Lecture 5
Plant hormones
and
Signal transduction
PLSC 452/552
1/23/14
Department of Plant Sciences
Neal Stewart Lab
Brain teaser
• Little Johnny nails a sign 90 cm off the ground
to a tree outside of his clubhouse that says
“no girls allowed.” If the tree grows 12 cm
every year, at what height will the sign be
when Little Johnny returns as Big Johnny in 15
years?
Overview
• Define “hormone” as a functional compound.
What is the function and importance of
hormones?
• Know the major plant hormones
• Understand real-world application of
hormones
• Importance of hormones for tissue culture:
introduction
What is a hormone?
• Biochemical which regulates growth based on
biological and environmental influences
• Synonyms: Plant hormones, plant growth
regulators (PGRs), phytohormones
• Regulate growth and development
• Mobile throughout plant
• Environment and stress responsive
Major plant hormones
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Auxin – Greek: auxein; to grow or increase
Cytokinin – cytokinesis (cell division)
Abscisic acid – abscission
Jasmonic acid – found in jasmine oil
Gibberellic acid – pathogen Gibberella
Ethylene – chemical brother to ethanol
Brassinosteroids – derived from Brassica spp.
Finding plant hormones
Observational:
Darwin stumbles on auxin
– Noticed grass tips grow toward light
– With tip growth responded to light
– Without tip growth had no response
Mutation screening: (aka forward genetics)
– Dwarf plants are can be hormone deficient
ABRC teaching tools website
General hormone biochemistry
• Present in all cells at various levels
• Classes of hormones work in signal cascades
– Hormone-receptor interactions
– Respond to a host of factors and biological needs
• Abiotic
– Water stress
– Light
– Nutrient deficiency
• Biotic
– Growth
– Development
– Herbivore stress
Hormone biosynthesis
Made from four biosynthetic pathways:
– Terpenoids
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AMP + IPP (cytokinins)
Carotenoid breakdown (abscisic acid)
Diterpene (gibberellic acid)
Triterpene (brassinosteroids)
– Fatty acids (jasmonic acid)
– Tryptophan (auxins)
– Methionine (ethylene)
EGG database:
tp://www.genome.jp/kegg/
Auxins
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Greek: auxein; to grow or increase
Apical dominance growth
Cell elongation
Hormone level very important
2, 4-dichlorophenoxyacetic Acid
http://pubchem.ncbi.nlm.nih.gov/
http://www.nsf.gov/news/news_images.jsp?cntn_id=104205
Taiz and Zeiger. 2002. Plant Physiology, 3rd Ed.
Auxin: Apical dominance
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Auxin.html
Cytokinin
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Cytokinesis (cell division)
Accidently added degraded DNA to medium
Organization and development of xylem tissue
Response to light
Kinetin
Lateral growth of shoots
Open stomata
http://pubchem.ncbi.nlm.nih.gov/
Application: cotton spraying
• Adding cytokinins to young cotton increase drought-resistance
• Arizona – 31% US cotton, highest yield acre-1
Auxin and cytokinin ratio importance
• Auxin alone = Large cells (no division)
• Cytokinin alone = Cells have no change
• Auxin + Cytokinin = Normal cell growth and
division
• Auxin + >Cytokinin = Shoot growth
• >Auxin = Cytokinin = Root growth
Auxin
Cytokinin
http://users.ugent.be/~p
debergh/pri/pri4ez04.jpg
Abscisic acid (ABA)
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Originally implicated in leaf and fruit abscission
Involved in leaf senescence
Maintains seed dormancy (opposed to GA)
Involved in stomata regulation (closes)
Single hormone unit
http://pubchem.ncbi.nlm.nih.gov/
http://www.rikenresearch.r
iken.jp/eng/research/6121
Jasmonic acid
• First identified in jasmine oil
• Response to biotic stress
– Wounding induces JA biosynthesis
– Microbial and fungal invasion
• Plant growth effects similar to auxin
– Specialty growth structures
http://pubchem.ncbi.nlm.nih.gov/
Jasmonic acid: Plant-insect co-evolution
Tri-trophic interactions
Degenhardt (2009)
Plant Physiology
149:96-102
Jasmonic acid: Pathogen response
http://park.itc.u-tokyo.ac.jp/biotec-res-ctr/kampo/eng/research_plant.html
Gibberellic acid
• Originally found in Gibberella (rice pathogen)
– Responsible for ‘foolish seedling’ phenomenon
– Uninhibited growth until breaking
• Involved in cell elongation
• Flowering and seed germination
http://pubchem.ncbi.nlm.nih.gov/
Modification of GA in rice
Miyako Ueguchi-Tanaka 2005 Nature 437, 693-698
Spraying GA for increased fruit yield
GA induces fruiting
in absence of seed
http://www.extension.org/pages/31607/using-plant-growth-regulators-toincrease-the-size-of-table-grape-berries#.Ut9EYbROmM8
Ethylene: the cell phone of PGRs
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A hydrocarbon gas
Involved in fruit ripening, stress response
Inhibition of growth in dark conditions
Excess ethylene inhibits callus growth
Conversational Plants?
• Ethylene production increases during stress
– Drought
– Heat
• Perceived by neighboring plants
• Unstressed plants induce stress pathways
Brassinosteroids
• Stress responses
– Switchgrass suspension cells have minor amounts
of lignin (small amounts of H monolignols)
– Addition of brassinolide induces normal lignin
formation and composition
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Stem elongation
Seed germination
Pollen tube growth
Cell differentiation control
Brassinolide
http://pubchem.ncbi.nlm.nih.gov/
Overview of hormone mutants
Cytokinin
mutant
Brassinosteroid
mutant
Cytokinin
mutant
closer look
Ethylene
mutant
grown in dark
Abscisic acid
mutant
Bishopp A et al. Development 2006;133:1857-1869
Hormones (PGR) pathways
• PGRs work in complex cascades to produce signals
– Synergistic
– Anatognistic
• Variety of actions in cell
– Gene transcription
– Protein degradation
http://chriscarterart.wordpress.com/2010/05/09/mo
thers-day-2/mikado-plant-watercolor-sketch-chriscarter-050910/
Arabidopsis histidine kinase sensing and signaling; 2) A histidine
phosphotransferase protein nuclear translocation; 3) A response regulation
transcription activation; and 4) a negative feedback loop through cytokinininducible ARR gene products.
http://molbio.mgh.harvard.edu/sheenweb/cytokinin_signaling.html
Introduction: the importance of manipulation
of hormones for tissue culture
• Auxins and cytokinins are very important
– Higher auxin induces root growth
– Equal ratio induces callus and cell enlargement
– Lower auxin induces shoot growth
• Gibberellic acid germinates difficult seeds
• Add brassinosteroids for cell wall induction
Lecture summary
• Hormones are biochemicals that regulate
plant growth based on biological and
environmental cues
• Auxin and cytokinin are key for plant growth
• Abiotic and biotic stress response is regulated
by hormones
• Cell signaling is regulated by specific receptors
on cell membranes