Plant Development: Ontogeny and Phylogeny Quiz

Development

• Root word: Develop
• Old French (1590s): Desvoloper
• Meaning: Unveil, reveal
• Development (Online etymology dictionary, 2021)
  • gradual unfolding
  • disclosure of the details of something
  • advancement through progressive stage (1836)

Ontogeny vs Phylogeny

• Both are developmental processes
• Ontogeny
  • History of an organism
  • From immature (youngest) to mature (old) stage
  • Example: From seed to tree
• Phylogeny
  • History of a species (evolution)
  • From a simple (primitive, earliest) form to advanced (advanced, recent) condition
  • Example: From unicellular to multicellular state (Choanoflagellaters → choanocytes)

Human Embryonic and Foetal Development

• Stages of development are shown in an image
• Fertilized egg, 2-cell stage, 4-cell stage, 8-cell stage, 16-cell stage, Blastocyst
• Foetus at 4 weeks, 10 weeks, 16 weeks, 20 weeks
• Image shows multiple stages leading to a fetus

Plant life cycle

• image shows maize and other plants life cycle with various stages

Angiosperm ontogeny

• Gametogenesis
• Double fertilization by pollination (zygote formation)
• Seed formation
• Embryo formation
• Proembryo formation
• Formation of 3 cell type layers (LI, L2, L3)
• Endosperm formation
• Dormancy and Germination
• Ageing, senescence, death

Animal ontogeny

• Fertilization (zygote formation)
• Cleavage (blastomere formation)
• Gastrulation (germ layer formation)
• Organogenesis (organ formation)
• Neurulation (Nervous system formation)
• Growth and histological differentiation
• Ageing, senescence, death

5 major overlapping developmental processes

• growth,
• cell division,
• differentiation,
• pattern formation,
• morphogenesis

Growth

• Increase in size of an organism
• Can result from increase in:
  • Cell size
  • increasing cell volume or by growth of extracellular structures (e.g. cartilage & bone)
  • Cell number via mitosis

Differentiation

• process that generates different cell types
• Results in cells becoming structurally & functionally specialized via the synthesis of different proteins

Pattern formation

• each cell must behave corresponding to its position in the embryo to form differentiated cell types
• Regional specification
• embryonic cells become specialised according to their position
• Axis specification: 1st patterning event in the embryo
• establishment of principal body axes (e.g., anteroposterior, dorsoventral)

Morphogenesis

• creation of shapes & structures
• governed by different forms of cell behavior, including:
  • differential rates of cell proliferation,
  • changes in cell shape & size,
  • movement of cells relative to each other (e.g. adhere; disperse),
  • cell fusion,
  • cell death (apoptosis)

Epigenesis vs Preformation

• Epigenesis: gradual developmental strategy,
• a simple embryo (few cell types organized crudely) → gradual refinement → a complex organism (many cell types with highly detailed organization)
• Preformation: early embryo comprised a miniature version of the adult, development consisted entirely of growth

Flower: an evolutionary strategy

• Primitive plants (thallophytes, bryophytes, pteridophytes)
  • naked, mobile sperm for sexual reproduction
  • requires water for sperm to swim to reach egg
• Tracheophytes (With vascular system (xylem, phloem) & elongated stem)
  • Exposure of most plant parts to hostile dry environment → reproductive structures cannot depend on free water to unite male & female gametes
  • Solution: Replacement of naked sperm with pollen grains to transfer to female organs by wind or other vectors
• Solution: Gymnosperms
  • Pollen disseminated by random wind currents, naked ovules (no ovary) lay on cone scales
• Solution: Angiosperms
  • include ~300,000 species, largest, most successful plant group
  • reproduce both sexually & asexually
  • depend on biological pollinators to transfer pollen to female organ

Sexual reproduction

• entails fusion of gametes (reproductive cells: eggs & sperm cells)
• Union of gametes occurs within the flower's ovary

Asexual reproduction

• does not usually involve flower, seed, & fruit formation
• offspring are formed when a vegetative structure (stems, leaves, or roots) of an existing plant expands → separates
• involves only one parent; no fusion of gametes
• offspring genetically identical to each other & to parent plant

Flower: Common plan

• flower is a reproductive shoot
• often consists of 4 kinds of organs: sepals, petals, stamens, carpels, arranged in whorls (circles) on the enlarged end of a stalk
• flower stalk supports a flower

Sepals & Petals

• Most leaf-like of the floral structures
• sterile organs/appendages
• not involved in the reproductive process
• Sepals comprise the outermost & lowest whorl on a floral shoot
• leaf-like in shape & form

Male reproductive structures (Stamen)

• collectively called androecium ("house of man")
• 2 Components (Stamen):
  • a 2-lobed anther
  • saclike structure
  • for pollen grains formation
  • stalk (Filament)
    • slender
    • modified leaf midrib in most angiosperms
    • bears the anther

Sexual reproduction

• pollen grains must be transferred from anther to carpel (often of another flower of same species)
• each pollen grain consists of two cells surrounded by a tough outer wall
  • one cell generates two male gametes (sperm cells)
  • the other cell forms a pollen tube allowing sperm travel to the ovule

Flower: Female Structures (Pistil)

• Simple: consists of 1 carpel

• Compound: a group of fused or free carpels

• Each carpel has 3 parts:

  • Stigma: traps pollen grains
  • Style: a neck-like structure through which the pollen tube grows
  • Ovary: a jug-like structure with 1 or more ovules which develop into a fruit -Each ovule (megasporangium) has 1 embryo sac (female gametophyte: megagametophyte)

• Egg & polar nuclei participate directly in fertilization

Carpel (megasporophyll): female reproductive structure

• Modified leaves
• located at center, topmost (1 or more) in most flowers
• collectively called as gynoecium ("house of woman")
• bears ovules which develop into seeds
• may be separate or fused into a single structure (Pistil)

Female reproductive structures

• Simple: consist of 1 carpel
• Compound: a group of fused or free carpels
• Each carpel/group of fused carpels has 3 parts:
  • Stigma: traps pollen
  • Style: pollen tube grows
  • Ovary: developing into a fruit with one or more ovules

Types of pistil

• 1 Carpel: Simple pistil- 1 carpel
• 2+ carpels: Compound pistil- 2 or more carpels fused

Female reproductive structures

• Stigma: located at the top of the carpel, receptive surface for pollen collection • Glandular tissue secretes a sugary solution helping to trap the pollen, facilitating germination & growth • Some have a sticky sugar and they help in trapping the pollen that lands on the stigma

Embryo sac

• part of the ovule
• develops into a seed after fertilization
• integuments, nucellus, embryo sac
• has the female reproductive cells

Floral-identity genes

• turned on by flowering-time genes
• commit the meristem to the production of floral primordia
• in turn activate the organ-identity genes, controlling the development of sepals, petals, stamens, carpels

Ovary types

• Superior ovary: other floral organs (sepals, petals, stamens) are free from the ovary, attached at the ovary's base
• Inferior ovary: sepals, petals, stamens are attached to the top of the ovary, located below the point of attachment of other floral organs, ovary is contained within the receptacle

From Leaves to Flowers

• Vegetative apical meristem: an indeterminate shoot, produces leaves
• indeterminate: continually renews itself producing an unlimited of leaves
• When flower signal reaches a vegetative apical meristem → switches from leaf primordia to floral part production → becomes a determinate shoot → floral meristem: limited to 4 whorls (sepals, petals, stamens, carpels)
• all four floral appendages are modified leaves

Flowering time genes

• determine when & how the apex of a species is susceptible to a flower stimulus

Floral-identity genes

• turned on by flowering-time genes
• commit the meristem for floral primordia production rather than leaf primordia
• in turn activate organ identity genes
• control the progressive development of floral organs

What flower part produces ...?

• Nectar
• Nectar glands (nectarries)
• secrete sugar sap (nectar) continuously from ~20 days
• derived from modified petals; may be located on receptacles, sepals, petals, base of filaments or carpels
• consists of a single layered epidermis, nectar-producing parenchyma & photosynthesising parenchyma
• Nectar secretion is holocrine; nectar is released by the rupture of wall & cuticle of each epidermal cell
• Fragrance
• Petals have essential oils and glucosides

Sporogenesis & Gametogenesis

• Microsporogenesis:
  • Stamen (anther, filament) = microsporophyll
  • Pollen sac = microsporangium
  • Microspore mother cell = pollen mother cell, microspocyte
  • Pollen grain = Male gametophyte, Microgametophyte
  • Sperm, pollen tube formation
• Megasporogenesis:
  • Carpel (stigma, style, ovary) = megasporophyll
  • Ovule = megasporangium
  • Megaspore mother cell = megasporocyte
  • Embryo sac = Female gametophyte, Megagametophyte
  • Egg cells, central cells, antipodals, synergid formation

POLLEN

• Stamen = microsporophyll
• Pollen sac = microsporangium

Microsporogenesis

• Meiosis→ immature pollen grains → Mitosis → mature pollen grains
• Generative cells (GC) & Tube cells (TC) →
• immature pollen grains (1), immature mature pollen grains (4).

Eggs

• Carpel = Megasporophyll • Ovule = Megasporangium • Megaspore mother cell = megasporocyte • Embryo sac = Female gametophyte = Macrogametophyte • Egg cell, central cell, antipodals, synergid formation

Integuments

• Layers of maternal protective cells that develop to become the seed coat

Egg production

• Megaspore Mother Cell (2n Megasporocyte) → Meiosis → 4 haploid daughter cells → 3 die, 1 survives and undergoes 3x Mitoses before cytokinesis to form 8 nucleated embryo sac (mature female gametophyte) → 1 egg cell; 1 central cell; 3 antipodal cells; 2 synergid cells

Life Cycle of Flowering Plants

• Pollen from anther transferred to the stigma (of carpel) → pollination
• Self-pollination, where pollen travels short between anther & stigma within a single flower → genetically homogenous
• Plants that can't self-pollinate need cross-pollination, where pollen of 1 plant carries to a flower of another plant

Coevolution of angiosperms & animal pollinators

• Insects see UV radiation (invisible to humans), used as color guides.
• Many flowers have UV markings (nectar guides) to attract insects to the center of flower.
• Nectar guides direct insects to the center of the flower where pollen grains and nectar are located.
• Insects have well-developed sense of olfaction.
• Bird-pollinated flowers are often red, orange or yellow, no scent for the birds.
• Birds do not have a strong sense of smell.
• Hummingbird-pollinated flowers has a long, tubular corolla with nectar glands at the bottom.
• Bat-pollinating flowers : feed at night, poor eyesight, important pollinators in many tropical regions, night blooming, often have dull white petals, a strong scent, often of fermented fruit.

Coevolution of "i'iwi" & tubular flowers

• I'iwi: Hawaiian honeycreeper with long, curved beaks for nectar collection
• Long, tubular corollas of flowers developed through coevolution.
• Rarity of some tubular flower species in the 20th century a result of grazing

Some angiosperms are wind-dependent to disperse pollen grains

• Grasses, ragweed, maples, oaks
• many small, inconspicuous flowers
• lack large, colorful petals scent, or nectar
• large, feathery stigmas for trapping wind-borne pollen
• Low pollen grain likelihood of landing on the stigma of the same flower species

Germination

• Pollen shed from an anther is dry → absorbs water upon landing on the stigma.
• Tube cell forms a pollen tube that grows via 1 of the exine pores through the style to the ovule.
• Pollen tube enters the micropyle of the ovule; breaches embryo sac wall.
• Generative cell divides → 2 non-flagellated sperm cells.
• Sperm cells move down the pollen tube, enter the ovule & release sperm for double fertilization

Double fertilization: unique to angiosperm sexual development

• One sperm fertilizes the egg cell to form the diploid zygote.
• The other sperm fuses with the polar nuclei to form the triploid endosperm
• Nutritive tissue surrounds the embryonic plant in a seed; rich in lipids, proteins, carbohydrates
• Ovaries with many seeds develop into fruits. Pollen & pollen tube required for each seed.
• Ovule develops into a seed, surrounding ovary develops into the fruit.
• Nutritive endosperm nourishes growing embryo

Fruits

• Ovary wall expands with seeds.
• In some plants, other tissues associated with ovary enlarge to form the fruit.
• Fruits protect seeds and aid in their dispersal. Example of seed dispersal methods in different seed types
  • Seeds in fruits dispersed by wind (e.g., dandelion seeds), -Animals (e.g., edible fruits), -Water (e.g., coconuts), -Exploding fruits (e.g., touch-me-nots)

Mature embryo in the seed

• 3 main parts:
  • Radicle (short embryonic root)
  • Embryonic shoot
    • Hypocotyl
    • Plumule
  • Cotyledons

Germination

• Under ideal conditions, the seed germinates (sprouts), embryo resumes growth
• Embryonic plants are non-photosynthetic initially and need nourishment
• Cotyledons of many plants contain stored food reserves for early growth.

Plants & People | Seed Banks

• Many counties/countries in the world collect plant germplasm (seeds, plants, plant tissues of traditional crop varieties)
• to maintain diversity of plants & revive extinct varieties.
• International Plant Genetics Resource Institute (Rome, Italy) oversees plant germplasm collections worldwide.
• National Plant Germplasm System (Fort Collins, Colorado) stores seeds from ~250,000 plant species and varieties.

Advantages of Seed Banks

• Seeds stored in a small space, safe from habitat loss & climate change, diseases & predators, neglect.
• Used to reintroduce extinct species
• Safeguard seeds for future use, maintain genetic diversity of crop species, ensure valuable traits
• Cryopreservation (seed storage at -196°C in liquid nitrogen)

Seed banks’ limitations

• Seeds are not indefinitely alive, maintenance requires labor & expense, some species don't tolerate dehydration or freezing

Plants & the Environment | Seed Dispersal by Ants

• The exchange between flowering plants & ants is mutualistic.
• Ants are attracted to food bodies (elaiosome), and consume nutrients while dispersing the seeds in other locations.

Seed Germination & Early Growth

• Seed and fruit development follow pollination and fertilization
• each seed develops from an ovule
• contains embryo and endosperm
• Many factors affect seed germination:
• presence of water & oxygen
• appropriate temperature
• light penetration

Water

• Seed germination can't occur without sufficient water absorbed through their tissues (due to osmosis).

Seed energy & subsequent growth

• Oxygen is needed for germination.
• Plants obtain energy by converting fuel molecules from stored endosperm/cotyledons through aerobic respiration.
• Some plants can survive without oxygen initially.

Temperature

• Optimal germination temperature vary for each plant species.
• germination can occur within a broad range of temperatures for many species
• Some seeds need cold stratification before they can germinate.

Some environmental factors are needed (seed germination)

• Prolonged cold period for some seeds to germinate in spring.
• Some plants with tiny seeds need light for germination to occur.

Breaking dormancy

• Some seeds have very hard coats that prevent water and oxygen from entering

Mechanical scarification

• involves nicking seed coat with knife, file, or sandpaper

Chemical scarification

• involves soaking seeds in concentrated acids (e.g., sulphuric acid) to weaken the seed coat

Biological scarification

• involves passing seeds through the digestive tract of animals, (e.g., acid in animal gut helps)

Eudicots & Monocots exhibit characteristic patterns of early growth

• Radicle (1st part to emerge), shoots (stems, leaves), cotyledons (seed leaves).

Seed longevity

• Duration that seeds remain viable/capable of germinating.
• Some seeds can survive several years in suitable conditions.

Record for longevity (oldest viable seeds)

• Lotus plant (Nelumbo nucifera) from Manchuria, 1300 years old.
• Scarified seeds coats and seeds moisturised to encourage germination.
• Also, Judean date palm, 2000 years old.

Growth Responses & Regulation of Growth

• Plant hormones regulate size and growth development
• Plant responses to growth and environmental changes
• Plant growth factors

Grape plant

• climbing deciduous woody vines with coiled tendrils to support growth
• Size of grapes is increased by spraying with gibberellins during development. -Gibberellins elongates branches that bear flowers to help flowers expose to more air, thus preventing fungal attacks and improving fruit production.

2 types of plant growth

• Indeterminate growth (ability to grow infinitely)
• Determinate growth (ability to grow for a set amount of time)

Genetic, Hormonal, Environmental Controls of Growth & Development

• Genetic factors: Genes determine plant's growth / development traits (leaf shape, flower color, root systems) & their expression
• Environmental cues affect gene expression.
• Cell location influences gene expression during development.

Hormones: organic compounds

• Chemicals act as signal between cells
• Regulates plant growth and responses to several environmental changes.
• Environmental cues (e.g., changing day length, temperatures) affect hormone production, which affect plant growth & development cycles.
• Plants have mechanisms to regulate many growth processes for optimal plant development under several conditions

Growth Movements: Tropisms

• Plant directional growth in response to environmental stimuli (e.g., light, gravity, touch)

Phototropism

• directional growth response resulting in a change in the position of a plant part.
• Plant grows toward light (positive phototropism) or away from light (negative phototropism)
• Phototropins are light-sensitive substances that absorb light for plants to respond.
• Auxin moves laterally to the shaded side of the stem
• Cells on shaded side elongate more than lighted side, stem bends toward light

Gravitropism

• Most stem tips grow away from Earth's center (+)
• Most root tips grow toward Earth's center (-)
• Root cap detects gravity
• Amyloplasts: starch-containing cells; collect at bottom in response to gravity, initiating gravitropic response

Thigmotropism or Seismonasty

• growth/response to touch of an object (e.g., twining or curling stems, help climb)
• Mimosa pudica

Ethylene

• Produced by plants
• Affects fruit ripening, cell elongation, seed germination, and plant response to pathogens & wounding.

Abscisic acid (aa) as environmental stress response hormone

• Promotes changes in water-stressed plant tissues.
• Increases dramatically in leaves exposed to droughts, high levels trigger stomata closing & saves water.
• Plants' chance of survival increases through transpiration.

Low temperature of the winter: type of plant stress

• Seed dormancy involves plant adaptations to withstand winter temperatures. Many seeds have high aa levels in tissues, preventing germination until spring.
• Some seeds germinate soon as embryos mature, while still attached to the ear.

Additional signaling molecules

• Plant hormones & hormone-like signaling molecules affect plant growth.
• These hormones include brassinosteroids, jasmonates, salicylic acid, & systemin
• Involve defensive responses against disease and insects.
• Involved in cell division, elongation, germination & development of many processes.

Florigen (Fg)

• Flower-promoting substance in plants.
• Induced in long-day plants and transported to day-neutral plants (e.g., tobacco).

Day Length & Flowering

• The length of day and night, or photoperiod, impacts flowering in many plant species.
• Long-day plants need longer days to flower, short-night plants need shorter nights to flower, while day-neutral plants do not rely on day length for flowering.

Other environmental factors affecting flowering

• Temperature, (e.g. some plants need cold temperatures to flower) light

Test your knowledge on the concepts of ontogeny and phylogeny in plant development. This quiz covers their definitions, differences, and significance in understanding plant evolution and relationships. Perfect for students and enthusiasts in plant sciences.