2. CELL SIGNALING MECHANISMS IN PLANTS
ARSHAD MAHMOOD KHAN
Lecturer (HED Punjab)
PhD Scholar
BOTANY DEPARTMENT
UAAR RAWALPINDI

3. 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
water
2. Which guide the plant’s cycle of
• growth and movements
• flowering and
• fruiting

4. Cont…
3. Thus plant cells also communicate to coordinate activities in their
• roots
• stems and
• leaves, flower and fruits

5. 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 Evolved
Independently in Plants and Animals
Although plants and animals are both eukaryotes, they
have had separate evolutionary histories for more than
a billion years
Their last common ancestor thought to have been a
unicellular eukaryote that had mitochondria but no
chloroplast

6. 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

7. Cont…
If multicellularity evolved independently in plants and
animals, the molecules and mechanisms used for cell
communication 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

8. Cont…
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.

9. Fig Alberts 5th Ed

10. Definition of plant hormone (phytohormone) and their role
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?

11. Different types of the Plant Hormones

12. Effects of plant hormones on plant growth and development
Embryogenesis
Senescence
(Cell division, expansion, differentiation and cell death)

13. Chronological events and persons involved in identification of different hormone receptors
Fawzi A. Razem
2007 (CHLH, GCR2)

14. Currently identified different plant hormone receptors
Nature 405: (2009)

15. Cellular locations of different plant hormone receptors
Nature 459: (2009)

16. 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
Introduction to the ethylene hormone
(history, synthesis, significance)
Genetic dissection of the ethylene signaling pathway (this provides for the genetic engineering of many responses to ethylene)
Summary

17. ETHYLENE is a gaseous plant hormone.
Various stimuli that produce plant responses through synthesis of signals
ETHYLENE is a gaseous plant hormone.

18. History Neljubov (1901): Gaseous hydrocarbon olefin
Triple response in etiolated pea seedlings
Cousins (1910):
Orange and banana in the same shipment
Gane (1934):
Ethylene as a natural plant product

19. Ethylene Biosynthesis
Wounding
Flooding
Biotic stress
Heat stress
Drought stress
Cold stress
Oxidative stress
Osmotic stress
Mechanical stress
UV stress
Pathogen attack
STRESS hormone
It has been shown that ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seedlings.
Elucidated yrs ago
Ethylene production is regulated by a variety of developmental and environmental factors. During the life of the plant, ethylene production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers. Ethylene production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators
Ethylene has it’s hands in everything 19

20. Ethylene biosynthetic pathway and the Yang cycle

21. Ethylene biosynthetic pathway and the Yang cycle
PP5e-Fig jpg

22. 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.

23. Ethylene responses/effects/significance
Developmental processes
Fruit ripening - ethylene is essential
Promotion of seed germination
Root initiation
Bud dormancy release
Inhibition/promotion of flowering
Sex shifts in flowers
Senescence of leaves, flowers
KNOWN AS STRESS HORMONE Pleiotropic effects.
What is actually caused by the gas may depend on the tissue affected as well as environmental conditions.
In flooding, root suffers from lack of oxygen, or anoxia, which leads to the synthesis of 1-Aminocyclopropane-1-carboxylic acid (ACC). ACC is transported upwards in the plant and then oxidized in leaves. The product, the ethylene causes epinasty of the leaves.
Responses to abiotic and biotic stress Abscission of leaves, flowers, fruits
Epinasty of leaves
Inhibition/promotion of cell division/elongation
Altered geotropism in roots, stems
Induction of phytoalexins/disease resistance
Aerenchyma formation 23

24. Constitutive triple response (CTR) by ethylene
For instance, when the shoot of germinating seedling
encounter an obstacle such as a piece of gravel
underground in the soil, the seedling respond to the
encounter 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 the
curvature of specialized hook structure
Thirdly, it reduces the shoot’s tendency to grow away
from the direction of gravity, so as to avoid the obstacle
This triple response is controlled by ethylene

25. Cont…

26. Ethylene has far-reaching consequences for agriculture and horticulture
Transport and storage of fruits and vegetables requires ethylene control
Flood-tolerant rice created by expression of ethylene response factor genes
“One bad apple spoils the whole bunch…” 26

27. Wounding induces ethylene production Ethylene causes senescence
Can block ethylene receptors with silver thiosulfate 27

28. Apple slices inducing ripening of persimmons
8 days in bag with apple slices
Controls, 8 days outside of bag 28

29. Perception by receptors
Genetic dissection of the ethylene signaling
pathway and receptors
Ethylene
Perception by receptors
Ethylene can reversibly bind
to its receptors present in ER membrane through a transition
metal (Cu)
Signal transduction
Responses

30. 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 related
They are dimeric, trans membrane proteins with a copper containing ethylene binding domain and a His-kinase domain that interacts with protein called CTR1

31. Ethylene signaling pathway

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

34. 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)

35. 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.

36. 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.

37. Signal perception and role of MAPK cascade family:
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

38. Further downward positive regulation of EIN2:
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.

39. Further downward positive regulation of EIN3 & EIL1:
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.

40. 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.

41. 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.

42. References
Aaron Santner & Mark Estelle, Nature 459, (25 June 2009)
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Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter Molecular Biology of the Cell, 5th edition, Garland science, New York, USA

43. 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

44. Questions and Comments?

45. Thanks for your attention!