Plant Structure, Embryogenesis & Seed Biology – BIOEB204 Lecture 2

Course Context & Learning Framework

• BIOEB204 Lecture 2: continuation of introductory module on plant structure (internal & external) before progressing to physiology.
• Builds on Lecture 1 recap: course starts with structural overview, then functional biology.
• Learning outcomes presented each lecture; not reiterated verbatim but guide study focus.
• No compulsory textbook; “Biology of Plants” (Raven et al.) recommended for depth; Campbell Biology useful for basic coverage.

Visualisation & Microscopy Techniques Introduced

• Confocal Laser Scanning Microscope (CLSM)
  • Uses lasers + pin-hole aperture to produce optical sections (non-invasive “slices”).
  • Multiple slices stacked → high-resolution 3-D reconstructions.
  • Demonstrated with multicoloured image of globular-stage plant embryo.
• Differential Interference Contrast (DIC) / Nomarski microscopy
  • Two prisms + polarised light split & recombine beams.
  • Phase differences in specimen create light/dark pseudo-3-D relief.
  • Ideal for thin, transparent, living tissues (e.g., early embryos).

Aesthetic & Mathematical Patterns in Plant Form

• Sunflower head: spiral arrangement of florets.
  • Emerges from iterative developmental program producing repeated units.
  • Spirals follow mathematical rules (e.g., Fibonacci phyllotaxis) ⇒ link between biology & mathematics.

What Defines a Plant? Key Diagnostic Characters

• Multi-trait definition required; no single exclusive feature.
• Eukaryotic organism.
• Photosynthetic capability via chlorophyll a & b—but photosynthesis alone not unique (cyanobacteria also perform).
• Cell walls composed of cellulose synthesised by plant-specific cellulose synthase complexes.
• Embryophyte life cycle:
  • Protected multicellular diploid embryo retained on/inside gametophyte.
  • Alternation of generations with two multicellular phases: gametophyte (n) and sporophyte (2n).
  • In angiosperms & gymnosperms the gametophyte is highly reduced but still present.
• Asexual as well as sexual reproduction common (e.g., stolons in strawberry).
• Indeterminate growth driven by meristems; enables plastic response to environment for sessile organisms.
  • Determinate growth also occurs but is organ-limited (e.g., leaves, flowers).

Basic Structural Terminology & Modular Construction

• Shoot system
  • Nodes: points where leaves attach.
  • Internodes: stem segments between nodes.
  • Axil: angle between leaf (petiole) & stem.
  • Axillary bud (lateral bud) located in axil; may develop into branch (new shoot).
  • Metamer/Phytomer (repeating unit): node + leaf + axillary bud + internode.
• Leaf anatomy
  • Petiole: leaf stalk; may be winged (diagnostic for species ID).
• Root system
  • Taproot: dominant central root with laterals (common in eudicots).
  • Fibrous/adventitious: numerous similar-sized roots arising from stem base (common in monocots, grasses).

Embryogenesis: Building the Primary Body

• Focus on sporophyte development from single-celled zygote.
• Four canonical stages (Arabidopsis model):
  • Globular stage
  • Zygote → asymmetric division: apical & basal cell.
  • Basal cell divides anticlinally → suspensor (stalk); apical cell divides → spherical embryo proper.
  • Heart stage
  • Two lateral bulges form → cotyledons (seed leaves) of eudicots.
  • Establishes bilateral symmetry.
  • Torpedo stage
  • Longitudinal elongation; clear root–shoot axis.
  • Procambium strands visible (future vascular tissue).
  • Maturation stage
  • Embryo desiccates, folds, accumulates storage compounds, becomes quiescent within seed coat.
• Development establishes two patterning frameworks:
  • Axial (apical–basal) pattern : shoot vs root poles.
  • Radial pattern : concentric tissue systems (dermal, ground, vascular).

Formation & Types of Meristems

• Meristem definition
  • Cluster of small, isodiametric, thin-walled cells with large nuclei.
  • Perpetually embryonic: one daughter remains an "initial"; the other becomes a "derivative" that divides/differentiates.
• Shoot Apical Meristem (SAM) & Root Apical Meristem (RAM) originate during embryogenesis.
• Primary meristems (immediately subjacent to SAM/RAM)
  • Protoderm → dermal (epidermis).
  • Ground meristem → ground tissue (parenchyma, collenchyma, sclerenchyma).
  • Procambium → primary xylem & phloem.
• Secondary meristems (post-embryonic)
  • Vascular cambium: secondary xylem/phloem (wood/bark) in woody species.
  • Cork cambium, pericycle (lateral root initiation), etc.
  • Formed by de-differentiation of certain primary derivatives; enable radial thickening & new organs.

Seed Structure & Storage Strategies

• Mature eudicot seed (e.g., bean, peanut)
  • Two large cotyledons (storage or absorptive depending on species).
  • Embryo axis between cotyledons.
  • Seed coat (testa) encloses.
• Eudicot variation: castor bean retains substantial endosperm; cotyledons mainly absorptive.
• Monocot seed (grass family, e.g., maize)
  • Single cotyledon (scutellum) functions as absorptive organ.
  • Endosperm (triploid) is primary nutrient store.
  • Protective structures: coleoptile (shoot sheath) & coleorhiza (root sheath).

Seed Dormancy & Desiccation Physiology

• Typical orthodox seeds
  • Lose water to <10 % fresh mass during maturation.
  • Metabolism arrests; remain viable for extended periods (weeks → years → centuries).
• Recalcitrant (unorthodox) seeds
  • Intolerant of desiccation; remain hydrated and metabolically active.
  • Cannot be stored dry; viability quickly lost.
• Approx. >90\% of species produce orthodox seeds; minority recalcitrant.
• Ecological & practical relevance
  • Forestry: Dipterocarpaceae (SE Asia) produce recalcitrant seeds; regeneration depends on persistent seedlings, not soil seed bank.
  • Conservation: Swamp maire (Syzygium maire) & Tainui (Pomaderris apetala subsp. maritima) have recalcitrant seeds ⇒ challenges for seed-banking.
  • Agriculture/Weed science: Velvetleaf (Abutilon theophrasti) forms long-lived seed bank (viable \approx50 years); dormancy complicates eradication.
• Types / mechanisms of dormancy (selection of examples)
  • Physical (coat-imposed): impermeable testa blocks water/oxygen (e.g., kowhai \textit{Sophora}; scarification enhances germination).
  • Physiological/chemical: embryo growth inhibited hormonally until conditions met.
  • Morphological: embryo under-developed at dispersal; requires after-ripening.
  • Combinational & environmental (light, temperature) cues.

Seed Germination Patterns

• Eudicots
  • Epigeal germination ("above ground"): hypocotyl elongates, lifts cotyledons into light (e.g., Phaseolus bean).
  • Hypogeal germination ("below ground"): epicotyl elongates; cotyledons stay subterranean (e.g., pea).
• Monocots / Grasses
  • Coleoptile pierces soil, protecting emerging shoot; coleorhiza protects root primordium.
  • Onion (Allium) example of monocot with seed + cotyledon emerging above soil.

Broader Significance & Integrative Themes

• Indeterminate, modular growth contrasts with determinate animal development; underlies ability to continually respond to environment.
• Embryogenesis research (Arabidopsis models, advanced microscopy) illuminates gene networks controlling body plan—parallels with animal developmental genetics.
• Seed biology intersects with:
  • Biodiversity conservation (ex-situ seed banks, restoration ecology).
  • Biosecurity & invasive-species management (weed seed longevity, dormancy cycling).
  • Climate-change resilience (germplasm preservation).
  • Ethical considerations: equitable sharing of seed resources, protecting indigenous taonga species, safeguarding food security.
• Practical applications reinforced through local NZ examples (AgResearch weed labs, Massey seed bank, threatened-plant gardens).

Key Take-Home Concepts for Examination

• Multi-criterion definition of “plant” & significance of alternation of generations.
• Microscopy modalities (CLSM vs DIC) and what structures they resolve.
• Metameric construction & terminology (node, internode, axil, petiole, taproot vs fibrous roots).
• Four embryogenesis stages (globular, heart, torpedo, maturation) and what anatomical patterns they set.
• Meristem hierarchy: apical, primary, secondary; role in indeterminate growth.
• Differences in seed structure/storage between eudicots & monocots; function of endosperm vs cotyledon.
• Definitions & ecological consequences of orthodox vs recalcitrant seeds; dormancy mechanisms.
• Epigeal vs hypogeal germination, coleoptile/coleorhiza terminology in grasses.
• Real-world relevance: forestry regeneration, weed management, conservation seed-banking.