Plant Biology - Growth in Plants

Growth in Plants

Understandings of Plant Growth

  • Observations of tropic responses in seedlings

  • Positive phototropism: directional growth response of plant shoots towards lateral light

  • Phytohormones: signaling chemicals in plants controlling growth and response to stimuli

  • Auxin efflux carriers: maintain concentration gradients of phytohormones

  • Auxin's role: promotes cell growth

  • Interactions between auxin and cytokinin: regulate root and shoot growth

  • Positive feedback: involved in fruit ripening and ethylene production

Hormones in Plants

  • Hormones: chemical molecules influencing other parts of the organism.

  • Phytohormones: plant hormones responsible for various functions such as:

    • Root and shoot growth (promotes/inhibits based on concentration)

    • Flowering

    • Fruit and leaf development

    • Response to wounds

  • Auxin: specifically controls stem growth

Tropisms

  • Definition: plant response to directional external stimuli

  • Types of tropisms:

    • Positive: growth towards the stimulus

    • Negative: growth away from the stimulus

  • Phototropism: response to light, where photoreceptors in the stem detect unidirectional light, leading to auxin movement to the opposite side for growth towards light

Gravitropism

  • Growth response to gravity: can be either positive or negative

  • Statoliths: cellular organelles in roots that accumulate according to gravitational pull

  • In roots: auxin inhibits growth due to statolith concentration

  • In shoots: auxin promotes elongation during gravitropic responses

Auxins

  • Mechanism: when bound to receptors, they promote transcription of specific genes

  • Cell wall modification: leads to secretion of H+ ions, loosening cellulose connections for cell expansion

  • Auxin transport: diffuses into adjacent cells, maintaining concentration gradients through efflux carrier proteins which revert auxin to its uncharged state

Growth Types

Determinate Growth

  • Defined juvenile/embryonic period

  • Growth halts upon reaching a certain size or structure completion

Indeterminate Growth

  • Continuous cell division indefinitely

  • Growth through increased cell size or mitosis

Meristems

  • Regions of active cell division called meristems where plant growth occurs.

  • Types of meristems:

    • Apical meristems: located at tips of stems and roots for vertical growth

    • Lateral meristems: increase thickness, notably in cambium

Axillary Buds

  • Shoots forming at the junction (node) of stem and leaf base

  • Hormonal inhibitors can block growth; removal activates meristems

Lateral Meristems

  • Support the increased mass of growing plants

  • Secondary growth produces extra xylem in a ring inside cambium leading to observable tree rings

  • Tree bark is also a product of lateral meristem growth.