Plant Hormones BL1004

Plant Hormones BL1004

• Presented by Prof. Marcel Jansen, Plant Sciences, BEES, Enterprise Centre, Distillery Field.
• Email: [email protected]

Campbell and Reece Editions

• Edition 7 Overview + Figure 39.3, Concept 39.2: Plant hormone discovery & survey
• Edition 7, 8, 9 & 10 include auxin, cytokinins, gibberellins, abscisic acid, and ethylene.
• Ethylene only is responsible for senescence and fruit ripening in all editions.

Plant Hormones - Control of Growth and Development

• Hormones control plant growth and development with many examples.
• Examples include: sink-source relation, seed dormancy, seed germination, flowering, development of seed/fruit, formation of tuber/bulb/etc., senescence. and many other processes.

Plant Hormones and Development

• Hormones control plant growth, which includes increasing size or weight.
• They also control plant development, which is the progression of the plants' life cycle.

Coordination of Plant Growth

• Plant hormones coordinate growth and development in cells, tissues, and organs.
• They are essential for all multicellular organisms.
• Sessile organisms such as plants must adapt to environmental changes, with hormones playing a key role in this process.

Plant Hormones or Plant Growth Regulators

• Produced by multicellular organisms as internal chemical signals.
• Present in minuscule amounts, traveling throughout the organism.
• Coordinate metabolic activities and environmental responses.
• Not produced in glands.

Hormone Activity at a Cellular Level

• Receptor: Hormone or environmental stimulus binds to the receptor located in the cell membrane.
• Signaling cascade: Reception, transduction (relay molecules, activating cellular response).
• Cellular response: Altered transcription of specific genes, altered enzyme activity by post-translational regulation; activation can be up or down.

Hormone Reception

• "Hormone" is of Greek origin, meaning "to excite."
• Specific receptor for each hormone type.
• Located in the cell membrane.
• Present at very low concentrations.

Identifying a Hormone Receptor

• Biochemical purification is difficult.
• Genetic approaches are successful, analyzing plants with altered phenotypes.

Signal Transduction

• Secondary messengers connect the receptor to the response system.
• They transduce and amplify hormone signals such as Ca-fluxes, cGMP, phosphorylation events.

Cell Response

• Altered transcription of specific genes through transcriptional regulation.
• Altered activity of enzymes, through post-translational regulation.
• Changes can mean upregulation or downregulation.

Hormone Metabolism

• Environmental factors alter hormone biosynthesis, transport, conjugation to sugars, perception, and degradation.

Five Main Classes of Plant Hormones

• Growth Promoters:
  • Auxin and gibberellin stimulate cell expansion.
  • Cytokinin stimulates cell division.
• Growth Inhibitors:
  • Abscisic acid slows growth (involved in dormancy).
  • Ethylene controls ripening and cell death.
• Multiple hormones control many responses.

Life Cycle - Apical Dominance

• Apex blocks growth of axillary buds, which is important for light competition.
• Auxin is produced in the apex and young leaves, blocking growth of lower axillary buds and is responsible for apical dominance.

Auxin

• Generic name for chemicals with auxin-like activity.
• Indole acetic acid (IAA) is the primary natural auxin.
• 2,4-Dichlorophenoxyacetic acid (2,4-D) is an auxin-like weed killer, and is a component of Agent Orange, a defoliant used in the Vietnam War.

Agent Orange

• Agent Orange was an auxin-containing defoliant used extensively in the Vietnam War.

Auxin continued

• Primarily produced in the shoot apex, young leaves, and embryos.
• Travels down to roots.
• Blocks axillary buds.
• Promotes formation of lateral roots.
• Commercially used as rooting powder.

Cytokinin

• Mostly produced in root tips and embryos.
• "Zeatin" is the most common type.
• Travels up from roots to shoots.
• Promotes cell division.
• Enhances sprouting axillary buds (branching).
• Has anti-aging properties.

Auxin & Cytokinin

• An extensive shoot produces lots of auxin, encouraging root growth. An extensive root system produces lots of cytokinin, encouraging shoot growth.
• Auxin and cytokinin maintain balance between root and shoot growth.

Life Cycle - Auxin & Fruits

• Fruit is an expensive investment.
• Auxin and/or gibberellin, produced in the embryo, stimulate fruit growth.
• Without a viable embryo, no fruit develops or young fruit is shed.
• Seedless fruits can be treated with synthetic auxin or gibberellin.

Seed Dormancy (Gibberellin vs Abscisic Acid)

• Seed Development: High levels of gibberellic acid promote embryo growth. High levels of abscisic acid facilitate embryo dormancy.
• Initial Stage Embryo Development: High levels of gibberellic acid activate growth, while low levels of abscisic acid have no effect.
• Second Stage Embryo Development - Seed Dormancy: Low levels of gibberellic acid may fail to activate growth. High levels of abscisic acid will slow down metabolism.
• Dormancy facilitates:
  • Dispersal
  • Survival.
• Some dormant seeds survive for hundreds of years.
• Third stage embryo development - germination
  • Low levels of abscisic acid combined with high levels of gibberellic acid facilitate metabolism.
• The balance between gibberellin and abscisic acid determines whether the seed/plant grows or remains dormant.

Life Cycle - Fruit Ripening (Ethylene)

• Timing of ripening is carefully regulated, switching from repelling to attracting consumers.
• Softening occurs (starch and acids convert to sugars).
• Scent and color change occur, controlled by ethylene
• Ripe fruit stimulates ripening of other fruits via ethylene in a positive feedback cycle.
• Ethylene is a gas.

Fruit/Flower Shelf Life

• Mixtures of fruit and flowers may have shorter shelf life, decreased flower quality & lower nutritional value of fruits and vegetables.

One Bad Apple...

• One bad apple spoils the entire bunch due to ethylene production.

Life Cycle - Ethylene

• Ethylene causes local cell death
• Programmed cell death, known as apoptosis.
• Example of xylem vessels dead when functional while surrounding cells stay alive.

Life Cycle - Ethylene and Auxin

• Programmed cell death (apoptosis).
• Leaf abscission involves increased ethylene and decreased auxin.
• Local cell death (weak point) happens at the abscission layer causing shedding of the leaf.

Ethylene, Auxin & Leaf Abscission

• Leaf abscission involves increased ethylene and decreased auxin.
• Not all plants display seasonal variations in hormone levels, with some, like evergreens, retaining leaves.

Explore the crucial role of plant hormones in controlling growth and development. This quiz covers concepts from the Campbell and Reece editions and discusses various hormones such as auxin and gibberellins. Test your knowledge on processes like seed dormancy, flowering, and fruit ripening.