the embryo and endosperm
• The nucleus also provides nutrition for the female gametophyte and young embryo
• After globular, we movve to the heart stage
• Heart stage: due to 2 cotyledons
• Beginning of shoot apical and root apical meristem tissues
• Suspensor is not the root; it's like an umbilical cord that degrades and goes away
• Notice the protoderm (becomes dermal tissue) and the procambium (becomes primary xylem and phloem)
• Ground meristem is the remaining cells that are not protoderm or procambium
• During the heart stage, the shoot apical meristem (SAM) and the root apical meristem (RAM) are first clearly present
• After the heart stage is the torpedo stage
• Torpedo stage: elongated and fills ovule, but not yet bent
• Eventually, cotyledons run out of room and bend back on themselves, filling up the entire ovule
• Suspensor starts to shrivel and it's gone by the time the ovule is filled
• Distinguish between root tip and suspensor
• Primary meristems- protoderm, ground meristem, and procambium

Monocot Embryogenesis

• Have a similar development to eudicots
• Main different is that they only have one cotyledon; they have all the same parts, suspensor, and primary meristems

Seed Anatomy

• Embryogenesis leads to seeds eventually
• Eudicots:
• Cotyledons have absorbd all the endosperm; two halves of a peanut or bean are those cotyledons
• If you were to open them up, you would see structure between those that a little bit of embrionic stem above those cotyledons (epicotyl); hypocotyl is embrionic stem tissue that is below cotelydons
• Integuments mature and become the seed coat
• Exceptions in eudicots: some don't absorb all the endosperm (ex: castor beans)
• Monocots:
• Onion has on cotyledons and doesn't take up all the endosperm
• Corn is a monocot that is very unique; corn is actually a fruit with the seed completely fuzed to the pericarp
• Maize has outgrowth called coleorhize that protects the radical
• Maize: scutellum = cotyledon
• Scutellum has a small outgrowth during development that eventually becomes the coleoptile

Seed Maturation Steps

• Once the embryo matures into a seed, there are steps that need to occur
1. Food storage. Seeds can't perform photosynthesis while underground in soil.
2. Desiccation. The seed dries out, water content is reduced, and respiration and cell division come to a halt.
3. Seed coat hardens.

Seed Germination

• Quiescence: seeds cannot germinate without water, oxygen (respiration), and warmth (enzyme function)
• Dormancy: some seed fail to germinate even under favorable conditions. Can be broken by:
• Scarification: mechanically wounding the seed coat (stepping on it, going through digestive tract, etc)
• Stratification: exposing the seed to cold temperatures
• Dormancy is evolutionarily beneficial; seeds which are ready in the fall need to go through stratification of the winter (prevents sprouting in the fall)
• Works better is there is some proportion of staggered dormancy, so they aren't all in competition at the same time

Eudicot Seed Development

• Epigenous germination: epicotyl sprouts up first
• Hypogenous germination: hypocotyl sprouts up first
• The very first sign of germination is the radicle breaks through the seed coat
• Radicle: embrionic root

Monocot Seedling Development

• Hypogenous development: coloeptile sprouts first
• Epigenous development: bent cotyledon sprouts first
• Maize: scutellum = cotyledon
• Scutellum has a small outgrowth during development that eventually becomes the coleoptile