Hormonal regulation in plants Plants do not have a nervous system or endocrine system However they do produce hormones These hormones are produced by specialised tissues

Hormone Produced in Transport in plant Effects Auxins Apical bud, young leaves Moves from tip to base through cells Growth through elongation of cells, tropisms, apical dominance, growth of fruit and delay of ripening Gibberellins Young leaves, roots, shoots and embryo in seed Xylem and Phloem Growth through cell division and cell elongation. Stimulates germination and flowering CytokininsRootsXylem Growth through cell division and differentiation. Breaks dormancy. Abscisic Acid Older leaves, root caps and stem Xylem and Phloem Dormancy, shedding leaves and fruit, closure of stomata Ethylene Stems, ripening fruit, damaged tissue, seeds Diffusion through intercellular spaces Fruit ripening, aging and shedding of leaves and flowers, germination

Auxins and tropisms Growth of a plant in response to a stimulus: –Light (phototropism) –Gravity (geotropism) Two types: –Positive – growing towards the stimulus –Negative – growing away from the stimulus

Phototropism When a plant moves or grows in response to light, it shows phototropism If it moves/grows towards the light, it shows positive phototropism This behaviour is controlled by a plant hormone called auxin

Phototropism The tip of the shoot shown in the diagram is called a coleoptile This is where auxin is produced The distribution of auxin within the shoot changes depending on the position of the light source

Phototropism Auxin causes the plant to bend towards the light source because: –It moves away from the light source, accumulating on the darker side –causing growth to occur more quickly at the darkest side –because of the difference in growth rates on the dark side and illuminated side of the shoot, it starts to bend If the light source is even, the shoot grows straight up Page 154-5, Fig 5.22

Light source evenly distributed over shoot Due to even light distribution, auxin is also evenly distributed in the shoot.  Growth rates equal on either side of the shoot and grows straight up Auxin concentrates on the dark/shaded side of the shoot causing growth to occur more quickly there Uneven growth causes the shoot to bend towards the light Light source not evenly distributed over shoot Phototropism

Phototropism experiments

Geotropism When a plant moves or grows in response to gravity, it shows geotropism If it moves/grows towards the gravity (like plant roots) it shows positive geotropism If it moves/grows away from gravity (like shoots) it shows negative geotropism This behaviour is also controlled by a plant hormone called auxin

Geotropism Auxin is produced in growing roots and shoots of a plant Auxin moves towards gravity, causing growth to occur more quickly in regions closer to the source of the gravity

Auxin accumulates here so that most growth occurs here, shoot grows upwards Auxin accumulates here so that most growth occurs here, root grows downwards Geotropism

Florigen? The hormonal control of flowering in plants is not well understood Some scientists suspect that a hormone which they have named florigen may be involved However florigen has yet to be isolated (extracted from plants) It’s existence is disputed

Signal Transduction Because plant hormones are generally water-soluble, plant hormones trigger signal transduction in order to carry out their effect on target cells Similar to signal transduction in animal cells