Plant Hormones

Overview Plant cells, like animal cells, respond to the actions of a variety of chemical signals, called hormones Just like animal hormones they are: often triggered by external environmental factors (stimuli) very effective at low concentrations

Overview The best understood plant hormones are: Auxins Gibberellins Cytokinins Abscisic acid Ethylene gas The same hormone can produce different responses in different parts of the plant

Auxins Auxins are water-soluble chemicals produced in growing tips of roots and shoots Auxin stimulates the growth of cells in shoots, but inhibits the growth of cells in roots!

Auxins Auxins produced in the growing tips of plants promote the elongation of the cells below

this selective pruning is how gardeners produce bushy plants Auxins When the bud at the apex is removed, the lateral buds lower down the stem begin to develop Indoleacetic acid (IAA), is responsible for apical dominance as it inhibits the growth of lateral buds near the growing tip at the apex of the plant this selective pruning is how gardeners produce bushy plants

increased concentration of auxins Tropism is when plant growth is in response to a stimulus like light, water or gravity Auxin moves away from light, causing plant stems to bend towards light! increased concentration of auxins cells in this region grow faster and longer, resulting in the plant bending towards the light phototropism

increased concentration of auxins Geotropsim is when plant growth is in response to gravity Gravity causes auxin to accumulate on lower side of horizontal shoots or roots This causes shoots to bend upwards and roots to bend downwards (as auxins inhibit growth in roots) increased concentration of auxins

Gibberellins Growth hormones that stimulate cell elongation and cell reproduction in stems and leaves They also instigate seed germination and bud development Best known gibberellin is gibberellic acid, which initiates a transduction pathway in seed germination induces production of gibberellic acid amylase starch glucose activated by water

Cytokinins Growth hormones that, when present in high concentrations, promote rapid cell reproduction in growing shoots, roots and young fruits Produced in roots and travel to stem via phloem Also produced in young fruits to stimulate their own growth

Abscisic Acid Growth-inhibiting hormone found in dormant buds This hormone also influences stomal closure to stop water loss Leaves produce, and accumulate, abscisic acid if they begin to wilt Abscisic acid somehow influences ion concentrations in guard cells, causing the stomata to close Therefore abscisic acid is important for water retention in plants

Abscisic Acid Also found in high levels in fruit and leaves that are about to fall from a plant – a process called abscission It was once though that abscisic acid caused abscission (hence the similarity in names); but it doesn’t Ethylene is now known to promote abscission in aging leaves

Ethylene Gas Ethylene is produced by some fruits as they ripen e.g. apples, bananas and tomatoes NOT cherries, grapes and citrus fruits Ethylene increases the rate of respiration, and thus the rate of ripening If one fruit produces ethylene, it can promote ripening in nearby fruits, due to the diffusion of the gas Ethylene gas is used to control the ripening of fruit so that they are ready to eat when they reach the shops

SUMMARY Plant hormones, like animal hormones, are chemical signalling molecules required for growth and development Different plant hormones affect different parts of a plant differently e.g. auxins in stems and roots Plant hormones, like animal hormones, result in signal transduction pathways in plant cells