Transcriptional Factors in Stress Tolerance

Kale Ravindra Ramrao

Outline

Stress

Function of drought inducible genes

Signal transduction

Introduction to transcription factors

TFs Motifs

Families of plant TFs

What is stress ? Types?

Stress resistance mechanisms

• Avoidance mechanisms

• Tolerance mechanisms

• Acclimatization(Bartels and Ramanjulu ., 2005)

Functions of Drought inducible genes

Drought

Detoxification enzymes (catalase)

Protein kinase

Protection factors of macromolecules(LEA proteins, chaperons

etc.)

Key enzymes for osmolyte biosynthesis

(proline , sugar etc.)

Water channel protein

( Agarwal et al.,2006)

Transcription factors (MYB ,

NAC etc.)

Introduction to Transcription Factors

TFs are proteins that binds to specific sequence of DNA(cis-

acting)elements in promoter region and regulate transcription

Number of transcription factors depends on genome size

Use different mechanism to up or down regulate gene expression

Basic Helix-loop-helix bZIP Zinc finger

Transcription Factor Motifs

Structure of TFs

TFs are modular in structure and contain 3 – domains:

DNA binding domain- DNA sequence binds to TFs is known as responsive elements

Trans-activation domain – Contain binding sites for other proteins

Signal-sensing domain – Sense external signals and transmits these to rest of transcription complex

8

Model of Hormone-dependent Gene Activation by Homodimeric Receptors

Different Families of plant stress TFs

ERF / AP2

WRKY

bZIP

NAC

MYB(Agarwal et al., 2006)

AP2 / ERF Family

It include DREB or CBF for DRE or C repeats

Arabidopsis AP2 proteins classified on the basis of similarities in their

DNA-binding domains

CBF genes coordinate both activation or repression of stress responsive

genes.

(Agarwal et al., 2006)

(Bartels and Sulumar., 2005)

Contd…

Phenotype for loss-of-function mutants have not yet been reported for any ERF genes

Tsi 1( tobacco stress-inducible gene ) provides direct evidence of regulation of responses t biotic and abiotic stresses through single ERF gene

(Singh et al., 2002)

Stress Response Related to ERF

(Singh et al., 2002)

Contd…

Contd…

(Yamaguchi and Shinozaki ., 2006)

WRKY Family

Presence of zinc fingure motif within WRKY domain

Classification done on the basis of number of WRKY domains and the

feature of their zinc-fingure-motif

Group I and II have same fingure motif(C H ) while group III have fingure ₂ ₂

motif(C HC)₂

cis-acting element for WRKY is W box having sequence (T)

(T)TGAC(C/T)which contain the invariant TGAC core

(Thomas et al., 2000)

Classification of WRKY

(Thomas et al., 2000)

(Khong et al., 2008)

Role of WRKY in Resistance Against stresses

MeJA

OsWRKY03

NH1

bZIP

ZB8 POX22.3 PR1b

OsWRKY45

GST

Resistance to bacterial blightx.oryza

Resistance to fungal blastM.grisea

light

OsWRKY31

auxin

Mangnaportae griseaSA BTH

PBZ1 OsSci2 Os1AA4 Arl1/Crl1

PAL peroxidase

Root development

wounding

RCl-1

Contd….

OsWRKY03 act upstream of the the NPR1/NH1 regulation pathway.

OsWRKY45 acts independent of NPR1/NH1

Expression of gene RCL-1 in response to BTH is independent of

OsWRKY45 and NH1

OsWRKY31 links activation and inhibition of plant growth

bZIP Family

Contain DNA-binding domain rich in basic residues adjacent to leucin zipper dimerization domain

Binds to ACGT (G –box motif ) core sequence

One class of it is TGA/OBF binds to activation sequence-1 / ocs element

TGA/OBF interact with NPR1 a key component of SA defense signaling pathway

(Bartels and Ramanjulu., 2005)

(Singh et al., 2002)

Contd….

(Singh et al., 2002)

Contd….

Different TGA / OBF members may involved in different stress responses

Other class include ABF/ AREB that work thorough ABA dependent signal transduction pathway

bZIP shows complexity in regulation that shows pathogen responses override UV protection

NAC Family Of TF

Plant specific NAC (NAM , ATAF1/2) TFs play diverse roles in plant

development and stress response

NAC family is essential to the activation of the ERD1 gene

OsNAC 1 involved in stress resistance in rice

Expression study of GFP placed under the control of OsNAC1 promoter in

plant , shows that this gene is normally expressed in ligulae , stamen and

in guard cells of stomata

(Khong et al., 2008)

Contd…

(Khong et al., 2008)

Network of rice tolerance to water deficit

MYB Family Of TF

MYB proteins from animal contain 3 repeats while plant MYB contain 2

repeats

It carry out expression of ABA-related gene and jasmonic acid-related

genes which shows cross talk pathway between abiotic and biotic stress

Shows conserved tryptophan residues that form core motif

In plants first tryptophan replaced by phenylalanine or isoleucine

(Abe et al,.2003)

Functions of MYB

1. Phenylpropanoid metabolism

2. Cell shape –

MIXTA and it’s orthologous PhMYB1 gene.

GL1 gene from Arabidopsis

3. Response to hormones –

GA application induce expression of barely MYB protein (GAMYB)

(Cathie and Javier ., 1997)

Analysis of AtMYB60 expression

Results :-

Light induced stomatal Opening is impaired in the atmyb60-1 Mutant

bHLH Family Of TF

In A.thalinia a Myb transcription factor encoded by PHR1 participate in Pi starvation signalling pathway

Os PTF1 from rice has been isolated from a cDNA library obtained from roots of plants cultured in normal or Pi-deficient condition

Os PTF1 acts on the cues of Pi uptake by stimulation root growth resulting in an increase in root surface area

Os PTF1 regulate genes that play ole in Pi starvation rescue but no high affinity Pi transporter genes found

(Yi et al., 2005)

(Schachtmn et al., 1998)

Networks involving bHLH transcription factors for tolerance to mineral nutrition

Contd….

CONCLUSION

DISCUSSION

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