Lecture19 - MCDB321 MCDB321 Introductory Plant Physiology...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: MCDB321 MCDB321 Introductory Plant Physiology March 29, 2011 1. Discovery of auxin 2. Biosynthesis & metabolism of auxin 3. Polar transport of auxin and PIN proteins 4. Physiological functions of Auxin 5. Auxin signaling Phototropism to the discovery of auxin Phototropism The apical tip is the site The to perceive light signal to Differential movement of a transmissible chemical A simple biological assay for measuring [auxin] simple In the mid 1930s, auxin was determined to be determined Indole-3-acetic acid (IAA), the most abundant and the physiological relevant auxin physiological Three requirements for auxin activity 1. A planar aromatic ring 2. A carboxylic group 3. A hydrophobic linker of ~0.5 nm synthetic auxin analogs A young leaf primordium IAA biosynthesis is associated with rapidly IAA dividing and growing tissues SAM, young leaves, and developing fruits and seeds are the primary sites of IAA Biosynthesis Plants may contain more than one auxin, Plants but the most active naturally occurring auxin is indole acetic acid, or IAA. is Regulation of [IAA] concentration by different pathways Regulation 1. de novo biosynthesis of IAA free 2/6. intra- and intercellular transport of IAA 4. conjugation 3. deconjugation 5. degradation IAA is structurally similar to tryptophan and plants can convert tryptophan IAA into IAA via several different biochemical pathways 1. orange pericarp mutants accumulate higher level of auxin level 2. fail to convert tryptophan to IAA 3. feeding radiolabelled anthranilate leads to 3. synthesis of radiolabelled auxin synthesis Conjugation and degradation of IAA Conjugation Polar Transport of Auxin Polar Requires Energy and Is Gravity Independent While other plant hormones are transported in the xylem or phloem, IAA is actively transported through parenchyma tissue such as the cortex, the pith, or the parenchyma cells associated with the vascular tissue (this is faster than just by diffusion). IAA moves down through the plant. Therefore, it moves toward the base in shoots and toward the root tips in roots. (Movement like this in a specific direction is termed "polar" movement). 1. IAA enters the cell either passively as the 1. non-charged form (IAAH) or by secondary non-charged active cotransporter in the anionic form 2. The cell wall is maintained at an acidic pH 2. by the activity of the plasma membrane by H+-ATPase. 3. In the cytosol, which has a neutral pH, the Anionic form (IAA-) predominates. 4. The anionic IAA exits the cell via auxin anion efflux carriers that are concentrated at anion the basal ends of each cell in the longitudinal the pathway Functions of Auxin Functions 1. 2. 3. 4. 5. 6. 7. 8. Stimulates cell elongation/division Mediates the tropistic response of bending in response to gravity and light The auxin from the apical bud suppresses growth of lateral buds ( apical dominance) Stimulates root initiation on stem cuttings and lateral root development in tissue culture Induce fruit setting and growth in some plants Stimulates differentiation of phloem and xylem Delays leaf senescence Inhibit or promote (via ethylene stimulation) leaf and fruit abscission Auxin induced proton extrusion acidifies the cell wall and increases cell extension. wall The “acid-growth” theory The H+ acts as the intermediate between auxin and cell wall loosening---> cell elongation Auxin treatment leads to Auxin 1) Increased gene expression 2) enhanced protein trafficking 3) stabilization of membrane-bound ATPase Phototropism is mediated by the lateral redistribution of auxin Phototropism Lateral redistribution of auxin during phototropism The gravitropism is also due to redistribution of auxin The 1. IAA is synthesized in the shoot and IAA transported to the root in stele. transported 2. When the root is vertical, the statoliths in When the cap settle to the basal ends of the cells. Auxin transported acropetally in the root via the stele is redistributed equally on all sides of the root cap. The IAA is then transported basipetally within the cortex to the elongation zone to regulate cell elongation elongation 3. 5. The high concentration of auxin The inhibits cell elongation, whereas the decreased [auxin] stimulates cell growth on the upper side growth 4. The majority of the auxin is then The transported basipetally in the cortex on the lower side of the root on In the horizontal toot the In statoliths settle to the side of the cap cells, triggering polar transport of IAA to the lower side of the cap side Auxin suppresses the growth of axillary buds in bean plants in Auxin promotes the Auxin formation of lateral and adventitious roots Auxin promotes fruit development Auxin IAA-induced xylem regeneration around IAA-induced The wound in cucumber stem tissue Xylem differentiation occurs around the wound, following the path of auxin diffusion Herbicidal Warfare: Agent Orange Herbicidal 50% 2,4D plus 50% 2,4,5T (dichlorophenooxyacetic acid and trichlorophenoxyacetic acid) 2,3,7,8-tetrachlorodibenzodioxin 21 millions gal. were sprayed 4.8 million Vietnamese were exposed 400,000 deaths/disabilities 500,000 children born with birth defects A model for auxin-regulated gene expression ...
View Full Document

Ask a homework question - tutors are online