Phytohormones and phototropism

phytohormones

• plant hormones

• involved in chemical signalling in plants

• for growth, development, response & reproduction

phototropism

• phototropism is a directional growth response to lateral light

• occurs in plant shoots due to unequal distribution of auxin

auxin characteristics

• auxin is produced in the shoot apex / apical bud

• IN = axn. influx carriers (passive transport/facilitated transport)

• OUT = axn. efflux carriers (active transport)

positioning of axn. efflux carriers

• can be positioned on one side of cell membranes

• redistributes auxin away from the side with lateral light

• creates concentration gradient

phototropism cellular mechanism

• axn. causes cell elongation

• auxin activates hydrogen ion transport proteins / proton pumps that secrete hydrogen ions into the apoplast

• cellulose fibres become loosened due to breakage of individual cellulose fibres (aided by activation of expansin proteins)

• allows cell to full w fluid and increase turgor to expand the cell

positive phototropism

cell expansion on the side away from lateral light, causes bending of stem towards light

cytokinin

• produced in root tips

• transported from roots to the shoots

growth of plant in relation to ratio of aux. and cyk.

high aux vs low cyk - increases root growth

low aux vs high cyk - increases shoot growth

less root tips - decrease cyk

less shoot tips - decrease aux

• integrate the growth of shoots and roots

ethylene

• gas phytohormone that causes fruit ripening

• works on positive feedback

ripening due to ethylene causes

• increased hydrolysis of starch to mono- & disaccharides → sweeter taste

• increased breakdown of cell walls → softens fruit

• color change: green pigments → yellow, orange, red, etc

• increased volatile compounds → aroma

positive feedback of fruit ripening

• positive feedback causes rapid ripening

  • due to exponential increase in ethylene

• positive feedback causes synchronized ripening

  • due to ethylene gaseous state

  • affecting nearby fruits