Topic 8: Grey matter
8.1 Sensitivity in plants
Photoreceptors: A structure in living organisms, especially a sensory cell or sense organ that responds to light.
The interaction of cells enables them to coordinate their activities appropriately. Organisms have specialised cells or molecules that are sensitive to changes in the environment (stimulus), called receptors. These then trigger events to bring about coordinated responses to environmental changes.
Plants use light as their sole energy source in photosynthesis. Light gives plants information about the seasons and respond by moving into different stages of their life cycles.
Plants respond to:
- Light intensity
- Light direction
- Light quality (wavelength)
- Light duration (day length)
Proteins that are sensitive to red light found in leaves. Phytochromes exist in two forms which are changed from one form to another when they absorb light. Phytochrome red P(R) absorbs red light and is changed to far-red P(FR) which absorbs far-red light. This changes it back to P(R) in a reversible reaction.
White light, including sunlight, contains both red and far-red light. This causes more P(FR) to be produced from P(R). In the dark, P(FR) slowly changes to P(R):
- After daylight there is more P(FR)
- After darkness there is more P(R)
In some plant species, phytochromes affect seed germination. Dormant seeds only germinate when they contain plenty of P(FR) which happens when normal sunlight falls onto them. This is valuable because it prevents germination in unsuitable conditions when there is not enough light for photosynthesis. If a light-sensitive seed is provided with the correct conditions (water, temperature) and then is given a short burst of far-red light, it won’t germinate. If the burst of far-red light is followed by a burst of red light it will germinate because the red light converts P(R) to P(FR).
Phytochromes also determine if the plant produces flowers. Some plants adapt to flower in spring, when the days are longer. Others are adapted to flower in late summer or autumn when the days are shorter. The photoperiod determines flowering.
During darkness, P(FR) converts back to P(R). Short-day plants require P(R) in their tissues to flower, which occurs when they have long uninterrupted nights. If white or red light is shone on them even briefly during the night, they will not flower because this converts P(R) back to P(FR).
Long-day plants need an abundance of P(FR) which only happens when they have short nights because not all of the P(FR) has been converted and the sun begins the conversion of P(R) back to P(FR) again.
Phytochromes activate other molecules in plant cells which affect various metabolic pathways. The phytochromes also act as transcription factors in the nucleus, switching genes on and off.
A growth response to directional light. Shoots are positively phototrophic, growing towards the source of light. Roots are negatively phototropic, growing away from the source of light. This response involves an auxin (IIA), a plant hormone promoting elongation of stems and roots. Auxin is a plant hormone/growth regulator. It is synthesised by cells in the meristem of a shoot and then transported downwards through the shoot tissues. Auxin binds with receptors in the plasma membranes in the zone of shoot elongation, producing a second messenger signal molecule that brings about changes in gene expression. An increased potential difference across the membrane enhances uptake of ions into the cell, causing uptake of water by osmosis causing cell elongation.
Auxin activates transcription factors in the nucleus, switching different sets of genes on or off. Some code for the production of proteins (expansins) which act on cellulose cell walls, enabling them to stretch and expand when the cell takes in water. This is probably due to the disruption of hydrogen bonds between the cellulose molecules. Auxin stimulates cell expansion.
The distribution of auxin in a shoot is affected by photoropin. When unidirectional light falls onto the shoot, this affects the auxin distribution, causing it to accumulate on the shady side. This causes the cells on the shady side to grow more rapidly, so the shoot bends towards the light.
When the tip is removed and placed on some agar jelly and then placed back on the plant, it started to grow again, showing the chemical diffused through the agar jelly.
Geotropism – a growth response by a plant to gravity. Shoots are negatively geotropic, roots are positively geotropic.