SIGNALS & SENSORS

Gravity receptors

In order to detect gravity plant cells must contain amyloplasts (also called "statoliths" in this context). These are heavier than the cytoplasm and tend to settle out under gravity. When they touch the endoplasmic reticulum some kind of signal is generated that results in correction of growth. Usually this means positive geotropism (or growth in response to gravity) in roots and negative geotropism in shoots.

Section of pea root, showing statoliths settled at bottom of cells

Secale root tip stained with iodine showing starch in columella of root cap

Allium root cap, showing columella.

Light receptors
The best known photoreceptor in plants is phytochrome, a pigmented protein that exists in two forms:

• Pr absorbs red light (660nm) and then is converted to Pfr
• Pfr absorbs far-red light (730 nm) and is converted back to Pr

Phytochrome is synthesized as Pr which is physiologically inactive. Pfr is the physiologically active form of phytochrome and apart from conversion to Pr in far red light, it is lost in the dark either by slow reversion to Pr or by degradation of the protein itself.

Many plant developmental processes are regulated by phytochrome, whereas growth responses are often regulated by a blue-light absorbing pigment, sometimes called cryptochrome. Shoots are usually positively phototropic whereas roots are negatively phototropic.

Brassica rapa seedling grown in dark

similar seedling grown in weak blue light from left

Auxin and tropisms
Growth responses to light and gravity are thought to involve changes in the concentration of IAA. IAA, coming from the stem apex, accumulates on the side of the organ away from the light or on the same side as the gravity stimulus. Because auxin stimulates shoot elongation, but inhibits root elongation this leads to unequal growth on the two sides of the structure and hence curvature.

This is the basis of the Cholodny-Went hypothesis of the mechanism of plant tropic responses, which is not universally accepted although it has been the subject of much research over the past 50 years.

Biological clock
Evidence of time-keeping is found amongst all groups of eukaryotes from single-celled protists to complex-multicellular organisms. Plants are no exception to this general rule but we do not understand how time is recorded. As in most organisms, the typical clock cycle is about a 24 hours, which leads to the term"circadian" in relation to rhythms of plant movement, metabolism or growth. Some features of the clock in plants are:

• it is set or"entrained" by daily light-dark cycles.
• phytochrome is involved in setting the clock
• rhythms die away in continuous light or dark
• the clock runs at the same speed whether it is warm or cold

So called"endogenous rhythms" are most readily seen among plants that close their leaves or flowers at night. Once the rhythm is set up or entrained it will continue for several cycles in total darkness. There are also diurnal cycles of plant growth (most plants grow mainly at night) and more subtle changes such as enzyme activity.

Mechanical stimulation
Some plants such as the senstive plant Mimosa pudica have obvious responses to touch or other mechanical stimulation. However, the growth of all plants is affected by mechanical stresses such as wind, trampling or brushing against other objects (animal, vegetable or mineral) Of course plants have no nervous system and it is not clear how they perceive these stimuli. Electrical signals seem to be involved, just as they are with nerve cells. Plants under mechanical stress usually produce more ethylene than unstressed plants, and since ethylene inhibits stem elongation it may be involved in the growth response of plants.

One of the best places to see plant response to touch or thigmotropism is in the tendrils of climbing plants such as Vitis. Gentle stroking of one side of a tendril will induce curvature towards that side in a matter of hours

Leaves of the insectivorous venus' fly trap (Dionaea muscipula) respond to touch by closing over their prey

Internal stresses
The thickening of horizontal stems (reaction wood in trees) is a response to a kind of mechanical stress. In this case the stress is generated by the weight of the plant itself.

At the cell level sensing and adjustment to the pressure of the plasma membrane on the cell wall is a necessary part of the cell's water relations. There seem to be pressure-sensing proteins in the cell wall so that water potentials inside and outside the cell and the elasticity of the cell wall can be balanced. The growing cell needs turgor pressure but avoids excessive turgor that would burst the cell wall.

Temperature
Plants respond directly to the effects of temperature on the rates of chemical reactions. They have mechanisms to cope with extremes of temperature, heat and cold. Processes such as seed germination, bud break, and flower initiation often require a period of cold, especially in temperate zone plants. We usually expect processes to be promoted by high temperature and it is not clear how cold exposure is perceived or translated into a positive signal.

Aesculus hippocastanum (Horse chestnut)

Copyright © Michael Knee, The Ohio State University
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