Plant Response to Signals and Feedback ₍˄•͈⚇•͈˄₎

The Darwin Jensen Experiment

  • highlights how plants communicate and transduce signals to coordinate growth responses

  • demonstrated that the tip of a plant shoot perceives environment stimuli like light/gravity

  • transmits a chemical signal that regulates growth direction

  • the process underpins tropisms → directional growth, movements in response to external stimuli

Tropism

  • directional growth response

  • gravitropism

    • refers to growth in response to gravity

    • shoot shows negative gravitropism by growing upward → not following the force of gravity

    • roots show positive gravitropism by growing downward → following the force of gravity

    • even in darkness, when placed on their sides, shoots curve up towards and roots downwards, indicating an important sensing mechanism

  • phototropism

    • the growth of plant organs in response to light, shoots display positive phototropism by growing toward light sources

    • roots show negative phototropism by growing away from light

    • the hormone auxin accumulates on the side of the shoot that is away from light → stimulates cell elongation which causes the stem to bend towards the light

  • thigmotropism

    • a directional growth response to touch stimuli

    • commonly seen in climbing plants and vines where tend rites coil around supports to provide structural stability

Role of Auxin in Tropism

  • auxin is a crucial plant hormone responsible for mediating growth responses during tropisms

  • it is water soluble chemical that moves through plant tissues in solution

  • auxin distribution is asymmetric during tropic responses and its accumulates more on one side of the organ to stimulate differential cell elongation

  • in phototropism auxin concentrates on the shaded side which promotes elongation and causing bending toward the light

  • in gravitropism auxin redistributes to regulate curvature of shoots and roots according to gravity’s direction

Circadian rhythms in plants

  • circadian rhythms

    • internal biological clocks (24 hrs)

    • allows plants to anticipate and adapt to daily environmental changes

    • these rhythms persist even under constant light/darkness

  • physiological processes

    • stomatal movements

      • opening and closing of stomata to regulate gas exchange and water loss follow a circadian pattern

    • photosynthetic enzyme production

      • enzymes that are critical for photosynthesis are produced rhythmically to optimize energy capture

  • light detection mechanisms

    • phytochrome

      • photoreceptors that primarily detect red light and regulate processes such as seed germination and response to day length (photoperiodism)

    • blue-light receptor

      • these receptors detect blue light

      • play a central role in phototropism by mediating growth toward light

  • integration of circadian rhythms and tropisms

    • plants integrate circadian rhythm with tropic responses to optimize growth and environmental responsiveness

Homeostasis and feedback systems

  • the maintenance of a stable internal environment despite external changes

  • relies on control systems primarily feedback mechanisms to regulate physiological conditions around a target set point

Negative Feedback

  • restores systems to their normal state when disrupted

  • operates molecular and cellular levels to maintain balance

  • antagonistic hormones often mediate this, where one hormone counters the effect of another

Positive Feedback

  • amplifies responses

  • moving variables further from the set point

  • initiates additional responses that enhance and perpetuate the system change

Examples and biological significance

  • blood glucose regulation

    • maintained by antagonistic hormone

    • actions from pancreatic islets

    • alpha cells → secrete glucagon when blood glucose is low

    • stimulating glycogen breakdown in the liver to increase glucose levels

    • beta cells → secretes insulin when blood glucose is high, stimulating glucose uptake and glycogen formation to lower glucose levels

    • negative feedback

  • body temperature regulation

    • maintains normal body temperature

    • if temperature falls, blood vessels constrict sweat secretion stops → muscle contractions (shivering) which generates heat

    • if temperature increases, blood vessels dilate and sweat glands secrete fluid to dissipate heat through evaporation

    • negative feedback

  • childbirth

    • uterine concentrations increases strength/frequency as the cervix stretches until delivery

    • positive feedback

  • fruit ripening

    • ethylene release promotes ripening, which increases ethylene production

  • lactation

    • baby suckling stimulates prolactin release, enhancing milk production

Cell signaling and glucose uptake

  • glucose uptake involves signaling pathways that mobilizes GLUT4 transporters to the plasma membrane

  • facilitating glucose entry into cells

Cytokines and immune response

  • cytokines

    • are chemical messengers coordinating immune cell activity and systemic response like fever and inflammation

    • excessive cytokine production can cause a positive feedback loop when as a cytokine storm leading to severe inflammation and organ damage

    • such storms can be life threatening by overwhelming organ like lungs and kidneys

    • early detection of inflammation markers aids in managing cytokine storm syndromes