Slide 1What to know?Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Final Exam Review 2015What to know?•Acid Growth Hypothesis – Auxin •Allelopath & Spotted Knapweed•GA Stimulation of Seed germination (molecular level)•Polar Transport of Auxin•Ethylene’s affect on seedlings.•ABA action in drought response – leaves and guard cells•Light phase of photosynthesis (Z- diagram, thylakoid membrane)•Nodule formation in legume-rhizobia mutualisms•Phytochrome signaling pathway for lettuce germination•CAM vs C4 pathways•Pathway of synthesis and function of each hormone!Function of growth promoting hormones Auxins Stimulate shoot growth by increasing cell size Tends to inhibit root growth at high concentrations (stimulates at low) Trigger growth in response to light and gravity Promote cell differentiation Gibberellins Stimulate shoot growth by activating auxin Promote seed germinationCytokinins Stimulate growth through cell division Promote cell differentiation Retard aging and leaf senescence**Both auxin and cytokinins are the only hormones that are required for viability. No mutants lacking either of these two hormones have been identified (lethal)Function of growth controlling hormones ABA Inhibits shoot growth Closes stomates Promotes seed dormancy Ethylene Ripens fruit Triggers leaf senescence (export of leaf nutrients prior to leaf drop in fall)Plant HormonesChemical messengers that are produced in one cell or tissue and modulate cellular processes in another cell by interacting with specific protein receptorsIn the absence of a nervous system plant hormones are responsible for coordinating plant development & adjustments based on conditionsHormones function at “vanishingly low” concentrationsHow is the action of hormones controlled?1) number of receptors2) Compatible cell signaling and response pathwaysHormone activity and action Cell responsiveness1) Concentration: controlled by synthesis and degradation pathways2) Form3) Conjugation state4) CompartmentationTransported via the vascular tissue- more abundant in phloemDrought -ABA transported from roots to leaves via xylemAnion trap: under well water conditions apoplastic pH is 6.3 which keeps ABA in its associated form (ABAH). Membrane permeable ABAH diffuses into the mesophyll cells and becomestrapped after dissociating (ABA-) under higher cytosolic pH. This keeps apoplastic ABA levels lowUnder drought conditions the apoplast becomes more alkaline (pH 7.2) which converts ABA intoits dissociated form increasing itsconcentration in the apoplastABA regulation: transport and compartmentationOsmotic adjustment Solutes accumulation in cell drives water uptake Solutes:Inorganic ions, especially K+Restricted to vacuoles to avoid cytosolic enzyme inactivationCompatible solutes-organic ionsAmino acidsSugar alcoholsGlycine betaineOsmotic adjustment develops slowly and helps plants maintain water balance during water deficitPlant responses to water deficitGibberellin activation of seed germinationEmbryo produces GA which diffuses to the aleurone cells surrounding the starch rich endospermIn response to GA the aleurone cells produce protein bodies that release hydrolytic enzymes α-amylase into the endosperm which convert the starch to sugars to fuel embryo growthModel of ABA control of stomatal closureABA binds to receptor which activates two separate signaling pathways that increase cytosolic Ca2+ concentrations by activating vacuole and plasma membrane Ca2+ transportersHigh cytosolic Ca2+ Inhibits proton pumping which slows K+ & Cl- uptake into guard cell Activates K+ and Cl- export transportersABA increases the root to shoot ratio under drought stressHow does increasing the root:shoot ratio help plants cope with drought stress?1) More roots increase soil moisture capture 2) Decreased shoots result in less water demand and transpirational water lossABA maintains seed dormancyVivipary: precocious seed germinationGas exchange limitationsWater uptake preventionWaxy cuticles, suberized layers, and lignified scleridsMechanical constraintInhibitor production- ABARetention of inhibitorsLeaching of inhibitors by precipitation often triggers germination- common in desert environmentsFive mechanisms of seed coat induced dormancyAuxin transport Polar transport: auxin mainly moves unidirectionally from the apical to the basal end of plants Auxin moves through a transmembrane pathway Velocity is 2-20 cm h-1, faster than diffusion Auxin influx: Permease influx transporter located at cell apex transports IAA- through secondary active transport IAAH diffuses in passively High cytosolic pH converts IAAH to IAA- which prevents it from diffusing back out Auxin efflux: Efflux transporters at basal end of cell export auxin (driven by negative membrane potential) Auxin can also be transported in the phloem Labeled efflux transportersGrow effects of Auxin: Acid growth hypothesis Auxin induces elongation by promoting proton pump (H+ATPase) activity by: 1) Increasing mRNA that encodes proton pump 2) Increasing H+ ATPase trafficking 3) Stabilizing H+ ATPase on the plasma membrane Cell walls acidification promotes cell elongation by: 1) Displacing Ca2+ which links cell wall polymers 2) Activating Expansins 3) generating proton motive force which facilitates solute transportGravitropism- growth in response to gravity Using gravity shoot orient growth upward and roots orient growth downward Gravity sensing: Starch-statolith hypothesis -dense plastids full of starch settle at the bottom of the root cell in response to gravity Growth effects of Auxin: Gravitropism How could statolith reorientation trigger a gravitropic response?Growth in response to gravity is mediated by polar transport of auxin and asymmetric growth High concentration on lower side of root inhibits growth Low concentration on upper side stimulates growthGravistimulation triggers pH changes in root cells (acid growth)Growth effects of Auxin: GravitropismGibberellin growth stimulation Mechanism that explain GA induced growth are not as well understood as they are for auxin 1) It doesn’t appear that GA increases growth through