Hormones, transcription factors and plant responses

What you’ll learn

• Compare neural and hormonal coordination.
• Explain extracellular and intracellular hormone signalling.
• Connect auxin, phytochrome and gibberellins to plant responses.

Understand the idea

Nervous coordination commonly uses rapid electrical signalling along neurones and short-range neurotransmission to specific effectors. Hormones travel in body fluids and affect cells with matching receptors; effects are often slower and longer-lasting, although speed is not an absolute distinction. Peptide hormones usually bind membrane receptors and trigger intracellular signalling. Lipid-soluble steroid hormones can cross membranes and bind intracellular receptors; hormone–receptor complexes can influence transcription.

Transcription factors bind particular DNA regions and alter the recruitment or activity of transcription machinery, switching gene expression on or off. The DNA sequence need not change. In plants, auxin such as IAA can change gene expression and promote shoot cell elongation by increasing wall extensibility. Unequal auxin distribution across a shoot can cause differential growth and bending. Responses depend on tissue and concentration, so a shoot response should not automatically be applied to roots.

Phytochrome occurs in interconvertible forms: red light converts Pr towards Pfr and far-red light converts Pfr towards Pr. The proportions influence transcription and developmental responses such as germination and flowering, depending on species and conditions. Gibberellins released by a germinating cereal embryo stimulate the aleurone layer to produce hydrolytic enzymes including amylase. Starch hydrolysis supplies soluble sugars for respiration and growth. A cereal-grain investigation can compare enzyme activity with suitable controls for living tissue, hormone availability and starch detection.

Key formulae

$$\mathrm{Pr}\xrightleftharpoons[\text{far-red light}]{\text{red light}}\mathrm{Pfr}$$

Worked example

A viable cereal embryo is associated with increased amylase production in surrounding tissue. Explain the hormonal link to seedling growth.

1. The embryo releases gibberellins as germination proceeds.
2. Gibberellin signalling in the aleurone layer activates expression of genes involved in hydrolytic enzyme production, including amylase.
3. Amylase digests stored starch to smaller sugars that support respiration and provide material for growth.

Answer: Hormone-dependent gene expression mobilises stored carbohydrate for the developing seedling.