EFB530 Plant Physiology

Discovery of a plant hormone-Auxin

Experiments were directed at the process of phototropism

• directional response of plant growth toward a light source
• typically was studied using oat seedlings (Avena)

First was to determine the site of light perception, then to determine how that signal is transmitted to the site of response

Charles and Francis Darwin (1880)

• studying curvature of seedlings toward a unidirectional light
• cut off the tip (of coleoptile), saw no bending
• covered the tip with foil, saw no bending
• therefore, signal was perceived in the tip

Boysen-Jensen (1913)

• used pieces of mica, inserted below tip on side opposite light=no curvature
• inserted on same side as light, curvature OK
• therefore, needed transport of signal along the opposite side as light

Frits Went (1926) (as a graduate student)

• cut off tips and put them on agar for a while
• then cut off the tip of another seedling, put the agar block on top, in the dark
• when he put it on an edge, the seedling curved away from the side with the block
• therefore, there was a signal diffusing from the tip into the block, stimulated growth on the side opposite from where light is perceived
• was used as a bioassay, more auxin = greater curvature
• aided in the purification of auxin and identification

Biochemistry

Structure of indole-3-acetic acid (IAA):

see Figs. 19.3, 19.4

• IAA is the primary natural auxin (others exist, but have unknown roles)
• IAA is synthesized mainly in the apical meristems-shoot tips, young leaves, developing fruit
• biosynthetic pathway is associated with tryptophan synthesis: either tryptophan-dependent (derived from tryptophan) or tryptophan-independent (uses some of the precursors to tryptophan)

• compounds with biological activity of auxins can be chemically synthesized (synthetic auxins)
• alpha-naphthalene acetic acid (a-NAA); 2,4-dichlorophenoxyacetic acid (2,4-D)

Plants can synthesize auxin, then store or transport it in an inactive form

• IAA can be conjugated (covalently bound) to other compounds
• conjugates are often sugars (IAA-glucose) or amino acids (IAA-aspartate)
• these are not biologically active until conjugate is cleaved off

IAA can move through the phloem from mature leaves, but its primary path of transport is through cells adjacent to the vascular bundles (parenchyma cells)

This movement is directional = polar transport

can do donor and receiver studies with coleoptile segments

• IAA in agar block is a donor, plain agar block is receiver
• with donor block on apical end of segment (tip end), observe IAA accumulation in receiver block at basal end - even if you flip the segment upside-down
• with donor block at basal end, there is no movement to the receiver at apical end
• movement is polar from apex to base in shoots, toward tip in roots

Polar transport requires energy (ATP), but indirectly

• in the cell wall (pH is approx. 5), IAA is protonated to IAAH
• IAAH can diffuse across membrane into cell, down a concentration gradient
• when inside (pH is approx. 7), IAAH is deprotonated to IAA^-
• IAA^- cannot pass through lipid membrane, it must go through a carrier protein
• the efflux carrier proteins are localized in the membrane on the basal side of the cell
• IAA^- diffuses into the cell wall space down a concentration gradient
• when in the cell wall, IAA^- is protonated to IAAH again, diffuses to next cell

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