9.3

• Auxins, Cytokinins, and Gibberellins are three groups of plant growth factors.
• Auxins are involved in trophic responses such as IAA, control cell elongation, suppress lateral buds to maintain apical dominance, and promote root growth in rooting powders.
• Auxins cause cell elongation through the Acid Growth Hypothesis 1, which involves IAA causing active transport of H+ ions into the cell wall, disruption to H-bonds between cellulose molecules, and the action of expansins making the cell more permeable to water.
• Cells with higher turgor pressure elongate faster.
• Gibberellins stimulate germination, elongation at cell internodes, fruit growth, and rapid growth or flowering.
• Gibberellins stimulate germination by the seed absorbing water, activating the embryo to secrete gibberellins, and gibberellins diffusing to the aleurone layer, which produces amylase.
• Amylase diffuses to the endosperm layer to hydrolyse starch, and hexose sugars act as a respiratory substrate to produce ATP as the energy currency.
• Plants use growth hormones synergistically to achieve the same effect, such as auxins and gibberellins.
• Long day plants flower when sunlight hours exceed a critical value and short day plants flower when darkness hours exceed a critical value.
• Phytochrome is a plant photoreceptor with a bilin chromophore group that converts between two forms: biologically inactive Pr absorbs red light and biologically active Pfr absorbs far-red light.
• Phytochrome controls flowering by converting from Pr to Pfr, which stimulates flowering, indicating that light intensity is high enough for photosynthesis.
• The ratio of Pr:Pfr enables a plant to detect how long days are.
• Plants use growth hormones antagonistically with inverse effects, such as auxins suppressing lateral buds and cytokinins stimulating lateral buds.
• Photomorphogenesis is the pattern of plant growth and development determined by light intensity.
• Phytochrome controls photomorphogenesis by controlling the transition from Pr to Pfr, which affects localization of proteins within cells, transcription of certain genes, and phosphorylation of proteins.
• The time of day when each form of phytochrome is most abundant is different: in darkness, Pr is abundant and in sunlight, Pfr is abundant.
• Cytokinins stimulate the development of lateral buds by promoting cell division at apical meristems and promote leaf abscission synergistically with ethene.
• Auxins, Cytokinins, and Gibberellins interact with plants in two ways: as signaling molecules and as growth regulators.