Hormone action and receptors

Lesson Prepared Under MHRD project “National Mission on Education Through ICT” Discipline: Botany Paper: Plant Physiology National Coordinator: Prof. S.C. Bhatla Lesson: Hormone action and receptors Lesson Developer: Anita Thakur Department/College: Department of Botany, University of Delhi Lesson Reviewer: Prof. S.C. Bhatla Department of Botany, University of Delhi Language Editor: Vinee Khanna Department/College: Department of Genetics, University of Delhi, South Campus Lesson Editor: Dr. Rama Sisodia, Fellow in Botany ILLL

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Hormone action and receptors

Table of contents Hormone action and receptors 

Introduction



Auxins 



Gibberellins 



Ethylene receptors

Cytokinins 



GA receptors and signalling

Ethylene 



Auxin signalling

Cytokinin receptors

Abscisic acid 

ABA receptors



Summary



Exercise/ Practice



Glossary



References/ Bibliography/ Further Reading

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Hormone action and receptors

Learning outcomes: The student will be able to 

Know about basics of hormones



Understand the importance of signal transduction pathways



Know about various kinds of phytohormones



Understand structures and pathway of auxin biosynthesis



Understand the role of auxin signalling



Know about gibberellins and its receptors



Know about triple response caused by ethylene



Understand the pathway of ethylene biosynthesis



Know about the types of cytokinins and their roles



Learn about the role of cytokinin receptors in signalling



Explain the pathway of abscisic acid biosynthesis



Know about the properties and applications of abscisic acid

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Hormone action and receptors

Introduction Hormones act as chemical messengers by way of their production in one part of the plant, get transported to target areas where they elicit responses. Endocrine hormones are the ones which are transported away from their site of synthesis to their sites of action in tissues whereas those hormones whose sites of synthesis and action are adjacent to each other are paracrine hormones. Plant hormones are signal molecules that are produced within the plant in extremely low concentrations that regulate the growth processes. The ability of a hormone to induce a specific response in a target cell is usually mediated by specific proteins, known as hormone receptor. These are linked to signal transduction pathways.

Did you know?

Signal transduction is a process where cells construct responses

to a signal either exterior or interior that triggers a cascade of events.

Figure: When and why does Signal transduction occur? Source:

http://www.authorstream.com/Presentation/Agriadda-2054799-indole-acetic-

acid-auxin-signal-transduction/

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Hormone action and receptors Efficient communication within the plants, among its various organs, tissues and cells, helps in the proper functioning of the systems. Increase or decrease in the levels of hormones depends on the signals perceived. The hormones then report to the sites of action or the target sites where they interact with the receptors. This interaction can induce a physiological or developmental response due to the involvement of some transcriptional or post-transcriptional events, such as phosphorylation, protein turnover, ion extrusion etc.

Figure: Possible pathways of transmission of signal by the interaction of a hormone with the receptor molecules. (Permission to be sought) Source: http://en.wikipedia.org/wiki/Nuclear_receptor (CC) Some major classes of hormones include: auxins, cytokinins, gibberellins, ethylene and abscisic acid.

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Hormone action and receptors

Figure: Some major classes of hormones. Source: http://chemwiki.ucdavis.edu/Wikitexts/UC_Davis/UCD_Chem_2C/UCD_Chem_2C%3A_L arsen/Unit_4%3A_Chemical_Kinetics/4.2_Reaction_Rates_and_Rate_Laws; http://photos1.blogger.com/blogger/4440/1858/1600/10.1371_journal.pbio.0020311.g0 01-M.jpg ; http://en.wikipedia.org/wiki/Strigolactone (CC)

Auxins Auxins were the first plant hormones to be discovered. Indole-3-acetic acid (IAA) is the principal auxin found in plants which is produced mainly in the meristematic tissue, shoot apex and young leaves. It shows polar movement i.e., from the apex to base (basipetal). Charles Darwin and his son Francis predicted the presence of auxins in plants for the first time in 1881. They examined the bending of coleoptiles and seedling hypocotyls towards the unidirectional light. According to them, growing tips of plants sense light and this sensing ability is not evident in the tissues behind the shoot apex. The chemical messenger is produced uniformly by growing tips of shoot but is Institute of Lifelong Learning, University of Delhi

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Hormone action and receptors transported to the unlighted or shaded side of the shoot. It causes cells on the shaded side to elongate more than cells on the illuminated side.

Figure: Bending of shoot tips towards the unidirectional light. 1. Normal phase under sun where auxin is equally distributed; 2. The change in position of sun causes the concentration of auxin on the other side of the plant; 3. The auxin concentration bends the plant in the direction; 4. Bending of the plant towards the sun Source: http://en.wikipedia.org/wiki/Phototropism (CC) Auxins promote elongation of young shoots, formation of adventitious and lateral roots, they affect secondary cell growth by inducing vascular cambium and secondary xylem, promotes cell division (with cytokinins), increases ethylene production, and enforces dormancy of lateral buds.

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Hormone action and receptors Figure: Synthesis of indole-3-acetic acid (IAA) from tryptophan. Source: http://plantphys.info/plant_physiology/auxin.shtml



Auxin signalling Auxin induces rapid changes in the growth of target tissues. Two types of auxin receptors have been identified: the auxin-binding protein (ABP) and the F-box protein TIR1 (Transport inhibitor Response 1). Many factors regulate the transcription of auxin-responsive genes which interact with their promoters. This also includes transcription factors such as auxin responsive factor (ARF) family of proteins. Binding of ARF to specific locations on the promoters allows activation of transcription. Transcription repressors, such as, AUX/IAAs, down regulate the transcription of auxin-responsive genes. Some IAA receptors, such as TIR1

(Transport inhibitor Response 1), are E3 ubiquitin

ligases, which upon binding to auxin, initiate the expression of these genes by activating complexes targeting AUX/IAA proteins for ubiquitination by the SCFTIR1 complex followed by its degradation by the 26S proteasome.

Figure: Auxin signalling pathway Source: http://staffweb.wilkes.edu/william.terzaghi/Bio369/lectures/PP1513.ppt

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Hormone action and receptors The auxin-binding proteins (ABP), on the other hand, are ER resident proteins with an endoplasmic reticulum retention signal (KDEL/HDEL). Some ABPs are located in the plasma membrane also. ABP exhibits much stronger affinity for auxin at acidic pH. Therefore, it functions as a receptor only after its transportation to the plasma membrane.

Gibberellins Gibberellins (GA) were first identified in the 1920s in the pathogenic fungus, Gibberella fujikuroi. It is the causal agent of the ‘foolish-seedling’ disease of rice. It causes excessive elongation of infected plants. Gibberellins are tetracyclic diterpene acids with 19 or 20 carbon atoms. The major bioactive GAs are GA1, GA3, GA4 and GA7, and these are derived from a basic diterpenoid carboxylic acid skeleton, and have a C3-hydroxyl group in common. They are produced in the plastids and are eventually they are modified and prepared for utilization after their transportation to the endoplasmic reticulum of the cell. More than 130 GAs have been identified in plants, fungi and bacteria since their discovery. GAs are important for many developmental processes in plants. They regulate seed germination, stem elongation, leaf expansion, trichome development, pollen maturation and flowering induction. Some GA deficient plants show a dwarf and late-flowering phenotype but GA treatment to these plants can restore their normal growth.

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Hormone action and receptors Figure: Elongated seedling with foolish seedling disease caused by a surplus of gibberellic acid from the metabolism of the fungus Gibberlla fujikuroi. Source: http://ccsbio.blogspot.in/2013/03/foolish-seedling-disease.html (CC) Gibberellins play many physiological roles in plants.

They cause light induced stem

growth inhibition in many plants, such as Pisum sativum. They promotes cell division in meristem, elongation of internodes, and enhanced activity of certain enzymes, like amylases. GA application in some plants, such as Pisum sativum, Viciafaba and Phaseolus multiflorus, causes elongation of internodes, which help in overcoming genetic dwarfism. In certain plants, like lettuce and tobacco, it breaks seed dormancy. It helps in induction of flowering and fruiting in long-day plants.

Figure: Biosynthetic pathway of gibberellic acid.

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Hormone action and receptors Source: http://www.scielo.br/scielo.php?pid=S010083582013000300025&script=sci_arttext (CC)



GA receptors and signalling The components of the GA signalling pathway were first identified in rice and Arabidopsis using molecular approaches. Key components include the GA receptor GIBBERELLIN INSENSITIVE DWARF1 (GID1), the DELLA growth inhibitors (DELLAs) and the F-box proteins. Elements of GA signalling lead to proteolysis of repressor proteins with the help of GID1 receptor which is a soluble nuclear protein. GID2 and SLY1 from rice and Arabidopsis respectively, are the F-box proteins involved in signalling.

Figure: Model depicting GA signaling in plants Source: http://journal.frontiersin.org/article/10.3389/fpls.2011.00107/full (CC) The GA receptor GID1 which perceives the signal is a soluble protein present in both cytoplasm and nucleus. DELLA proteins repress GA signalling thereby restricting the growth of plants by causing transcriptional reprogramming. Binding of GA to the receptor enhances the interaction between GID1 and DELLA. This, in-turn, can lead to rapid degradation of DELLA proteins via the ubiquitin proteasome pathway with the help of a specific ubiquitin E3 ligase complex (SCFSLY1/GID2). This complex is essential since it recruits

DELLA

for

polyubiquitination

and

subsequent

degradation

proteasome. Institute of Lifelong Learning, University of Delhi

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by

the

26S

Hormone action and receptors

Ethylene Ethylene is a simple, gaseous hormone which regulates growth and development in plants. It was the first gaseous signaling molecule to be discovered. Initial evidence for ethylene was provided by a Russian scientist-Neljubov in 1901, when he reported ethylene as an active compound in illuminating gas that causes altered growth of pea seedlings. Ethylene may also serve as a signal to promote cell expansion as well as play an important role in plant responses to biotic and abiotic stresses. A well-known effect of ethylene on plant growth is the ‘triple response’ of etiolated dicotyledonous seedlings which is characterized by the inhibition of hypocotyl and root cell elongation, thickening of the hypocotyl, and pronounced curvature of the apical hook.

Figure: Triple response of seedling mediated by ethylene Source: https://www.scienceopen.com/document/vid/0c40c71b-234c-49fd-9f27105c4295c891;jsessionid=lp3pj6jGCsdnLXPPwU5Fb0fA.slave:so-app2-prd?0 (cc) Ethylene is synthesized in almost all plant tissues. It is synthesized from methionine in three enzyme steps: adenosyl methionine syntheses, ACC synthase and ACC oxidase.

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Hormone action and receptors

Figure: Biosynthesis of ethylene

Source: http://en.wikipedia.org/wiki/Ethylene#/media/File:Yang-cycle.png (CC) ACC synthase and ACC oxidase are represented by large multigene families and are highly regulated enzymes in vivo. 

Ethylene receptors Ethylene receptors were identified in Arabidopsis and ETR1 (ethylene response1) was the first ethylene receptor to be identified and cloned from plants. Later studies showed that there are four other receptor isoforms called ETR2, ERS1 (ethylene

response

sensor1),

ERS2,

and

EIN4

(ethylene

insensitive4)

in

Arabidopsis that bind to ethylene with high affinity. All ethylene receptors are predicted to contain three transmembrane α-helices at the N-terminus which form the ethylene-binding domain, followed by a GAF domain and a histidine kinase domain or receiver domain in the carboxyl half of the molecule. Isolation and sequencing of etr1 gene showed that the receiver domain at the C-terminus is similar to bacterial histidine kinase two-component response regulator. EIN4, ETR2 and ERS2 lack histidine kinase activity. The GAF, kinase and receiver Institute of Lifelong Learning, University of Delhi

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Hormone action and receptors domains represent the output domains of the receptors. The receptors form homodimers which are stabilized at the N-termini by two disulphide bonds. Various studies in Arabidopsis, melon and tobacco have shown that these receptors are predominantly localized to the membranes of ER. The binding of copper is required for ethylene receptor function. In the absence of ethylene, ethylene receptors inhibit the ethylene response pathway.

Figure: Ethylene receptor structures Source: http://openi.nlm.nih.gov/detailedresult.php?img=3485614_pls03102&req=4 (CC) The mechanism for ethylene signal transduction remained unknown until early 1980s when ethylene-binding sites in plants and plant cell extracts were characterized. Various experiments have revealed a model of ethylene signal transduction where the receptors signal to and stimulate CTR1 (constitutive triple response1), a serine/threonine kinase, which inhibits downstream signaling. CTR1 is a major component of ethylene signaling and the receptors directly interact with CTR1.

Cytokinins Cytokinins (CKs) are the phytohormones which promote cell division, or cytokinesis. They have been detected in all plants, mosses, fungi, and bacteria. Among higher plants most cytokinins are produced in the root apical meristem (region actively promoting cell division) and in fruits. They are transported to the other tissues through the xylem or Institute of Lifelong Learning, University of Delhi

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Hormone action and receptors vascular strands. They enhance cell division by stimulating the production of proteins needed for mitosis. There are two types of cytokinins: i)

Adenine-type cytokinins, such as kinetin, zeatin, and 6-benzylaminopurine, and

ii) Phenylurea-type cytokinins, which includes diphenylurea and thidiazuron (TDZ).

Kinetin

Zeatin

Figure: Chemical structures of adenine-type cytokinins kinetin and zeatin. Source: http://commons.wikimedia.org/wiki/File:Kinetin.png; http://commons.wikimedia.org/wiki/File:Zeatin.png (CC) Kinetin was originally isolated from autoclaved herring sperm DNA by Miller and Skoog et al. It was a compound that had cell division-promoting activity. Zeatin is a purine derivative that is most abundant natural cytokinin. It was discovered as a DNA breakdown product and helps to stimulate cell division/growth. It shows inhibition of aging/senescence.

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Hormone action and receptors

DID YOU KNOW? The ratio of cytokinin/auxin regulates organogenesis. In concert with auxin, cytokinins promote cell division and shoot development in tissues growing on sterile cultures. In the mediums containing both the components i.e., cytokinins and auxins, callus of undifferentiated cells can be obtained from the plant cells. On a medium with higher CK content, callus produces shoots while with higher auxin concentrations, roots are generated.

Figure: Plant responses to cytokinin/auxin ratio Source: http://www.nicholls.edu/biolds/Biol156/Lectures/Plant%20Hormones.pdf (CC)

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Hormone action and receptors

Figure: Model of de novo biosynthesis of cytokinin in plants. iP: isopentenyladenine; Z: zeatin. Source: http://www.pnas.org/content/97/26/14778/F1.expansion.html

Cytokinin receptors The cytokinin receptor appears to be related to the bacterial two-component histidine kinase family receptor proteins. It consists of a. Histidine kinase sensors (with input and transmitter domains) b. Response regulator (with receiver and output domains) The

importance

of

this

system

lies

in

the

activation

of

the

transmitter

and

autophosphorylation i.e., passage of signal by transferring the phosphate group from a histidine (H) to aspartate (D). It can lead to activation of transcription in bacteria. In plants, the receptor is called a phosphorelay two-component system. Some CK receptors that have been isolated from Arabidopsis are CRE1 (Cytokinin Receptor 1); AKH2 (Arabidopsis hybrid sensor kinase 2) and AHK3. These receptors function as dimers and have a membrane spanning domain. These are hybrid sensor histidine kinases (both sensor and receiver domains). Due to autophosphorylation, the sensor transfers the phosphoryl group to the histidine phosphotransfer protein (Hpt). This further leads to the transfer of phosphoryl group to the Asp residue on the regulator (Arabidopsis response regulator-ARR).

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Hormone action and receptors

Figure: Two-component histidine kinase systems. Source: http://rspb.royalsocietypublishing.org/content/276/1665/2133 (CC)

Abscisic acid Abscisic acid (ABA) is a plant hormone which functions in many plant developmental processes, including bud dormancy, seed germination and development and abiotic stress tolerance, particularly drought resistance. It affects gene expression, may cause the inhibition of shoot and root growth, promotes leaf fall, plant ageing. It was earlier known as abscisin II and dormin. These are tetraterpenes that are synthesized in plastids. They were believed to be involved in abscission. They are produced in aging leaves and fruits and cause yellow spots and premature aging if applied on leaves. ABA may cause induction of winter buds, seed dormancy, and repression of the growth of dormant lateral buds (with ethylene). It also counters the effects of gibberellins. By countering auxin, ABA stimulates the process of senescence. It is one of the phytohormones that is produced in plants under stress conditions, and is called a stress phytohormone. It may lead to loss of K + from guard cells and control opening and closing of stomata. It also helps in root-to-shoot communication. It is synthesised in leaves as well as in roots. The precursor for their synthesis is carotenoids. They are distributed in xylem, phloem, apoplast and symplast. It is released from leaf mesophyll cells and is transported in the form of free ABA or ABA conjugates. It helps in expanding the hydraulic permeability in roots.

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Hormone action and receptors

DID YOU KNOW?? ABA affects osmotic potential of guard cell by inhibiting the channels for K+ transport into guard cells (K+in) and stimulating the channels for K+ transport out of guard cells (K+out). It allows the transport of anions out of guard cells by stimulation of slow and rapid anion channels and also helps in membrane. ABA induces changes in structure of actin filaments or microtubules and therefore, alters the mechanical properties of guard cells.

Figure: Effect of ABA on guard cells Source: http://creating-a-new-earth.blogspot.in/p/special-desertplant-container-which.html (CC)

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Hormone action and receptors

Figure: Biosynthesis of abscisic acid Source:

http://en.wikipedia.org/wiki/Abscisic_acid#mediaviewer/File:Xanthtoaba.svg

(CC)



ABA receptors There are multiple ABA receptors. Three most important classes include: a) G proteins GTG1 and GTG2 localized in plasma membrane b) An enzyme that is plastid-localised and it helps in co-ordinating nucleus to plastid signal c) ligand-binding proteins of the START domain superfamily localised in cytosol.

In Arabidopsis, the PYR/PYL/RCAR family of soluble proteins were found that indicate one type of ABA-binding receptor-like proteins. There are 14 members in this family of proteins. All these can form an ABA-receptor complex that can activate the transcription of ABA-responsive genes.

Summary Hormones act as chemical messengers and help in transmitting signals even at very low concentrations. Their receptors are present throughout the cells for the effective transfer of chemical messengers from the site of synthesis to target sites. Transduction of signals in response to internal and external stimulus gives rise to both short and long-term responses. In multicellular systems, a network of various signal transduction pathways helps in the regulation of many important processes such as, cell differentiation, cell expansion, responses to stress, growth and development.

Glossary Cell signalling Effective transmission of molecular signals from the exterior of a cell to its interior with the help of cell-surface receptors. Stimulus

An agent, action, or event

that elicits a specific functional reaction or

physiological or psychological activity or response. Institute of Lifelong Learning, University of Delhi

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Hormone action and receptors Dormancy Conservation of energy by lowering down the metabolic activity and temporarily ceasing the growth and development due to presence of non-conducive environmental conditions, such as temperature, moisture, nutrient availability. Ubiquitination Addition of ubiquitin (protein) to a substrate protein. It

is a post-

translational modification. Proteasome Multisubunit enzyme complexes that plays a significant role in regulating the concentration of particular proteins and degrade misfolded proteins. Diterpene Organic derivatives of geranylgeranyl pyrophosphate composed of four isoprene units. Dicotyledons Seeds with two embryonic leaves or cotyledons. Etiolation Process of growing plants in partial or complete absence of light. Characters of plants undergone this process include usually long, weak stems; smaller, sparser leaves due to longer internodes; and a pale yellow colour (chlorosis). Abscission The

natural

shedding

of

various

parts

of

a

plant

typically dead leaves, ripe fruits, flowers or seeds. Seed germination It is a physiological process of growth of plants from a seed under favourable conditions leading to radical emergence. Trichomes Hairy or other type of outgrowths from epidermis of a plant. They are unicellular and glandular.

Exercises 1. State whether the following are true or false and justify your answer. i.

Signal transduction is a process where cell responds to exterior signals only.

ii.

Indole-3-acetic acid is the principal auxin found in plants.

iii.

Gibberellins have a well known effect on plants called as triple response.

iv.

Kinetin was originally isolated from autoclaved herring sperm DNA.

v.

Higher concentration of auxin can lead to the generation of roots. Ans. i.False, ii. True, iii. False, iv. True, v. True

2. Fill in the blanks i.

Kinetin and zeatin are examples of ______________ cytokinins.

ii.

__________ is also known as abscisin II and dormin.

iii.

Key components for GA signalling include ______________ receptor, the DELLA growth inhibitors (DELLAs) and the F-box proteins.

iv.

Auxin shows polar movement i.e., from _____ to ______. Institute of Lifelong Learning, University of Delhi

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Hormone action and receptors v.

The

cytokinin

receptor

appears

to

be

related

to

bacterial

_______________________ family receptor proteins. Ans. i. Adenine-type, ii. Abscisic acid iii. GIBBERELLIN INSENSITIVE DWARF1 (GID1), iv. Apex, base v. two-component histidine kinase. 3. Multiple choice questions i) Which of the following is a gaseous hormone? a. Auxin

b. Ethylene

c. Abscisic acid

d. Cytokinin

Correct answer: b ii) Which of the following is a diterpene? a. Auxin

b. Gibberellins

c. Ethylene

d. None o the above

Correct answer: b iii) The hormone responsible for triple response is: a. Ethylene

b. Auxin

c. Cytokinin

d. All of the above

Correct answer: a

iv) Precursor for the ethylene biosynthesis is: a. Tryptophan

b. Sulphur

c. Aminocyclopropane

d. Methionine

Correct answer: d v) Which of the following hormone is responsible for opening and closing of stomata? Institute of Lifelong Learning, University of Delhi

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Hormone action and receptors a. ABA

b. Auxins

c. Cytokinin

d. Gas

Correct answer: a 4. Match the following Column A

Column B

Stomatal closing

Ethylene

Thidiazuron (TDZ)

Auxin synthesis

Tryptophan

DELLA growth inhibitors

Foolish seedling disease

Phenylurea type cytokinin

GA signalling

GA

Gaseous hormone

Production of shoots

Higher cytokinin content

ABA

Ans: 1 (g), 2 (d), 3 (b), 4 (e), 5(c), 6(a), 7(f) 5. Short answer type questions i. Write a short note importance of signal transduction in plants. ii. Explain the role of auxin signalling in plants. iii. What is triple response? iv. Write the main steps of biosynthetic pathway of gibberellins. v. What is the mechanism involved behind the closure of stomata due to ABA? vi. Write a short note on different types of cytokinin receptors. vii. Illustrate GA signalling model in plants.

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Hormone action and receptors

References 

Plant physiology and development (2014) Taiz L., Zeiger E., Max Moller I. and Murphy A. Sixth Edition. Sinauer Associates, Inc.



The molecular life of plants (2013) Jones R., Ougham H., Thomas H. and Waaland S. John wiley and sons, Ltd., Publication.

Weblinks https://www.scienceopen.com/document/vid/0c40c71b-234c-49fd-9f27105c4295c891;jsessionid=lp3pj6jGCsdnLXPPwU5Fb0fA.slave:so-app2-prd?0 http://rspb.royalsocietypublishing.org/content/276/1665/2133

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