Coordination and control in plants

Plant growth regulators include:

• Auxins - stimulate cell elongation in zone of elongation

• Cytokinins - promote cell division

• Gibberellins - stimulate elongation of internodal regions

• Auxins are produced in the shoot tip and diffuse down to the zone of elongation.

• They stimulate the pumping of hydrogen ions into the cellulose cell wall

• The acidification of the cell wall activates agents which loosen the linkages between cellulose microfibrils - making the wall more flexible

• The cells absorb water by osmosis and the flexible cell walls allow the cells to expand as the extra water exerts increased hydrostatic pressure against them

• Auxins are involved in producing the positive phototropic response in plant shoots.

• As the auxins diffuse down from the tip, unidirectional light causes auxin to move towards the shaded side promoting greater cell elongation on the shaded side and a resultant curvature of the shoot towards the light.

• Plants have phytochromes that detect the photoperiod.

• They are interchangeable: P660 and P730.

• P660 (Pr) maximally absorbs red light, while P730 (Pfr) maximallly absorbs far-red light.

• During darkness, there is a slow conversion of P730 to P660 - P660 accumulates

• During daylight, P730 accumulates

• P730 is a physiologically active form, and can either stimulate or inhibit flowering

• LONG DAY PLANTS - require a critically short period of darkness, so that high levels of P730 accumulate and stimulate flowering

• SHORT DAY PLANTS - require a critically long period of continuous (uninterrupted) darkness, so that the inhibitory effect of P730 is removed

In both plants and animals:

• Chemicals are used to communicate between receptors and effectors

• Receptors receive the stimulus (e.g. phytochromes detects photoperiod in plants, while osmoreceptors detect changes in water potential and retina in eye detects light in animals).

• Effectors bring about the response (e.g. plants may bud into a flower, and cells elongate in response to auxin, while in animals muscle contracts)

• However, plants lack a nervous system, which animals require for rapid communication in the body and is important in coordinating movement.