LESSON 2

• Signal transduction pathways


  Link signal reception to response
• Cell communication


  • Cells can signal to each other and interpret the signals they receive from other cells and the environment
  • Signals may include light and touch, but are most often chemicals
  • Studying cell communication has provided evidence for the evolutionary relatedness of all life
  • The same small set of cell-signaling mechanisms shows up again and again in diverse species, in processes ranging from bacterial signaling to embryonic development to cancer
• Plant cell communication


  • Plant cells usually communicate via signaling molecules targeted for cells that may or may not be immediately adjacent
  • Plant cells may communicate by direct contact, one type of local signaling
  • Plants have cell junctions that directly connect the cytoplasms of adjacent cells
  • Signaling substances dissolved in the cytosol can pass freely between adjacent cells
  • Local signaling in plants is not as well understood as in animals due to plant cell walls
• Plant hormones (plant growth regulators)


  • Often travel in vessels but more often reach their targets by moving through cells or by diffusing through the air as a gas
  • Vary widely in size and type, as do local regulators
• Plant hormone


  • Ethylene, a gas that promotes fruit ripening and helps regulate growth
• Mammalian hormone


  • Insulin, which regulates sugar levels in the blood
• Cell signaling process


  1. Reception - Signals are first detected by receptors, proteins that undergo changes in shape in response to a specific stimulus

  2. Transduction - The binding of the signaling molecule changes the receptor protein, initiating the process of transduction which converts the signal to a form that can bring about a specific cellular response

  3. Response - The transduced signal finally triggers a specific cellular activity
• Ligand


  A molecule that specifically binds to another molecule, often a larger one
• Second messengers


  Small molecules and ions in the cell that amplify the signal and transfer it from the receptor to other proteins that carry out the response
• Post-translational modification


  Activates preexisting enzymes
• Transcriptional regulation


  Increases or decreases the synthesis of mRNA encoding a specific enzyme
• Plant sensory response


  • Relies on chemical messengers (hormones)
  • Affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall
  • Present in very small amounts, transported throughout the plant body, and only elicit responses in cells which have the appropriate hormone receptors
  • Travel throughout the body via the vascular tissue (xylem and phloem) and cell-to-cell via plasmodesmata
  • Potentially every cell in a plant can produce plant hormones, unlike animal hormones which are produced only in specific glands
• Phototropism


  Growth or movement in the direction determined by the direction of a light stimulus
• Geotropism

  Growth or movement in the direction determined by the direction of earth's gravity
• Hydrotropism


  Growth or movement in the direction determined by the availability of water
• Thigmotropism


  Directional growth in response to touch
• Plant Hormones


  • Auxin
  • Cytokinins
  • Gibberellins (GA)
  • Abscisic acid (ABA)
  • Ethylene
  • Brassinosteroids
  • Jasmonates
• Auxin


  Produced in shoot apical meristems and young leaves

  Stimulates stem elongation (low concentration only)

  Promotes the formation of lateral and adventitious roots

  Regulates development of fruit

  Enhances apical dominance

  Functions in phototropism and gravitropism

  Promotes vascular differentiation

  Retards leaf abscission
• Cytokinins


  Synthesized primarily in roots and transported to other organs

  Regulate cell division in shoots and roots

  Modify apical dominance and promote lateral bud growth

  Promote movement of nutrients into sink tissues

  Stimulate seed germination

  Delay leaf senescence
• Gibberellins (GA)


  Produced in meristems of apical buds and roots, young leaves, and developing seeds

  Stimulate stem elongation, pollen development, pollen tube growth, fruit growth and seed development, germination

  Regulate sex determination and the transition from juvenile to adult phases
• Abscisic acid (ABA)


  Synthesized in almost all plant cells and transported in phloem or xylem

  Inhibits growth

  Promotes stomatal closure during drought stress

  Promotes seed dormancy

  Inhibits early germination

  Promotes leaf senescence

  Promotes desiccation tolerance
• Ethylene


  Produced by most parts of the plant, especially during senescence, leaf abscission, and fruit ripening

  Promotes ripening of many types of fruit

  Promotes leaf abscission

  Promotes the triple response in seedlings

  Enhances the rate of senescence

  Promotes root and root hair formation

  Promotes flowering in the pineapple family
• Brassinosteroids


  Present in all plant tissues, with different intermediates predominating in different organs

  Promote cell expansion and cell division in shoots

  Promote root growth at low concentrations, inhibit root growth at high concentrations

  Promote xylem differentiation, inhibit phloem differentiation

  Promote seed germination and pollen tube elongation
• Jasmonates


  Derived from the fatty acid linolenic acid, produced in several parts of the plant and travel in the phloem

  Produced in response to herbivory and pathogen invasion

  Regulate fruit ripening, floral development, pollen production, tendril coiling, root growth, seed germination, and nectar secretion
• Drought


  • Water loss by transpiration exceeds water absorption from the soil
  • Prolonged drought will kill a plant
  • Plants have control systems that enable them to cope with less extreme water deficits
  • Responses to water deficit help the plant conserve water by reducing the rate of transpiration
  • Water deficit in a leaf causes stomata to close, thereby slowing transpiration dramatically
• Flooding


  • Overwatered houseplant may suffocate because the soil lacks the air spaces that provide oxygen for cellular respiration in the roots
  • Some plants are structurally adapted to very wet habitats, e.g. mangroves with aerial roots exposed to oxygen
  • Oxygen deprivation stimulates the production of ethylene, which causes some cells in the root cortex to die, creating air tubes that function as "snorkels" to provide oxygen to the submerged roots
• Salt Stress


  • Excess sodium chloride or other salts in the soil can cause a water deficit in plants even though the soil has plenty of water by lowering the water potential of the soil solution
  • Sodium and certain other ions are toxic to plants when their concentrations are too high
  • Many plants can respond to moderate soil salinity by producing solutes that are well tolerated at high concentrations to keep the water potential of cells more negative than that of the soil solution without admitting toxic quantities of salt
  • Most plants cannot survive salt stress for long, except for halophytes with adaptations like salt glands that pump salts out across the leaf epidermis
• Heat Stress


  • Hot, dry weather tends to dehydrate many plants; the closing of stomata in response to this stress conserves water but then sacrifices evaporative cooling
• Cold Stress


  • When the temperature of the environment falls, cell membranes lose their fluidity as the lipids become locked into crystalline structures, altering solute transport across the membrane and adversely affecting the functions of membrane proteins
  • Plants respond to cold stress by altering the lipid composition of their membranes
• Defense against pathogens


  1. Physical barrier presented by the epidermis and periderm of the plant body

  2. Mechanical wounding of leaves by herbivores opens up portals for invasion by pathogens

  3. Pathogens can also enter through natural openings in the epidermis, such as stomata

  4. PAMP-triggered immunity - plant recognizes pathogen-associated molecular patterns (PAMPs) and mounts a chemical attack to isolate the pathogen and prevent its spread

  5. Hypersensitive response - local cell and tissue death at and near the infection site, which may restrict the spread of the pathogen
• Defense against herbivores - Molecular level


  • Plants produce chemical compounds that deter attackers, such as terpenoids, phenolics, and alkaloids
  • Some terpenoids mimic insect hormones and cause insects to molt prematurely and die
  • Some phenolics, like tannins, have an unpleasant taste and hinder the digestion of proteins
• Defense against herbivores - Cellular level

  • Trichomes on leaves and stems hinder the access of chewing insects
  • Laticifers and central vacuoles may serve as storage depots for chemicals that deter herbivores
  • Idioblasts are specialized cells that contain needle-shaped crystals of calcium oxalate called raphides, which penetrate the soft tissues of the tongue and palate, enabling an irritant produced by the plant to enter animal tissues and cause temporary swelling
• Defense against herbivores - Tissue level


  • Some leaves deter herbivores by being especially tough to chew as a result of extensive growth of thick, hardened sclerenchyma tissue
• Defense against herbivores - Organ level

  • Spines (modified leaves) and thorns (modified stems) provide mechanical defenses against herbivores
  • Bristles on the spines of some cacti have fearsome barbs that tear flesh during removal
  • Some plants mimic the presence of insect eggs on their leaves, dissuading insects from laying eggs there
• Defense against herbivores - Organismal level

  • Mechanical damage by herbivores can greatly alter a plant's entire physiology, deterring further attack, e.g. a wild tobacco plant changing the timing of its flowering to avoid herbivorous moth larvae
• Defense against herbivores - Population level

  • Some plants can communicate their distress from attack by releasing molecules that warn nearby plants of the same species, e.g. lima bean plants releasing chemicals that signal "news" of the attack to noninfested neighbors
  • Masting - a phenomenon in some species where a population synchronously produces a massive amount of seeds after a long interval, overwhelming local herbivores
• Defense against herbivores - Community level

  • Some plant species "recruit" predatory animals that help defend the plant against specific herbivores, e.g. parasitoid wasps that inject their eggs into caterpillars feeding on plants
  • A leaf damaged by caterpillars releases compounds that attract parasitoid wasps