HORT301 Introduction to Plant Hormones

Michael Van Oosten

Wednesday, September 24th

What Defines a Plant Hormone? Regulate Growth (cause changes in basal pattern) Site of synthesis differs from site of action Occur at very low concentrations (10-6 to 10-5 mol/L) Terminology

Phytohormones = Plant Hormone Plant Growth Regulators

Can be natural or synthetic (2,4-Dichlorophenoxyacetic acid ) If it was discovered in the 50’s or 60’s, it’s a plant hormone (the

classical five) anything after that is arguable between PGR or PH

New hormones proposed every few yearsNature 455, 195-200 (11 September 2008) | doi:10.1038/nature07272; Received 14 June 2008; Accepted 21 July 2008; Published

online 10 August 2008Inhibition of shoot branching by new terpenoid plant hormonesMikihisa Umehara1, Atsushi Hanada1, Satoko Yoshida1, Kohki Akiyama2, Tomotsugu Arite3, Noriko Takeda-Kamiya1, Hiroshi

Magome1, Yuji Kamiya1, Ken Shirasu1, Koichi Yoneyama4, Junko Kyozuka3 & Shinjiro Yamaguchi1 RIKEN Plant Science Center, Tsurumi, Yokohama 230-0045, Japan Graduate School of Life and Environmental Sciences, Osaka Prefecture University, 1-1 Gakuencho, Naka-ku, Sakai, Osaka 599-

8531, Japan Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo, Tokyo 113-8652, Japan Weed Science Center, Utsunomiya University, Utsunomiya 321-8505, JapanCorrespondence to: Shinjiro Yamaguchi1 Correspondence and requests for materials should be addressed to Sh.Y.

(Email: shinjiro@postman.riken.jp).Abstract

Shoot branching is a major determinant of plant architecture and is highly regulated by endogenous and environmental cues. Two classes of hormones, auxin and cytokinin, have long been known to have an important involvement in controlling shoot branching. Previous studies using a series of mutants with enhanced shoot branching suggested the existence of a third class of hormone(s) that is derived from carotenoids, but its chemical identity has been unknown. Here we show that levels of strigolactones, a group of terpenoid lactones, are significantly reduced in some of the branching mutants. Furthermore, application of strigolactones inhibits shoot branching in these mutants. Strigolactones were previously found in root exudates acting as communication chemicals with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Thus, we propose that strigolactones act as a new hormone class—or their biosynthetic precursors—in regulating above-ground plant architecture, and also have a function in underground communication with other neighbouring organisms.

What Defines a Plant Hormone? Hormone: Signal molecules produced in

small concentrations by cells in one location that affect cells in other parts of the organism

Animal Hormones use specialized organs (glands) to produce hormones

Any plant cell is capable of producing a hormone

Role of Phytohormones Two general categories of hormone action

Regulate Growth, essential for pattern, tissue and organ formation Germination Senescence Cell division, expansion, and differentiation

Environmental Response, essential for the plant to react to its environment Abiotic stress: toxic ions, salinity, heat, cold, etc Biotic stress: pathogens, herbivores

Phytohormones (the classical five) Abscisic Acid (stress hormone) Gibberellins (growth regulator) Auxin (growth regulator) Cytokinins (cell proliferation regulator) Ethylene (maturation regulator)

Classical Five are classical because of when they were discovered and characterized

Other Phytohormones Brassinolides (growth regulation) Salicylic Acid (pathogen defense signal) Jasmonates (wounding/defense signal) Nitric Oxide (defense signal) Systemin (herbivore defense signal) Polyamines (nuclear division regulation)

Why study plant hormones? Name a plant process that doesn’t involve

hormones? Hormones control critical processes Perturbing (disruption) of hormone signaling

typically has visible phenotypes Hormone signals are triggered by

environmental or developmental signals

Khush (2001) Nature Reviews GeneticsPlant Phyiology Online Web Essay 20.2

Dwarf Varieties (GA insensitive)

Mode of Action

Stimulus

ProductionTransport

Perception

Gene Activity (transcription)

Physiological Activity(release of secondary signals, direct effects)

Stimulus Ends Removal of Signal(degradation, conjugation,translocation)

Ways Plant Hormones are Regulated Biosynthesis (production of hormone) Catabolism (degradation of hormone) Conjugation (typically inactivation) Structural Diversity Translocation (transport and sequestration) Perception

Regulation via Biosynthesis

HO

O

9-cis-viotaxanthin9'-cis-neoxanthin

HO

HO

OH

O2

CHOHO

O

Xanthoxin

CH2OH COOH

CHOOH

AB-aldehyde

OHO

ABA

OHO

AB-alcohol

O O

OH

O

Regulation via Catabolism

Pathway of ABA Catabolism, Van Oosten (2004)

Regulation via Conjugation

Cohen: http://www.horticulture.umn.edu/vd/cohen/

Auxin conjugated to a glucose molecule

Auxin conjugated to myo-inositol

Auxin conjugated to an amino acid

IAA (free form)

Free

Conjugated

Inactive

Active

Up to 95% of IAA present in a plant is conjugated and inactive

Variety!

Fischbach & Clardy, 2007 Nature Chemical Biology

136 known GAsLess than 10% have attributed activity

Translocation Main forms of hormone movement

Diffusion Ethylene

Cytoplasmic streaming (cell-to-cell) Cytokinins

Vascular flow (xylem or phloem) ABA

Trafficking (active transport) Auxin

Hormones can be conjugated, sequestered in vacuole, or both

Free salicylic acid is biologically active and synthesized in the chloroplast in response to biotic and abiotic oxidative stress via isochorismate synthase 1 (ICS1). Free SA is cytotoxic, SA accumulates predominantly as the SA-2-o-B-D-glucoside in the plant vacuole. Hydrolysis of SAG to free SA upon subsequent infection results in rapid induction of defense responses and systemic acquired resistance.

Wildermuth et al. (2001) Nature

Perception Not all plant cells are programmed to respond

to hormone signals the same way Specific cells respond to signaling hormones

Hetherington & Woodward (2003) Nature

In rice, ABA causes guard cells to close, but pericycle cells to initiate lateral roots

Abscission Zones

Conrad (2007) Deciduous & Evergreen Leaves McGraw Hill, Encyclopedia of Science and Technology 5th ed.

The Five Classical Hormones Abscisic Acid Gibberellins Auxins Cytokinins Ethylene

Non-Classical Plant Hormones Salicylic acid - in some plants activates genes that

assist in the defense against pathogenic invaders. Jasmonates - are produced from fatty acids and seem

to promote the production of defense proteins that are used to fend off invading organisms. They are believed to also have a role in seed germination,

the storage of protein in seeds and seem to effect root growth.

Nitric oxide (NO) - has been found to serve as a signal in hormonal and defense responses to pathogen attack.

Polyamines

Putrescine

Cadaverine

Calvero, 2008, Open License

Spermidine

Polyamines are strongly basic molecules of low molecular weight that have been found in all organisms. Act as growth factors in all eukaryotes & prokaryotes Essential for plant growth and development and affect the process of mitosis and meiosis.Highly up-regulated under stress, especially cereal grainsDelay onset of senescence

Galston & Sawhney 1990, Plant Phyisology

Systemins Systemin - a polypeptide consisting of 18

amino acids, functions as a long-distance signal to activate chemical defenses against herbivores Solanceae family only Only peptide hormone

Hormones function within interconnected networks

A simplified scheme for the interactions between ethylene, auxin, and BR in the hook of etiolated seedlings in the dark. Biosynthetic pathways for the regulators are indicated by dashed lines. Plant

Physiology Web Topic: Essay 24.1

END

Abscisic Acid (ABA) Lecture ## 13, #30,38 & #39

PLANT PHYSIOLOGY 4rd Edition (2006) by Taiz and Zeiger

Wayne - wayne@sparkleberrysprings.com

ABA maintains Seed Dormancy

Gibberellins (GAs) Lecture #29

GAs regulate cell expansion

The Scientist 2002, 16(14):20

Auxin Coming Soon! Lecture #28

Cytokinins Lecture #29

Ethylene Lecture #29