1. Chapter 39: Plant responses to internal & external signals
Plants= sessile
must respond to environmental changes/cues by adjusting patterns of growth & development
Signal transduction pathways in plants
(ex. De- etiolation/greening)
Environmental signal conformational change in protein
Ex. Phytochrome (photoreceptor) in cytoplasm detects light
Second messengers amplify signal
Ex. Phytochrome activation increases levels of cGMP & Ca2+ changes in ion channels activating protein kinasesactivate other enzymes
Regulation of one or more cell activities
usually by activating enzymes or stimulating gene transcription
Ex. Secondary messengers activate transcription factors stimulate photosynthesis enzymes, chlorophyll production, or affect hormones that regulate growth

3. Plant hormones & their responses
Hormones=chemical signals that coordinate different parts of an organism
Types:
Auxin (AA)
Cytokinins
Gibberellins
Brassinosteroids
Abscisic acid (ABA)
Ethylene

4. Auxin (AA)/Indoleacetic acid (IAA)
Functions:
Stimulate elongation of cells in young shoot
Stimulates proton pumps action potential activates expansin enzymesallow cell membrane to expandmore H2O enters
Increases gene expression of growth proteins
Induces cell division in vascular cambium & differentiation of secondary xylem
Regulates fruit development
Produced in:
Embryo of seed
Apical meristems

5. Cytokinins Functions: Works in conjunction with auxin
Stimulates cytokinesis
Influences differentiation
Stimulates germination
keeps plant green longer
Works in conjunction with auxin
Most common cytokinin=zeatin
Produced in:
Actively growing tissues, particularly roots, embryos, & fruit

6. Gibberellins Functions: Produced in:
Elongate stems (stimulates expansin enzymes)
Stimulate leaf growth
Stimulate fruit growth
Stimulate seed germination (break dormancy)
Produced in:
Roots
Young leaves
Embryos of seed

7. Brassinosteroids Functions:
Induce cell elongation & division in stem segments & seedlings
Slow leaf abscission
Promote xylem differentiation
Inhibit root growth
Chemically similar to cholesterol & animal sex hormones
Produced in:
Seeds
Fruit
Shoots
Leaves
Floral buds

8. Abscisic Acid (ABA) Functions: Produced in: Slows growth
Inhibits growth
Closes stomata during H2O stress (alters Ca2+ levels which effects K+)
Promotes seed dormancy
Produced in:
Leaves
Stems
Roots
Green fruit
Slows growth
Inactivated by large amounts of H2O, light or prolonged cold exposure
ABA to giberellin ratio often determines whether seed germinates or remains dormant

9. Ethylene Functions: positive feedback mechanism Produced in:
Promote fruit ripening (attracts animals to spread seeds)
Causes enzymes to break down cell walls to soften fruit
Causes enzymes to convert starches to sugars for taste
Activates scent & color to display ripeness
May promote or inhibit development of roots, leaves, & flowers (dependant on species)
Opposes auxin affects
Activates enzymes to break down chemical components of cells leading to leaf abscission or plant death after flowering
positive feedback mechanism
Produced in:
Tissues or ripening fruit
Nodes of stems
Aging leaves & flowers

10. Photomorphogenesis effects of light on plant morphology
Light effects plant growth & development
Plants detect light presence, direction, intensity, & wavelength (color)
Blue-light photoreceptors
Initiates responses in plants including
Phototropism (phototropin blue pigment)
Light induced stomata opening (carotenoid based zaxanthin)
Light induced retardation of hypocotyl elongation upon emerging from soil (cryptochrome pigment)
Phytochromes photoreceptors
Photoreceptor switches between Pr & Pfr forms
When Pr active- growth inhibited; Pfr active- growth stimulated
Responses include:
Stimulating seed germination
Aid in tracking seasons
Shade avoidance

11. Biological clocks & circadian rhythms
Physiological processes of plant including production of photosynthesis enzymes, stomata opening/closing, raising/lowering of leaves follow a 24 hour pattern even under controlled conditions
Circadian rhythms
Cycle in 24 hour frequency
Not necessarily connected to an environmental variable
Without environmental signal rhythm occurs in a hour frequency
Light places rhythms on a 24 hour cycle
In plants a result of blue light photoreceptors & phytochrome
Light causes phytochrome to switch between Pr & Pfr
Allow plants to adjust to seasonal differences in day & night

12. Photoperiodism
Physiological response of plant to photoperiod (relative length of day or night)
Seasonal responses of plant
Short day plants (long night)
Flower only if light period is shorter than critical length
Flower in late summer, fall, or winter
Long day plants (short night)
Flower only if light period is longer than critical length
Flower in late spring or summer
Day neutral plants
Flowering unaffected by photoperiod
***Plants really respond to night length not day length as names suggest***
Leaves detect photoperiod
signal buds to develop flowers through signal florigen
Florigen activates organ identity genes

14. Other environmental influences on plant growth: abiotic
Gravity
Shoot grows upward (negative gravitropism)
Roots grow downward (positive gravitropism)
Statoliths (starch dense plastids) settle at lower portions of cell triggering redistribution of Ca2+ & thereby affecting auxin
Help determine up vs. down in plant
Mechanical stimuli
Thigmomorphogenesis- plant changes from due to mechanical perturbances
Viny plants grasp & grow around supports it encounters to stabalize itself
Touch sensitive plants can send action potentials similar to animal nerve impulses to neighboring organs

15. Other environmental influences on plant growth: abiotic
Drought
Low H2O in leaf- guard cells lose turgor- stomata close
H2O deficit increases abscisic acid in leaf to keep stomata closed
Cell expansion inhibited by lack of H2O; young leaves do not grow- decreases surface area for H2O loss
Conserves H2O but minimizes photosynthesis thereby decreasing crop yield
Flooding
Too much H2O suffocates roots- no O2 for respiration
Excess H2O- increased ethylene – root cortex undergoes apoptosis creating air tubes for O2 to reach submerged roots
Salt stress
Too much salt lowers H2O potential of soil causing roots to lose H2O to soil
Plant responds by producing organic compound solutes that keep Y of cells lower than soil

16. Other environmental influences on plant growth: abiotic
Heat stress
High heat denatures enzymes of plant, damaging plant metabolism
Transpiration lowers leaf temps through evaporative cooling
Heat shock proteins produced which protect enzymes from denaturing
Cold stress
Cold causes problems with membrane fluidity & transport
Plants increase unsaturated fatty acids & membrane solutes to prevent dehydration & keep membrane fluid

17. Other environmental influences on plant growth: biotic
Defenses against herbivores
Thorns
Chemical defenses
Distasteful or toxic compound production
Chemicals that attract defensive predators
Defenses against pathogens
First line: epidermis & periderm
Second line: chemicals released to kill pathogen & prevent its spread from infection site
R genes produce proteins that bind to pathogen protein
Elicitors produced
Oligosaccharins- released from damaged cell wall to stimulate production of antimicrobial phytoalexins
PR proteins activated that directly attack pathogen
Stimulate strengthening of cell wall to prevent spread