Plant Growth Regulators (PGRs) - Overview & Discovery
Plant growth regulators (PGRs / phytohormones / plant hormones) are small, simple molecules that control growth and development. They fall into two broad groups:
- Growth promoters - auxins, gibberellins, cytokinins (cell division, enlargement, flowering, fruiting).
- Growth inhibitors - abscisic acid (ABA) and the gaseous ethylene (dormancy, abscision, stress, senescence). Ethylene fits both groups but is mostly an inhibitor.
Landmark discoveries (learn who did what)
- Auxin - Charles Darwin & Francis Darwin observed that a grass coleoptile bends towards light and that the tip perceives the light; F.W. Went isolated auxin from oat (Avena) coleoptile tips.
- Gibberellin - traced to 'bakanae' (foolish-seedling) disease of rice, caused by the fungus Gibberella fujikuroi (E. Kurosawa).
- Cytokinin - kinetin (a modified adenine) was obtained from autoclaved herring sperm DNA; the natural cytokinin zeatin was isolated from corn kernels and coconut milk.
- Abscisic acid (ABA) - discovered as an inhibitor ('inhibitor-B'/'dormin').
- Ethylene - a simple gaseous PGR.
Auxins
- Natural: IAA (indole-3-acetic acid), IBA; Synthetic: NAA, 2,4-D. First isolated from human urine.
- Effects/uses: control apical dominance (the shoot tip suppresses lateral buds); promote rooting (IBA/NAA in cuttings); induce parthenocarpy (e.g., seedless tomato); prevent early fruit and leaf drop but promote abscission of older parts; 2,4-D is a weed-killer/herbicide (kills dicot weeds, not mature monocots) and is used to prepare weed-free lawns; induce xylem differentiation.
Gibberellins
- GA3 is the most studied; all are acidic (over 100 known).
- Effects/uses: cause stem/internode elongation and 'bolting' (elongation of the internode just before flowering in rosette plants); increase fruit size and shape (e.g., grapes); delay senescence; speed up the malting process in the brewing industry; hasten maturity in juvenile conifers; increase sugarcane yield (spraying elongates the stem).
Cytokinins
- Natural cytokinins (e.g., zeatin) occur where rapid cell division happens (root apices, fruits, endosperm).
- Effects/uses: promote cell division (cytokinesis); help overcome apical dominance (promote lateral bud growth); delay leaf senescence by nutrient mobilisation; promote new leaves, chloroplasts and lateral shoots.
High-yield pairings: apical dominance/rooting/2,4-D herbicide = auxin; bolting/malting/grape-size/sugarcane = gibberellin; cytokinesis/delay senescence-by-nutrient-mobilisation/overcome apical dominance = cytokinin.
Visual - Plant Growth Regulators at a Glance
The five PGRs - auxins, gibberellins and cytokinins (promoters) and ethylene and ABA (inhibitors) - with their signature effects and applications.