Fix the mistake: Plant Growth and Environmental Responses

Types and Functions of Plant Hormones (Auxin, Gibberellin, etc.)

Plants do not have the nervous systems of animals, but they precisely regulate growth, development, and responses to the environment through chemical substances called plant hormones. Let us understand the types and functions of the main hormones.

Basic Knowledge

Auxin (IAA = indoleacetic acid): Synthesized in the shoot apical meristem, young leaves, and seeds. Transported polarly from the tip toward the base (PIN transporters). Lowers the pH of the cell wall and softens it → promotes cell elongation. The effect depends on concentration: in stems, low concentrations promote growth and high concentrations inhibit it. Roots are more sensitive, so even low concentrations may inhibit growth. The main hormone involved in tropisms.
Gibberellin (GA): Promotes stem elongation and seed germination (amylase induction in the aleurone layer), and fruit enlargement. The phenotype of dwarf plants is often caused by GA deficiency or receptor mutations.
Cytokinin: Synthesized in roots, promotes cell division, and delays senescence (maintaining chloroplasts). The auxin/cytokinin ratio determines organ differentiation (high ratio → roots; low ratio → shoots).
Ethylene (gaseous hormone): Promotes fruit ripening, leaf abscission, and flower senescence. Triple response (in darkness: inhibition of stem elongation, thickening, and horizontal growth). A single ripe apple causes surrounding fruits to ripen early because of ethylene diffusion.
Abscisic acid (ABA): Closes stomata (drought response), induces seed dormancy, and inhibits germination. It increases rapidly during water stress.
Other hormones: Brassinosteroids (steroid hormones; cell elongation and differentiation), salicylic acid (induction of disease resistance), and jasmonic acid (wound response and insect resistance).

Example Problem
Explain how auxin is involved in the phototropism of stems, based on the relationship between unilateral illumination and cell elongation.
Answer: In the shoot apex exposed to unilateral light, auxin is redistributed laterally to the side opposite the light (the shaded side). The cells on the shaded side undergo acidification of the cell walls due to the high auxin concentration and elongate greatly, making the shaded side of the stem longer than the lighted side. This unequal elongation causes the stem to bend toward the light (positive phototropism).
Key Points
  • Auxin (IAA) = promotes cell elongation, apical dominance, and tropisms
  • Gibberellin = stem elongation, promotion of germination, and amylase induction
  • Cytokinin = promotion of cell division and inhibition of senescence
  • Ethylene (gas) = fruit ripening, leaf abscission, and triple response
  • Abscisic acid (ABA) = stomatal closure, seed dormancy, and water-stress response
  • Auxin/cytokinin ratio = determines root (high ratio) and shoot (low ratio) differentiation
  • Salicylic acid = disease resistance; jasmonic acid = insect resistance

Points to Note

① Apical dominance: a phenomenon in which auxin from the shoot apex is transported to the lateral buds, reaching a high concentration and inhibiting their growth. When the shoot apex is removed, the lateral buds elongate. ② Ethylene is a gaseous hormone and can diffuse between cells and between plants. ③ ABA stands for “abscisic acid” and plays the most important role in stomatal closure under drought stress.

Practice

  1. What is apical dominance? Explain it including the role of auxin.
  2. Explain the mechanism by which abscisic acid (ABA) closes stomata during drought stress.
  3. What is ethylene’s “triple response”? Also describe the relationship between fruit ripening and ethylene.

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