Plant Hormones

• Phytochrome is a light-sensing pigment that flips between an inactive form (P_R) and an active form (P_FR) depending on the colour of light it absorbs.
• Phytochrome controls seed germination — red light triggers it, far-red light blocks it.
• Phytochrome also controls flowering, by "measuring" night length through the balance of P_R and P_FR.
• P_FR acts as a transcription factor — it enters the nucleus, binds to PIF3, and switches genes on.
• Growth factors (plant hormones) like auxin (IAA), gibberellins, cytokinins, abscisic acid and ethene each control different aspects of growth.
• IAA (auxin) drives phototropism (bending towards light) and geotropism (roots growing down, shoots growing up) — but has opposite effects in shoots vs. roots.
• Gibberellin triggers germination in barley seeds by switching on the gene for amylase in the aleurone layer.
• Core Practical 18 uses a starch-agar assay to measure how gibberellin concentration affects amylase production.
• Animals, unlike plants, coordinate responses using both the nervous system (fast, electrical, short-lived) and the hormonal system (slower, chemical, longer-lasting).

Think about it from the plant's point of view: it can't move, so every decision it makes has to be based on signals from its environment. Should I germinate now, or wait? Should I put energy into flowers this month? These are life-or-death decisions, and the plant needs a reliable way to "read" the light around it. That's exactly what phytochrome does.

Phytochrome is a pigment — a molecule that absorbs specific wavelengths of light — and it exists in two interconvertible forms:

Here's the clever bit: this conversion is reversible, like a switch that flips back and forth depending on which colour of light hits it last.

• short burst of red light _{R FR}
• _{FR R} reverses

_{R FR}