Coordination & Control in Plants

Coordination & Control in Plants

• Plant hormones, also known as growth substances or plant growth regulators, are organic chemicals.
• They exist in very low concentrations in plant tissues.
• They act as messengers, stimulating, inhibiting, or modifying growth and development.
• They are usually synthesized in specific regions of the plant, such as embryos, apical meristems of shoots and roots, and young growing leaves and developing seeds.

Plant Hormones vs. Animal Hormones

• Plants lack specialized glands for hormone synthesis.
• Plant hormones are transported from the production site to other parts of the plant.
• Plant hormones are active at their site of production.
• Transport and action are generally slow in plants.
• Plant hormonal actions are less specific, potentially affecting different tissues and organs.
• Plant hormones are primarily involved in growth and development.
• Animal hormones are produced by specialized endocrine glands.
• Animal hormones are mainly transported by the bloodstream to target cells/organs.
• Animal hormones are active primarily at their target cells/organs
• Transport and action are generally rapid.
• Animal hormonal actions are more specific, effecting only specific target cells/organs.

Major Plant Hormones

• Five major plant hormones recognized in modern plant biology: Auxin, Abscisic Acid, Cytokinin, Gibberellins, and Ethylene.
• These hormones play crucial roles in plant growth and well-being.
• Hormones may interact individually, have opposing effects (antagonism), or exhibit synergistic interactions (where the combined effect is greater than the sum of individual effects).

1. Auxins

• Plant hormones influencing cell division and elongation (via vacuolation and elongation).
• Natural auxins, such as indole-3-acetic acid (IAA), are produced naturally by plants.
• Auxin (IAA) binding to cells decreases cell pH, loosening cell walls, allowing for more water uptake.
• This increased turgor pressure leads to cell swelling and elongation promoting plant growth.
• Auxin effects depend on the target tissue.
• Examples include auxin's role in shoot elongation, cell division and differentiation in vascular cambium, fruit development in ovaries, and lateral root formation in roots.
• Phototropism - plant growth towards a light source
  • Auxin is produced more on the shaded side of a shoot or coleoptile.
  • Cells on the shaded side elongate faster than cells on the illuminated side.
• Geotropism - Plant growth in response to gravity
  • Auxin gathers in the lower half of the stem and root, slowing growth in the root and curving downward.
  • Auxin stimulates growth in the shoot and stem, curving upward.
• Promotes fruit development-stimulates fruit development and is used for parthenocarpy (fruit development without fertilization), producing seedless fruits.
• Auxins also have inhibitory effects, suppressing lateral bud growth, which is known as apical dominance.
• Inhibits abscission-causes dropping of leaves, flowers, and fruits from the plant
• Young leaves and fruits produce auxins, and production of auxin reduces in an ageing leaf or ripened fruit, increasing production of abscisic acid.

2. Gibberellins

• Plant hormones promoting cell elongation and stem length growth.
• Synthesized in meristems of apical buds, roots, young leaves and embryos in seeds.
• Gibberellins have a terpene structure, a group of plant chemicals related to lipids.
• Gibberellins stimulate stem growth by interacting with IAA and sometimes substituted for red light in promoting flowering in long-day plants and inhibits it in short-day plants.
• Promotes fruit growth and parthenocarpy.
• Used to break seed dormancy.
• Inhibits root initiation
• Stimulates leaf growth

3. Cytokinesis

• Plant hormones that promote cell division and differentiation in the presence of auxin.
• Produced in actively growing tissues, such as embryos in seeds, fruits, and roots.
• Transported in the xylem to various parts of the plant.
• Promotes lateral bud growth (inhibition of apical dominance).
• Delays senescence (aging) of plant cells.
• Acts as a signal to shoots about the health and activity of roots.

4. Abscisic Acid (ABA)

• Plant hormone that slows metabolism and inhibits growth in most plant parts.
• Works antagonistically to auxins, gibberellins, and cytokinins.
• Synthesized in leaves, stems, fruits, and seeds.
• Mainly translocated in the phloem and can diffuse from the root cap to the root cells.
• Causes stomata closure during water stress by stimulating the removal of K+ ions from guard cells.
• Causes dormancy in some seeds, particularly in desert plants; ABA inhibits germination which helps plants survive adverse conditions.

5. Ethylene

• Plant hormone in the form of a gas.
• A metabolic by-product in most plant organs, especially ripening fruits and aging leaves.
• Increased level of ethylene triggers fruit ripening (positive feedback).
• Hydrolytic enzymes are released, breaking down cell wall components, chlorophyll, and softening fruits, converting starches and fruit acids to sugars.
• The bright colors, scents, and sugars produced during fruit ripening help attract animals to eat the fruits and disperse the seeds.
• Used commercially to allow the shipping of green, still-hard fruit (minimizing bruises and rotting) by applying ethylene after transportation.

Other Hormones

• Brassinosteroids - first discovered in Brassica pollen, stimulating cell division and elongation.
• Broad spectrum of physiological effects on plants, promoting apical dominance, promoting leaf senescence, and enhancing seed germination, gravitropism.
• Oligosaccharins - Oligosaccharides with hormone-like functions released from the cell wall by enzymes secreted by pathogens. The signal for defense responses.

The Effect of Light on Flowering

• Light is vital in directing plant growth and development.
• Sunlight resets biological clocks in plants by activating and inactivating photoreceptors (phytochromes).

Phytochromes

• Phytochromes are photoreceptors for red light responses, present in low concentration in various plant organs, especially leaves.
• Each phytochrome is a homodimer consisting of two identical protein molecules linked to a chromophore.

Photoperiodism

• Photoperiodism – response to the relative lengths of daylight and darkness.
• Many plants flower at specific times of the year due to photoperiod differences.
• Plants exhibiting this phenomenon are classified into short-day plants, long-day plants, or day-neutral plants.

Phytochrome Roles in Controlling Flowering

• Photoreceptor in photoperiodism is phytochrome.
• The balance between Pr and Pfr forms of phytochrome controls flowering in short-day and long-day plants.
• Pfr (active form) triggers or inhibits flowering in response to different light conditions.
• Accumulation or high amounts of Pfr stimulates flowering in long-day plants, and inhibits flowering in short-day plants.
• An accumulation of florigen( a chemical messenger that transmits information from leaves to flower buds) in the leaves is also needed for flowering. Florigen is secreted in response to relative amounts of Pr and Pfr in the leaves.

Short-day Plants (SD)

• Flower when the dark period exceeds a certain critical length.

Long-day Plants (LD)

• Flower when the dark period is less than a specific critical length.

Day-neutral Plants

• Plants unaffected by photoperiodism, such as tomatoes and cucumbers.

Explore the fascinating world of plant hormones in this quiz. Test your knowledge on how these growth regulators operate, their role in plant development, and how they differ from animal hormones. Delve into the intricate processes that guide plant growth and response to stimuli.