Plant Hormones BL1004

Plant Hormones BL1004

• Plant hormones control plant growth and development
• They control processes like sink-source relations, seed dormancy, seed germination, flowering, development of seed/fruit, formation of tubers/bulbs, senescence, and many more
• Hormones increase plant size or weight (growth) and direct progression through the plant's life cycle (development)
• They coordinate growth and development of cells, tissues, and organs
• Plant hormones are essential for all multicellular organisms
• Sessile organisms, like plants, must adjust to environmental changes, with hormones playing a key role in this adjustment

Plant Hormone Characteristics

• Produced by multicellular organisms
• Internal chemical signals
• Present in minute amounts
• Travel throughout the organism
• Coordinate metabolic activities
• Coordinate environmental responses
• Not produced in glands

Hormone Activity at Cell Level

• Receptor: Hormone or environmental stimulus binds to a receptor
• Signaling Cascade: Relay molecules transmit the signal
• Cellular Response: Activation of cellular responses, altering gene transcription, or enzyme activity

Hormone Reception

• "Hormone" comes from a Greek word meaning "to excite"
• Specific receptors exist for each type of hormone
• Located in the cell membrane
• Present in very low concentrations

Identifying a Hormone Receptor

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

Signal Transduction

• Secondary messengers connect the receptor with the response system
• Secondary messengers transduce and amplify hormone signals
• Examples include calcium fluxes, cyclic GMP, and phosphorylation events

Cell Response

• Altered transcription of specific genes (transcriptional regulation)
• Altered activity of enzymes (post-translational regulation)
• Changes can be up or down regulated

Hormone Metabolism

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

Five Main Classes of Hormones

• Growth Promoters:
  • Auxin and gibberellin stimulate cell expansion
  • Cytokinin stimulates cell division
• Growth Inhibitors:
  • Abscisic acid slows growth, promoting dormancy
  • Ethylene controls ripening and cell death

Life Cycle - Apical Dominance

• The apex blocks the growth of axillary buds
• Important for light competition
• Auxin is produced in the apex and young leaves
• Blocks growth of axillary buds lower the stem
• Responsible for apical dominance

Auxin

• Generic name for chemicals with auxin-like activity

• Indole acetic acid (IAA) is the main natural auxin

• 2,4-D (2,4-Dichlorophenoxyacetic acid) is an auxin-like weed killer

• Agent orange (a component of it) was used as a defoliant in the Vietnam War

• Mostly produced in shoot apex, young leaves and embryos

• Travels down to the roots

• Blocks axillary buds

• Promotes lateral root formation

• Used commercially as rooting powder

Cytokinin

• Mostly produced in root tip, as well as in embryos
• Most common type is "zeatin"
• Travels up from roots to shoots
• Promotes cell division
• Enhances sprouting of axillary buds (branching)
• Anti-ageing

Auxin & Cytokinin

• An extensive shoot produces a lot of auxin, encouraging root growth
• Extensive root systems produce a lot of cytokinin, encouraging shoot growth
• Together, auxin and cytokinin balance root and shoot growth

Life Cycle - Auxin & Fruits

• Fruits are costly investments for the plant
• Auxin and/or gibberellin, produced in the embryo, stimulate fruit growth
• No embryo = no auxin/GA= no fruit
• Flowers/young fruits are shed if there's no viable, hormone-producing embryo

Life Cycle - Fruits (Auxin and Gibberellin)

• Seedless fruits are treated with synthetic auxin or gibberellin, or a genetic mutant is used
• Synthetic auxins/gibberellins make seedless fruits possible

Seed Dormancy (Gibberellin vs Abscisic Acid)

• Seed development
  • High levels of gibberellic acid promote embryo growth
  • High levels of abscisic acid facilitate embryo dormancy
• Dormancy facilitates seed dispersal and survival; some dormant seeds can survive for hundreds of years
• Different stages of seed development are affected by different hormone levels
  • Initial stage – high levels of gibberellic acid activate growth, while low levels of abscisic acid don't affect growth
  • Second stage – Low levels of GA fail to activate growth, while high levels of ABA slow down metabolism
  • Third stage – Low levels of ABA with high levels of GA facilitate metabolism
• The balance between the two hormones determines whether a seed grows or remains dormant

Life Cycle - Fruit Ripening (Ethylene)

• Timing of ripening is carefully regulated

• Plants switch from repelling to attracting consumers

• Softening occurs as starch and acids convert to sugars

• Scent and color changes occur

• Ethylene controls the process

• Ethylene burst; Positive feedback cycle; Ethylene is a gas, ripe fruit stimulates ripening in other fruits

• Worldwide trait in fruits possible by removing ethylene gas

• Ripe fruit stimulates fruit ripening and flower aging

Fruit/Flower Shelf Life

• Mixing fruits and flowers results in shorter shelf life, reduced quality, and lower nutritional content for both
• The best practice is to keep items separate

One Bad Apple Spoils The Bunch

• Ethylene is responsible for this process

Life Cycle - Ethylene

• Causes local cell death (apoptosis)
• Example of this is xylem vessels dying while surrounding cells stay alive

Life Cycle - Ethylene and Auxin

• Programmed (regulated) cell death (apoptosis)
• Leaf abscission involves local increases in ethylene and local decreases in auxin
• Local cell death at the abscission layer results in leaf shedding

Ethylene, Auxin & Leaf Abscission

• Leaf abscission involves local increases in ethylene and local decreases in auxin
• Seasonal variations in hormone levels differ among species; not all species shed their leaves in winter