1. Chapter 14 Physiology – Hormones, signals, growth and development

2. Topics What is a hormone? – method of action
Roles in survival of a sessile organism in the environment
Mostly studied five plant hormones
Other hormones
Specific growth and developmental effects
Hormone interactions
Environmental signaling thru hormones
Agricultural uses

3. Hormones - Chemical Messengers
Regulate growth, differentiation and development (among other roles e.g. stomatal activity, environmental signaling)
Organic molecules produced in one part of the plant and transported to other parts where they initiate a response – 3 common characteristics
Act in very low concentrations and in collaboration
Released into general circulation - not carried specifically to the target
At the site of response, receptor molecules, usually on plasma membrane, bind to them triggering a variety of gene expressions

5. Hormones

6. Hormones
Other compounds show hormone-like activity - much less studied than the big five
Brassinosteroids (first isolated from rapeseed (Brassica napus) = complex chemicals involved in leaf morphogenesis, root and stem growth, and vascular differentiation – auxin-like effect – works thru auxins?
Jasmonic acid (JA) - involved in defense against animals and fungi
Salicylic acid (SA - related to aspirin) - involved in resistance to pathogens, especially viruses

7. Auxin: Activation/Inhibition of Shoots
Auxin - produced as shoot apical meristems (SAM) grow and initiate new cells
Auxin flows mainly downward, surrounds all stem cells, causing 3 unique responses:
Cell elongation
Apical dominance
Differentiation of vascular tissues

8. Auxin: Apical dominance
Apically produced auxin induces dormancy in axillary buds
Each shoot tip gets only one active apical meristem
In lower buds where auxin level is below a threshold – buds grow branches, flowers etc.
Spring - buds release auxins to stimulate vascular cambium in stem

9. Hormone Interactions Auxins and Cytokinins:
In some species, apical dominance is established by interplay of 2 or 3 hormones
Active roots synthesize cytokinins
Cytokinins are transported to the shoot and stimulate axillary buds
Ratio of cytokinin and auxin determines whether buds become active or remain dormant

10. Hormone Interactions Auxins and Gibberellins:
Auxin alone activates vascular cambium and elicits differentiation of xylem
Gibberellin is also present in a healthy stem and causes some new cells to differentiate as phloem

11. Hormones: Environmental signaling
Hormones help communicate with various plant parts when one part has encountered an environmental change
If a leaf is damaged by animal feeding or water stress, auxin production and transport drop - abscission zone does not stay quiescent
Autumn conditions stimulate production of ethylene

12. Hormones: Environmental signaling
Ethylene suppresses auxin production and transport in time for abscission before winter
Premature abscission of fruits is prevented by the presence and export of auxin through the pedicel

13. Hormones: Environmental signaling
Auxin causes the developing fruits to release ethylene
At maturity, high concentration of ethylene stimulates the pedicel abscission zone, overriding auxin concentration

14. Hormones: Tropisms Blue - most effective wavelength for phototropism
Small pigment protein called phototropin in stem tips or coleoptile (outermost protective leaf at the stem tip) absorbs blue light
Darker side of the stem receives extra auxin
That side grows more rapidly and bends toward the light

15. Hormones: Tropisms
In gravitropism - root cap acts as the organ of perception of gravity by statocytes with their statoliths
Lower side of the root is detected by sinking statoliths - extra auxin transported to that lower side of the root cap and then into the root
Growth inhibition on lower side
Root grows downward

16. Flowering: Vernalization and Photoperiodism
Conversion from juvenile to adult is phase change – flowering (fruits, seeds – survival or agricultural significance)
Cold temperatures stimulate phase change in some – vernalin production – flowering
Photoperiod (day length – night length really) triggers flowering in some – phytochrome perceives length of night – next slide
Some need both critical low T and night length
Hormones mediate these responses too – e.g. GA can substitute vernalization and night length requirements in some

17. Photoperiodic Induction to flower more common in temperate spp.
Pr - regulates circadian and seasonal phenomena
Day neutral plants - no phytochrome regulation

18. So many agricultural uses – growth regulators
Seed germination - GA
Propagation by cuttings, rooting - Auxin
Partheniocarpic fruits - Auxin, GA, Cytokinin
Flower and fruit set or fall - Auxin induced Ethylene, Cytokinin
Fruit size - Auxin
Fruit ripening, abscision - Ethylene
Synchronized flowering - friuting - Ethylene
Cut flower, vegetable postharvest shelf life - Cytokinin
Stem/stalk length - GA
Weedicides - Auxin
Hybrid plant production e.g. cucumber hybrids - GA
Antitarnspirants, enhanced color in red grapes - ABA
Tissue culture - Auxin, cytokinin