BOTA30005 W1-3

• Long term plant responses

  • Response to mechanical stress over many years
  • Response to wind
  • Compression
  • Modify development and change plant architecture
• Medium term plant responses

  • Phototrophisms (grow towards light)
  • Gravitropism (against force of gravity)
  • Can modify behaviour and generate outputs
• Rapid plant responses


  • Movement detected on hairs of venus flytrap
  • Mimosa pudica can detect touch
  • Involve electrochemical processes
• Multicellularity


  • Evolved independently in plants and animals
  • This has lead to similarities and differences in plant and animal signal behavior
• Signal transduction


  Process by which a cell responds to substances outside the cell through signaling molecules found on the surface of and inside the cell
• Steps in signal transduction

  1. Signal initiates a plant response by binding to receptor

  2. Receptor receives signal and initiates a signal transduction

  3. Signal transduction converses the signal and amplifies it through cascades

  4. A responses occur through the induction of cellular molecules
• Role of cascades


  • Often cause a change in gene expression
  • Activate or deactivate transcription factors to modulate gene expression and a response
  • Amplify initial signal as signal is diluted in cytosol when traveling to nucleus
• Role of receptors


  • Perceive signals in membrane or cytoplasm
  • Hydrophobic signals pass through membranes to reach intracellular receptor
  • Hydrophilic signals perceives at plasma membrane
• Animal membrane receptors


  • G-protein-coupled receptor
  • Enzyme coupled receptor
  • Ion channel-coupled receptor

  
• Role of transcription factors


  • Able to activate/repress genes via elements in promoters
  • Gene products undertake various functions
• Plant receptors

  • Not all in plasma membrane
  • Some hormones transported into cell or activated in the cell
  • Receptors that have evolved from bacterial origins have "two component" system
  • Ethylene and cytokinin
  • Receptors can be similar to animals and fungi
• Plant receptor kinases


  • Large extracellular domains
  • A membrane-spanning domain
  • Intracellular kinase domain
  • Differs to animals which are dominated by G-protein-coupled receptors
  • Arabidopsis encodes for 340 of these and most have no function
• MAP Kinase Cascade


  1. Animals transmit mitogenic signals

  2. Plants transmit a broader range of stimuli

  3. MAP3K and MAP2K activated via phosphorylation

  4. MAP2K intersection and integration between converging signals from MAP3Ks and divergent outputs from MAPK

  5. MAPK activated via dephosphorylation by MAPK phosphatase

  6. MAPK Enables signal specificity through substrate specificity
• Generalised MAPK Signalling steps


  1. Signal/Stimulus

  2. Receptor

  3. Secondary messengers

  4. MAP3K activated

  5. MAP2K activated

  6. MAPK activated

  7. Phosphorylation of target proteins via MAPK to induce transcriptional response
• WRKY


  • Amino acid repeats
  • WRKY TFs mainly present in plants
  • Known to respond to abiotic stress
  • Many MAP Kinase cascades operate through WRKY TFs
• MAPK Targets


  • Ser with Pro repeats
  • MAPKs are proline-directed kinases
• MAPK Drought response


  Drought -> MAP4K -> MAP3K -> MAP2K -> OsMPK3 -> OsWRKY3 phosphorylated -> activated drought response
• MAPK Drought response is not commercialized
• Lignocellulosic Biofuels


  • Cell wall material in plants
  • Plant + CO2 -> Biomass + pretreatment -> cellulose +enzyme -> sugar + microbe = fuel
  • More complicated
  • More bonds and energy
• Issues with biomass (cell walls)

  • Biomass composition many different sugars
  • Biomass recalcitrance difficult to break down
  • Biomass density, low energy compared to oil
• Plant stems


  • Lots of space between cells
  • Primary cell walls: space for oxygen, laid first
  • Secondary cell walls: dense microfibre contains a lot of energy, thicker
• Thickening cell walls to increase density

  • Model system: arabidopsis
  • Cell cultures treated with the hormone brassinosteroid
  • Develop into xylem cells and produce secondary cell walls
  • Xylan biosynthesis instead of 35S promoter is used
• NST and VND


  • NAC secondary wall thickening promoting factor 1
  • Vascular-related NAC domain protein
  • Overexpress NST/VND to increase biomass
• Roles of plant cell walls

  • Determine identity
  • Provide mechanical strength
  • Joins neighboring cells together
  • Acts as a diffusion barrier
  • Defense against pathogens
  • Source of fuels
  • Timber for building, fibre for clothes
  • Quality characteristics of plant-derived foods
• Types of plant cell walls

  • Primary cell walls
  • Secondary cell walls
• Primary cell walls


  • Found around growing cells
  • Thinner
  • High water content
  • Architecturally simple
  • Leaves and herbs
• Secondary cell walls


  • Form after growth
  • Thicker
  • Low water content
  • Architecturally and compositionally complex
  • Trees
• Cell wall polysaccharide components


  • Cell wall is a polysaccharide-rich extracellular matrix
  • A polymer made up of carbohydrates
  • 9:1 polysaccharide: protein
• Monosaccharides


  • D-glucose, linear form, 6 membered pyranose ring form
  • L-arabinose, linear form, 5 membered furanose ring
  • Can have different ring forms
  • 𝞪: hydroxyl group attached to C1 and -CH2OH group at C5 lies opposite sides of ring
  • 𝛃: they are on the same side
  • Can be interconverted, occurs when they are nucleotide sugars
  • Rotation of C4 hydroxyl
  • Oxidation at C6
  • Rotation of C2 hydroxyl

  
• Polysaccharide Biosynthesis


  • NDP-sugar(sugar donor) + sugar(sugar acceptor) -> sugar(n+1)(growing polysaccharide) +NDP
  • Glycosyl transferases are responsible for polymer extension

  
• Sugar Acceptor


  • Linking monosaccharides together
  • Cellulose: made up of D-glucose units linked together through 𝛃(1-> 4) glycosidic bonds
  • Cellobiose is basic repeating unit of cellulose
  • 𝛃(1-> 4) linkage inverts adjacent sugars relative to each other
  • Most stable configuration
  • Cellulose & starch are polymers of glucose
  • Starch: 𝞪(1->4) bond
  • Cellulose: 𝛃(1->4) bond
  • There are fewer opportunities for hydrogen bonds to form in starch -> polymer is less crystalline
• Nucleotide sugars (Sugar Donor)

  • Used in cell wall biosynthesis
  • Activated forms of monosaccharides
  • Glycosyl donors in glycosylation reactions
  • UDP(uridine diphosphate) and GDP(guanine diphosphate) most common nucleoside diphosphates (NDPS)
• Plant cell wall network

  • Complex network of polysaccharides
  • Cellulose microfibrils = strength, basic scaffold
  • Crosslinking hemicelluloses (xylogucan, mixed linkage glucan) = link cellulose microfibrils together
  • Cellulose-xyloglucan network can be embedded in a gel of pectin
  • Pectin = wall hydration and porosity
• Polymer extension


  Linking monosaccharides together

  
• Sugar Acceptor


  \

  • Linking monosaccharides together
  • Cellulose : made up of D-glucose units linked together through 𝛃(1-> 4) glycosidic bonds
• Cellulose


  • Made up of D-glucose units linked together through β(1→4) glycosidic bonds
  • Cellobiose is basic repeating unit
  • β(1→4) linkage inverts adjacent sugars relative to each other
  • Most stable configuration

  
• Cellulose and starch


  Polymers of glucose, linked through (1→4) glycosidic linkage

  
• Starch


  α(1→4) bond
• Cellulose


  β(1→4) bond
• There are fewer opportunities for hydrogen bonds to form in starch, so the polymer is less crystalline