Mechanisms of Chemical Control in Plants

• Plant hormones are chemical messengers that regulate sensory responses of plants to external stimuli.
• Plant hormones, together with water and other metabolites, are usually transported to different plant tissues through the vascular bundles.
• Cell-to-cell transport of hormones is possible through the plasmodesmata.
• Auxins control cell elongation in stems and leaves as induced by light exposure and in roots as a response to gravity. 
• Cytokinins are mainly responsible for the regulation of cell division.
• Gibberellins are responsible for shoot elongation, seed germination, and fruit and flower maturation.
• Abscisic acid or ABA is the primary hormone for stress response and stomatal closure.
• Ethylene is responsible for promoting fruit ripening, flower wilting, and leaf fall.
• Phototropism refers to the movement of plant parts toward or away from a light source. 
• Light exposure of the apical meristem causes the accumulation of auxin on the shaded side of the tissue.
• Seed Germination - Upon exposure to light, light-sensitive proteins signal the production of the enzyme amylase that breaks down the starch in the seed to produce simple sugars.
• The intake of too much water activates gibberellin that signals amylase production.
• Gravitropism refers to the movement of plant parts as a response to gravity. 
• When the plant is tilted, the starch granules drop to the bottom cell wall, which triggers auxin production in the root apical meristem.
• The action of light-sensitive proteins that detect blue light causes stomata to open to allow the gas exchange for photosynthesis to occur.
• Abscisic acid triggers stomatal closure to prevent excessive water loss.
• Thigmotropism refers to the movement of plants in response to a touch stimulus.
• Similar to other plant responses, thigmotropism may also be mediated by hormones.
• Despite the pathways that lead to stomatal closure at night, the small gap between guard cells can still allow small molecules such as gases to pass through. 
• Basta Cytokinins

  A) A

  B) B

  C) C

  D) 1

  E) 2

  F) 3

  G) 4