Lecture 14

• Photoperiodic Flowering


  Plants that flower earlier or only when day length is above/below a threshold
• Long-Day Plants


  • Flowering occurs earlier or only when day length is above a minimum threshold
• Short-Day Plants

  • Flowering occurs earlier or only when day length is below a maximum threshold
• Day Neutral Plants

  • Flower at the same time regardless of day length
• Florigen

  Mobile signal that stimulates flowering in both short-day and long-day plants
• Florigen


  Protein encoded by the gene FLOWERING LOCUS T
• Photoperiod is measured by external coincidence

  1. Florigen gene expression is controlled by upstream genes that promote or repress its expression

  2. Some of these genes are light-sensitive and their expression is circadian, peaking during a specific interval each 24 hours

  3. They only function to promote florigen in LD plants or repress it in SD plants in long day lengths
• Night breaks (short periods of light exposure at night) inhibit flowering by short-day plants and stimulate flowering by long-day plants in short days
• Vernalization


  Promotion of competency to flower by prolonged exposure to cold
• Juvenile-to-adult phase transition

  • Juvenile plants are not competent to respond to environmental signals that promote flowering
  • Long juvenile phases are a common obstacle to breeding and production of tree crops
• Many tree crops are propagated asexually by grafting to circumvent age-based brakes on flowering
• Fruit development

  1. Auxin produced by developing seeds triggers cell division and expansion in ovary

  2. Seed position determines fruit shape
• Ethylene


  Gaseous plant hormone important for many commercial applications
• Balance between ethylene and auxin

  Regulates the abscission (detachment at a defined zone) of autumn leaves from deciduous trees
• Fruit ripening


  Process by which a tart, hard, green fleshy fruit becomes sweet and soft to attract animals for dispersal
• Ethylene is auto-catalytic; it triggers its own production
• Many fruits are picked green, e.g. grocers ripen bananas in distribution centers by increasing temperature and adding ethylene to accelerate and synchronize ripening
• Plant Biotic Interactions


  • Positive Interactions: pollinators, dispersers, bacterial mutualists, mycorrhizal fungi
  • Negative Interactions: parasitic plants, bacterial pathogens, fungal pathogens, herbivores
• Ethylene

  Regulates fruit ripening and fruit abscission in many crops
• Ripening

  Tart, hard, green fleshy fruit becomes sweet and soft to attract animals for dispersal
• Ethylene

  Auto-catalytic; it triggers its own production
• Fruit ripening

  Many fruits are picked green, then ripened in distribution centers by increasing temperature and adding ethylene to accelerate and synchronize ripening
• Positive plant biotic interactions

  • Pollinators
  • Dispersers
  • Bacterial mutualists
  • Mycorrhizal fungi
• Negative plant biotic interactions

  • Parasitic plants
  • Bacterial pathogens
  • Fungal pathogens
  • Herbivores
• Rhizobium

  • Nitrogen-fixing bacteria associated with legumes (beans, peas, etc.)
  • Association induces formation of root nodules
  • The plant supplies nutrients to the nodule and withdraws nitrogen rich compounds from the nodules
• Ectomycorrhizal fungi

  • Only associated with ~5-10% of plants
  • Common among woody plants in boreal, temperate, and tropical ecosystems; relevant for ecosystem restoration and forestry
  • The specificity of fungal symbiont - plant host associations varies widely
  • Some associations can be obligate
• Arbuscular mycorrhizal fungi

  • Associated with ~85-90% of plant species
  • Plants emit the hormone strigolactone into the soil to attract arbuscular mycorrhizae, kicking off a cascade events to promote and allow hyphal entry into the root
  • Arbuscules increase area of nutrient exchange but remain isolated from the cytosol; are sheathed by invaginations of the plasma membrane
• The "Wood Wide Web"

  • Mycorrhizal fungi networks can connect many plants like a giant underground circulatory system
  • Tracking of radioisotopes shows that materials can be transferred between plants by this network
  • Can direct resources from sources to sinks
  • Can also be hijacked to spread toxins
• Each year, plant diseases cost the global economy $220 billion and are major impact on global food security
• Plant pathogens

  • May include viruses, bacteria, fungi, nematodes, oomycetes, or even other plants
  • A disease requires a pathogen, a susceptible host, and a favorable environment
  • Climate change and other anthropogenic impacts threaten to make existing diseases worse or lead to the emergence of new diseases
• Parasitic plants

  • Have partly or wholly given up making their own sugars or obtaining their own water or nutrients. They steal them from other plants or the fungi associated with them instead
  • ~1% of all plants are parasitic
  • Example: Striga (witchweed) ravages over 100 million hectares of cereal and legume crops annually, causing severe losses throughout Africa and parts of Asia
  • Germinates in response to strigolactones
• Fungal pathogens

  • Smuts, rusts, blights, mildews (though not downy mildew)
  • Evidence of fungi plaguing grain crops dates back thousands of years
  • e.g., rice blast fungus destroys enough rice to feed 60M people / year
  • e.g., flour from rye infected with ergot can cause gangrene, spasms, hallucinations, and temporary insanity
• PAMP-triggered immunity (PTI)

  General features of pathogens (e.g., flagellin, chitin) are recognized and responded to
• Effector-triggered immunity (ETI)

  • NLRs (nucleotide-binding/leucine-rich-repeat proteins) recognize effectors OR plant proteins that have been altered by effectors
  • NLRs often then act to elicit immune responses including means of subcellular resistance and cell death, i.e. the hypersensitive response
• Hypersensitive response

  • Programmed cell death of infected cells and their neighbors
  • Includes production of antimicrobial molecules, adding more lignin to cell was to seal of infection, and destruction of cell contents. Prevents pathogen spread by depriving it of nutrients.
• Hypersensitive response

  1. Prior to death, the infect cells release a signal, methylsalicylic acid, into the phloem which moves through the plant

  2. In rest of plant, methylsalicylic acid is converted to salicylic acid (SA), which induces systemic acquired resistance

  3. Systemic acquired resistance is a state of elevated resistance to infection not just by the triggering pathogen but all pathogens. It can last for days to months. It can reduce growth rate, however.
• Key terms

  • long-day / short-day / day neutral plant
  • facultative / obligate
  • florigen
  • external coincidence
  • vernalization
  • juvenile -> adult phase transition
  • ethylene
  • ripening
  • abscission
  • Rhizobium
  • strigolactone
  • Striga
  • PAMP-triggered immunity
  • effector / effector-triggered immunity
  • NLRs
  • hypersensitive response
  • systemic acquired resistance