Plant responses and plant hormonesEdexcel International A Level Biology: Revision notes

Plants respond to their environment by tropisms (directional growth responses), by germinating when conditions suit and by flowering at the right time of year. These responses are coordinated by plant hormones, chemicals made in tissues rather than in glands. They are effective at very low concentrations, move by diffusion or in the transport tissues, and act only on target cells that have the right receptor.

This topic covers three controls: auxin (IAA), gibberellins and the light-sensing pigment phytochrome. All three work largely by changing which genes are transcribed.

Auxin, mainly indoleacetic acid (IAA), is made in the shoot tip and moves down the stem. It promotes cell elongation in two ways.

• Acid growth: IAA activates proton pumps that move H⁺ into the cell wall. The fall in pH activates expansins, which loosen the bonds between cellulose microfibrils. Water enters by osmosis, the vacuole expands and the cell elongates.
• Transcription: IAA binding leads to the breakdown of repressor proteins, so transcription factors can switch on genes needed for elongation.

In phototropism, light from one side causes IAA to move to the shaded side. Cells there elongate more, so the shoot bends towards the light.

Gibberellins (GAs) promote stem elongation and seed germination. A gibberellin binds to its receptor, which leads to the breakdown of DELLA proteins. DELLA proteins inhibit transcription, so their removal allows transcription of genes for cell elongation and division in the internodes. Dwarf plants often lack gibberellin, and spraying gibberellin onto such a dwarf restores normal height.

To describe the effect on transcription, name the step: the hormone removes a transcription inhibitor, so genes are switched on.

When a barley grain takes up water, the embryo makes gibberellin. It diffuses to the aleurone layer, where it stimulates transcription of the amylase gene. Amylase is secreted into the endosperm and hydrolyses stored starch to maltose, which is converted to glucose. The glucose is respired or used to build new tissue by the growing embryo.

Phytochrome is a light-sensing pigment protein with two interconvertible forms.

• Pr absorbs red light (660 nm) and is converted to Pfr.
• Pfr absorbs far-red light (730 nm) and is converted back to Pr. In the dark Pfr also slowly reverts to Pr.

Pfr is the active form. It moves into the nucleus, where it affects transcription factors and so changes which genes are transcribed. Responses include germination of light-sensitive seeds, flowering and stem elongation in shade. Shade under leaves is rich in far-red light because chlorophyll absorbs red, so Pfr falls and shaded plants elongate.

In a short-day plant, flowering needs a long uninterrupted night in which Pfr falls low enough. A brief red flash in the night makes Pfr again and prevents flowering.

Method: cut barley grains in half across the middle. Place half grains with and without the embryo cut side down on starch agar, some on water and others on gibberellin solutions of different concentrations. Incubate (for example 48 hours at 25 °C), flood the plates with iodine solution and measure the width of the clear zone around each half grain.

• Where amylase has hydrolysed the starch the agar stays clear; elsewhere it is blue-black.
• Embryo half grains and embryo-less half grains with gibberellin give clear zones. Embryo-less half grains on water give none.
• Controls: same grain size and variety, same agar thickness and volume of solution, same temperature and time; a boiled-grain control shows that the zone is due to an enzyme.
• Compare areas (πr²) rather than widths, and repeat to calculate a mean.