Intro to Plant Growth Regulators

Question 1

what are plant growth regulators?

Answer 1

chemical used to modify plant development and growth, often small amounts needed
- incl both naturally-occurring hormones (endogenous) and synthetic substances (exogenous)

Question 2

growth regulators: signal-transduction pathways

Answer 2

• growth regulators act by binding to a specific regulator, thereby initiating a series of biochemical events (response)
• response can be positive (turn on) or negative (off)

Question 3

plant hormones share 3 basic elements with mammalian hormones

Answer 3

1. synthesis in one part of body
2. transport to another part of body
3. induction of a chemical response to control a physiological event

Question 4

hormones direct growth and development: (3)

Answer 4

1. hormones both promote and inhibit responses
2. effects of a particular hormone depends on concentration, location, and timing
3. key component of plant’s communication system (cell-cell or long-distance comms)

Question 5

“effects of a hormone depends on ___, ___, and ___”

Answer 5

concentration
location
timing

Question 6

Plant Hormones: Auxin

Answer 6

first plant hormone discovered
- charles darwin and son francis studied grass seedling phototropism
- tip of sheath covering shoot (coleoptile) critical to response
- Fritz Went found a substance accumulating in agar & causes growth on coleoptiles w removed tips

Question 7

auxin structural formula?

Answer 7

indoleacetic acid (IAA)
- modified AA from tryptophan and the most common of natural auxins
- mutants entirely lacking auxin have never been isolated (essential to development + function)

Question 8

auxin and cell elongation (detailed)

Answer 8

• activates H+ ATPase in cell membrane to pump protons into cell wall
• resulting decrease in cell wall pH activates expansion enzymes
• expansins cleave bonds between cellulose and hemicellulose weakens cell wall
• process called “acid growth hypothesis”

Question 9

acid growth hypothesis

Answer 9

expansins temporarily disrupt hemicellulose strands to relax tension, increase space between cellulose fibers
- cell wall expansion IRREVERSIBLE

Question 10

auxin and etiolation

Answer 10

• response of angiosperms grown with little to no access light; plant attempts to reach potential light source

features of an etiolated plant incl:
- elongation of stem and leaves via increased auxin levels
- longer internodes (fewer leaves)
- chlorosis (lack of chlorophyll)

Question 11

auxin functions

Answer 11

• short-term effect is stimulate cell elongation

Promotes:
- root-shoot axis during embryogenesis
- lateral (adventitious) root formation
- differentiation of procambium and de-differentiation of vascular tissues
- fruit development

Inhibits:
- axillary buds and secondary branching
- lateral root length
- abscission in young leaves and immature fruit (when levels drop with maturity, abscission can occur)

Question 12

Auxin synthesis and transport

Answer 12

ALL plant tissues can make IAA
- higher production in meristems, buds, young leaves, actively growing parts
- highest levels of auxin synthesized in shoot tip, maxima in leaf primordia
- auxin synthesized in leaves can be transported in sap of phloem sieve tubes (non-polar mass flow) to rest of plant

Question 13

auxin polar transport

Answer 13

in addition to phloem transport, active polar transport occurs
- polar auxin transport facilitated by IAA influx and efflux transporters
- main route of polar auxin transport in stems and leaves is through parenchyma cells surrounding vascular bundles
- polar auxin transport is unidirectional

Question 14

auxin synthesis and transport in shoots

Answer 14

IAA concentration gradient forms, with highest levels in shoot tip
- shoot-root orientation determined by polar auxin transport as IAA moves away from source (shoot tip) towards roots due to unidirectional placement of efflux/influx transporters

Question 15

auxin and apical dominance

Answer 15

auxin from shoot tip suppresses axillary buds from becoming lateral branches
- removal of shoot tip, reduces auxin levels

auxin involved in apical dominance, suppression of axillary bud growth closest to shoot tips

Question 16

auxin transport and synthesis in roots

Answer 16

auxin moves from shoot through sieve tubes (bulk flow) in vascular cylinder to root tip (acropetal)
- once at root tip, auxin redirected to epidermis and cortex via polar transport and back towards to root-shoot junction (basipetal)
- in addition to polar transport fro shoot, IAA also made in RAM

Question 17

auxin promotes lateral root formation

Answer 17

accumulation of auxin in pericycle stimulates cell expansion and later root formation
- too much auxin will inhibit root length

Question 18

auxin and vascular tissues

Answer 18

• auxin synthesis promotes differentiation in procambium (formation of primary xylem/phloem) in shoot and root meristems
• auxin transport and synthesis from surrounding cells initiates de-differentiation of vascular tissue in formation of vascular and cork cambium in secondary growth

Question 19

auxin and fruit development

Answer 19

• auxin promotes fruit development through expansion and by inhibiting fruit ripening and abscission
• developing seeds are a source of auxin to promote maturation of ovary wall
• parthenogenic fruit formation (fruit without fertilization) can be stimulated in some species by exogenously applying auxin

Question 20

synthetic auxin

Answer 20

synthetic vers:
2,4-D (2,4-dichlorophenoxyacetic acid), NAA (1-naphthaleneacetic acid)

NAA induces adventitious roots in plant cuttings, reduce fruit absission

synthetic auxins are not as readily degraded by IAA oxidase as IAA, so plants retain artificially high levels of compounds

can be lethal; 2,4-D first herbicide (kill broad-leaf plants, allow grass crops to live)

broad-leaf plants (dicots) absorb and transport synthetic auxins more than grasses (monocots), more susceptible to effects of ethylene production (senescence and abscission)