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CELL SIGNALING MECHANISMS IN PLANTS ARSHAD MAHMOOD

KHAN

Lecturer (HED Punjab)

arshadbotanist@gmail.com

PhD Scholar

BOTANY DEPARTMENT

UAAR RAWALPINDI

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General Introduction

Cells in plants like in animals remain in constant communication with one an other

1. Plant cells communicate to coordinate their activities in

response to the changing conditions of• light and dark • gravity• temperature• water2. Which guide the plant’s cycle of• growth and movements• flowering and• fruiting

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Cont…

3. Thus plant cells also communicate to coordinate activities in their

• roots• stems and• leaves, flower and fruits

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PROBLEM

• Less is known about the receptors and intracellular signaling mechanisms involved in cell communication in plants than is known in animals

Hypothesis

Multicellularity and Cell Communication EvolvedIndependently in Plants and Animals• Although plants and animals are both eukaryotes, theyhave had separate evolutionary histories for more thana billion years• Their last common ancestor thought to have been aunicellular eukaryote that had mitochondria but nochloroplast

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Cont…

• The plant lineage acquired chloroplasts after plants and animals diverged (Endo-symbiont Hypothesis)

• The earliest fossils of multicellular animals and plants

date from almost 600 million years ago• Thus, it seems that plants and animals evolved

multi-cellularity independently, each starting from a different unicellular eukaryote, sometimes between 1.6

and 0.6 billion years ago

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Cont…

If multicellularity evolved independently in plants andanimals, the molecules and mechanisms used for cellcommunication will have evolved separately and would be expected to be different

However, there should be some degree of resemblance because the genes in both plants and animal genes diverged from those contained their last common unicellular ancestor. For example

Like animals, plants make extensive use of cell surface receptors

Whereas most cell-surface receptors in animals are G-protein linked, most found so far in plants are enzyme linked

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Moreover, the largest class of enzyme linked receptors in animals is tyrosine kinases, this type of receptor is extremely rare in plants

Whereas plants seem to rely largely on serine/threonine kinases cell membranes receptors.

Cont…

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Fig 15.81 Alberts 5th Ed

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Definition of plant hormone (phytohormone) and their role

1. The word hormone is derived from the Greek verb meaning to excite.

2. Hormones are organic substances synthesized in one tissue and transported out where their presence results in physiological responses ( not always true; may act at or close to synthesis site). They are required in minute amounts (10-6 to 10 -8M).

3. Each hormone may result in multiple effects -- the particular effect depending on a number of factors:

(a) The presence of other hormones and the presence of activator molecules ( calcium, sugars)

(b) The amount of the hormone (dosage or concentration)

(c) The sensitivity of that tissue to the hormone. (d) The condition of the plant itself is critical: what is

the condition of the plant? its age?

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Different types of the Plant Hormones

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Effects of plant hormones on plant growth and development

Embryogenesis

(Cell division, expansion, differentiation and cell death)

Senescence

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Chronological events and persons involved in identification of different hormone receptors

2007 (CHLH, GCR2)Fawzi A. Razem

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Nature 405:1071-1078 (2009)

Currently identified different plant hormone receptors

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Cellular locations of different plant hormone receptors

Nature 459:1071-1078 (2009)

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ETHYLENE SIGNALING PATHWAY

As the detail discussion about the signaling pathways of all phyto-hormones is too lengthy; only the ethylene signaling pathway is discussed here OUTLINE

1. Introduction to the ethylene hormone(history, synthesis, significance)

2. Genetic dissection of the ethylene signaling pathway (this provides for the genetic engineering of many responses to ethylene)

3. Summary

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ETHYLENE is a gaseous plant hormone.

Various stimuli that produce plant responses through synthesis of

signals

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HistoryNeljubov (1901): Gaseous hydrocarbon olefin Triple response in etiolated pea

seedlingsCousins (1910): Orange and banana in the same

shipmentGane (1934): Ethylene as a natural plant product

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Ethylene Biosynthesis

Cold stressOxidative stressOsmotic stressMechanical stressUV stressPathogen attack

Biotic stressFlooding

WoundingHeat stress

Drought stress

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Ethylene biosynthetic pathway and the Yang cycle

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Ethylene biosynthetic pathway and the Yang cycle

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Biosynthesis of ethylene

The precursor for ethylene biosynthesis is methionine, which is converted sequentially to S-adenosylmethionine, ACC, and ethylene. ACC can be transported and thus can produce ethylene at a site distant from its synthesis.

Two key enzymes: ACC synthase and ACC oxidase Ethylene biosynthesis is stimulated by environmental factors, other

hormones (auxin), physical and chemical stimuli The biosynthesis and perception (action) of ethylene can be

antagonized by inhibitors, some of them have commercial applications ACC can be converted to a conjugated form, N-malonylACC (MACC) to

avoid over production Ethylene can travel through diffusion (short transport) or in the form

of ACC when long distance transport is required.

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Ethylene responses/effects/significanceDevelopmental processesFruit ripening - ethylene is essential Promotion of seed germinationRoot initiationBud dormancy release Inhibition/promotion of floweringSex shifts in flowers Senescence of leaves, flowers

Responses to abiotic and biotic stress Abscission of leaves, flowers, fruitsEpinasty of leaves Inhibition/promotion of cell division/elongationAltered geotropism in roots, stems Induction of phytoalexins/disease resistanceAerenchyma formation

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Constitutive triple response (CTR) by ethylene

For instance, when the shoot of germinating seedlingencounter an obstacle such as a piece of gravelunderground in the soil, the seedling respond to theencounter in three ways:• Firstly, it thickens its stem which can then exert more force on the obstacle• Secondly, it shields the tip of the shoot by increasing thecurvature of specialized hook structure• Thirdly, it reduces the shoot’s tendency to grow awayfrom the direction of gravity, so as to avoid the obstacleThis triple response is controlled by ethylene

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Cont…

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“One bad apple spoils the whole bunch…”

Transport and storage of fruits and vegetables requires ethylene control

Ethylene has far-reaching consequences for agriculture and horticulture

Flood-tolerant rice created by expression of ethylene response factor genes

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Wounding induces ethylene production Ethylene causes senescence

Can block ethylene receptors with silver thiosulfate

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Apple slices inducing ripening of persimmons

8 days in bag with apple slices

Controls, 8 days outside of bag

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Ethylene

Perception by receptors

Signal transduction

Responses

Genetic dissection of the ethylene signaling pathway and receptors

Ethylene can reversibly bind to its receptors present in ER membrane through a transition metal (Cu)

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Genetic dissection of the ethylene signaling pathway and receptors

Plants have various ethylene receptors like Ethylene receptor 1 & 2 (ETR1, ETR2), Ethylene response sensor 1 & 2 (ERS1, ERS2) and Ethylene insensitive protein 4 (EIN4) which are located in the endoplasmic reticulum and are all structurally relatedThey are dimeric, trans membrane proteins with a copper containing ethylene binding domain and a His-kinase domain that interacts with protein called CTR1

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Ethylene signaling pathway

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33Arrows and T-bars represents positive and negative control respectively

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Genetic dissection of the ethylene signaling

pathway and receptors

Most of the ethylene signaling pathways studies was performed in the model plant Arabidopsis thaliana.

Receptors: In Arabidopsis ethylene is perceived by a family of

five receptors viz. ETR1,ETR2,ERS1,ERS2 and EIN4. All these dimeric receptor molecules are integral part of ER cell membrane.

The receptors family is further divided into Type 1 subfamily (It includes ETR1 and ERS1) Type 2 subfamily (It includes ETR2,ERS2 and

EIN4)

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Cont…

Components of a receptor: Each receptor molecule (e.g. ETR1 or ERS1 of

Type1 subfamily) have two domains like Amino terminal, also called sensor domain

where ethylene binding can occur. Carboxyl terminal, or Histidine kinase domain

or receiver domain RAN1 protein transfer or deliver copper ions to

the sensor domain of receptors that acts as cofactor.

In the absence of ethylene each dimeric receptor molecule is functional or active (due to phosphorylation of receiver domain) and hence negatively control the ethylene responsive genes.

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Cont…

CTR1: It is a serine/threonine kinase receptor (commonly

called as constitutive triple response protein) CTR1 also have two domains;

The sensor domain and the receiver or active kinase domain.

The sensor domain of CTR1 is bonded with the receiver domains of initial ethylene receptor molecules. (-COOH terminal of receptor and –NH2 terminal of CTR1)

Hence CTR1 is not transmembrane bounded directly. In the absence of ethylene both receptor and CTR1

receiver domain are active and negatively controlling the ethylene response pathway.

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Cont…

Signal perception and role of MAPK cascade family:

Under biotic or abiotic stress, when ethylene binds with the initial receptors present in the ER membranes, it causes the following effects downstream; Initial receptors becomes inactive, thus causing a

conformational change at the receiver ends. This will release CTR1 into cytosol and it also become

inactive, as they further phosphorylate MAPK cascade.

This cascade includes SIMKK and MPK6

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Cont…

Further downward positive regulation of EIN2:

The positive activation of MAPK cascade family members in the cytosol causes; The positive activation of EIN2 that are nuclear

membrane bounded. Further downward EIN2 positively regulate the

concentration of transcription factors like EIN3 and EIL1.

Ubiquitin-proteosome complex (Ub/26S) and EBF1 & 2 negatively control the concentration of EIN3 within the nucleoplasm.

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Cont…

Further downward positive regulation of EIN3 & EIL1:

EIN3 and EIL1 transcription factors acts on the immediate target genes (like ERF1, EDF1, EDF2, EDF3 and EDF4) by binding with promoter called PERE.

Above mentioned transcription and then translation results in ERF1, EDF1, EDF2, EDF3 and EDF4 proteins or secondary transcription factors.

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Cont…

Further downward positive regulation of ERF1:

ERF1 a secondary transcription factor then binds with the GCC box present in the promoter of other genes (PDF1.2,Hls1 and ChiB).

The product of above mentioned genes acts as metabolic protein and control the various responses in plant body. For example PDF1.2 shows defensive response against the viral or various microbial infections whereas Hls1 protein is responsible for differentiation and growth in plants.

An unidentified JA transcription factor also binds to the promoter of ERF1 to activates its expression.

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Summary

Although ethylene is the simplest of all plants hormones, it has a strong influence on many different developmental processes, from germination to senescence. In the last decade, molecular and genetic investigations have contributed enormously to the understanding of ethylene perception and signal transduction.

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References

Aaron Santner & Mark Estelle, Nature 459, 1071-1078 (25 June 2009) Liu Q, Zhou GY, Wen CK. Zhi Wu Sheng Li Yu Fen Zi Sheng Wu Xue Xue Bao. 2004

Jun;30(3):241-50.Wang ZF, Ying TJ. Zhi Wu Sheng Li Yu Fen Zi Sheng Wu Xue Xue Bao. 2004 Dec;30(6):601-8. Chang C. Trends Plant Sci.2003 Aug;8(8):365-8. Zimmerli L, Stein M, Lipka V, Schulze-Lefert P, Somerville S. Plant J. 2004 Dec;40(5):633-46. Guo H, Ecker JR. Cell. 2003 Dec 12;115(6):667-77. Alonso JM, Stepanova AN. Science. 2004 Nov 26;306(5701):1513-5. Bleecker AB, Kende H: Ethylene: a gaseous signal molecule in plants. Annu Rev Cell Dev Biol

2000, 16:1 18. Alonso JM, Ecker JR: The ethylene pathway: a paradigm for plant hormone signaling and

interaction. Sci STKE 2001, 2001:RE1. Wang KL, Li H, Ecker JR: Ethylene biosynthesis and signaling networks. Plant Cell 2002,

14(Suppl):S131-S151. Klee HJ: Control of ethylene-mediated processes in tomato at the level of receptors. J Exp Bot

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23:619-627. Xie C, Zhang JS, Zhou HL, Li J, Zhang ZG, Wang DW, Chen SY: Serine/threonine kinase activity

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Hongwei Guo and Joseph R Ecker1 2004 Current Opinion in Plant Biology 2004, 7:40–49Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.

2002 Molecular Biology of the Cell, 5th edition, Garland science, New York, USA

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Abbreviations ACC 1-aminocyclopropane-1-carboxylate AdoMet Adenosyl methionine ChiB chitinaseB CTR1 Constitutive triple response1 (serine/threonine

kinase) EBF1,2 EIN3-Binding F Box protein1,2 EDF1,2,3,4 Ethylene response DNA binding factor1,2,3,4 EIL1 EIN3 like1 EIN2,3,4 Ethylene insensitive2,3,4 ER Endoplasmic reticulum ERF1 Ethylene Response factor1 ERS1,2 Ethylene response sensor1,2 ETR1,2 Ethylene receptor1,2 His Histidine Hls1 Hookless1 JA Jasmonic acid/Jasmonate MAPK Mitogen-activated protein kinase MET Methionine MPK6 Arabidopsis MAPK6 PDF1.2 Plant defensin factor1.2 protein PERE Primary ethylene response elements RAN1 Responsive to antagonist1 SAM Sulfur- Adenosyl methionine SIMK Salt-stress inducible MAPK SIMKK SIMK Kinase TF Transcription factor

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Questions and Comments?

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Thanks for your attention!