Plant respond to the attack of diseases by triggering various bio-molecules insider their system to combat the infection and establishment of the pathogens. these response operate in specified pathways mediated by many enzymes starting from the infection site to the nucleus which together constitute the signal transduction pathway.

2. BY
K.T. RAVI KIRAN
TAD/13-20

3. SIGNALTRANSDUCTION
 Elictors/ligands - microbial proteins, small peptides, and
oligo-saccharides
 Host receptors - many of which may be encoded by R
genes
 Signal transduction cascade - protein phosphorylation, ion
fluxes, reactive oxygen species (ROS), and other signaling
events
 Subsequent transcriptional and/or post-translational
activation of transcription factors - induction of plant defence
genes.
 Pathogen signals may be amplified through the generation
of secondary plant signal molecules such as SA.

4. SIGNALTRANSDUCTION
 Both primary pathogen elicitors and secondary endogenous
signals may activate a diverse array of plant protectant and
defence genes, whose products include:
 Glutathione S-transferases (GST),
 Peroxidases,
 Cell wall proteins,
 Proteinase inhibitors,
 Hydrolytic enzymes (e.g., chitinases and p-l,3-glucanases),
 Pathogenesis-related (PR) proteins, and
 Phytoalexin biosynthetic enzymes, such as phenylalanine
ammonia lyase (PAL) and chalcone synthase.

5. Componentsof signaltransduction
pathway
 Host recognition of pathogen elicitors
 Ion fluxes
 G proteins
 Protein kinases
 Reactive oxygen intermediates
 Endogenous secondary signals
 Integration of signaling pathways and activation of
plant defense responses

6. Host recognition of pathogen
elicitors
 Gene – for – gene interaction between a dominant
avirulence [avr] gene in the pathogen and a corresponding
dominant R gene in the host (Flor, 1971)
 Some avr genes may be important for pathogen fitness
and/or Pathogenicity
 Dozens of avr genes have been isolated and characterized -
biochemical function of their products remains unknown

7. Host recognition of pathogen
elicitors
 Plant R genes encode receptors for the recognition of
specific elicitors or ligands
 Most of the R genes encode one or more common structural
motifs – LRRs, serine/ threonine kinase domains, nucleotide
binding sites, leucine zippers and Toll/interleukin-1 receptor-
like domains
 Such structural conservation within R genes from a wide
range of plant taxa - a common molecular mechanism
underlying many gene-for-gene interactions.

8. Host recognition of pathogen
elicitors
 Based on sequence analyses, it has been predicted that
some of the plant R gene products are localized
extracellularly
 In general, these products consist of a putative extracellular
LRR, a transmembrane region, and a small cytoplasmic
domain
 Rice Xa21 gene encodes a protein with an extracellular
LRR motif as well as an intracellular serine/threonine protein
kinase domain

9. Host recognition of pathogen
elicitors
 Many fungal and bacterial oligosaccharides, proteins, and
glycoproteins can function as nonspecific elicitors to induce
defense responses in the plants carrying no specific R
genes.
 Host recognition of general fungal elicitors is likely mediated
by high affinity receptors present in plasma membranes.

10. Molecular recognition in gene for
gene interaction

11. Ion fluxes
 The earliest detectable cellular events are ion fluxes across
the plasma membrane and a burst of oxygen metabolism
 Elicitor increases the open probability of plasma membrane-
located ion channel and may thereby stimulate elevated
cytosolic calcium levels, as well as activate additional ion
channels and pumps
 Mediated through the regulation of plasma membrane-bound
enzymes. These include changes in Ca2+-ATPase and H+-
ATPase activities

12. G proteins
 Molecular signal transducers whose active or inactive states
depend on the binding of GTP or GDP, respectively
 Include two major subfamilies, the heterotrimeric G proteins
and the small G proteins
 α subunit has the receptor-binding region and possesses a
guanosine nucleotide binding site and GTPase activity
 Both classes of G proteins use the GTP/GDP cycle as a
molecular switch for signal transduction

13. G proteins

14. Protein kinases
 Phosphorylation cascades are involved in defense signaling
at many different levels
 Some plant resistance genes encode receptor-like protein
kinases themselves - activate downstream signaling
elements by phosphorylation, others - secondary messenger
such as Ca2+ may trigger the protein kinases
 Rice Xa-21 and wheat Lr10 encodes receptor-like kinases

15. Some important protein kinases
 MAP kinase (ERM kinase)
 Calmodulin-like domain protein kinase (CDPK)
 Protein kinase C (PKC) and a Ca2+/ CaM-dependent protein
kinase
 A salicylate-responsive MAP kinase, SIP kinase
 A wound-responsive MAP kinase from tobacco, WIP kinase

16. Reactive oxygen intermediates
 Transient elevation of cytosolic calcium levels necessary for
elicitor stimulation of the oxidative burst
 Extracellular generation of ROS is a central component of
the plants defense machinery
 ROS act as direct toxicants to pathogens, catalyze early
reinforcement of physical barriers and are involved in
signaling later defense reactions, such as phytoalexin
synthesis and defense gene activation, programmed cell
death and protective reactions .

17. A hypothetical model of the early signal
transduction events in plant-pathogen
interactions

18. Endogenous secondary signals in
plant disease resistance
 Salicylic acid:
 Plays a critical role in the activation of defense responses
 Increases in the levels of SA and its conjugates have been
associated with the activation of resistance responses in a
wide variety of plant species
 These increases slightly precede or parallel the expression
of PR genes in both the infected tissue as well as the
uninfected tissues exhibiting SAR

19. Endogenous secondary signals in
plant disease resistance
 Salicylic acid:
 Inhibit the activity of catalase and ascorbate peroxidase
 By serving as a one-electron-donating substrate – SA free
radicles
 Phenolic free radicals are potent initiators of both lipid
peroxidation and protein oxidation.
 Lipid peroxides, the products of lipid peroxidation, were
shown to induce PR-1 gene expression - SAR

20. Endogenous secondary signals in
plant disease resistance
 Ethylene, jasmonates and systemin:
Ethylene levels increase during the HR
Ethylene treatment induces the expression of PAL and the
basic PR genes, as well as several wounding-induced
genes
Ethylene can also enhance the SA-induced expression of
PR-1 in Arabidopsis.

21. Endogenous secondary signals in
plant disease resistance
 Ethylene, jasmonates and systemin
 The activation of defense responses after mechanical
wounding and insect attack - mediated by systemin, an 18-
amino-acid peptide, as well as by JA and its ester, methyl
jasmonate (MeJA), collectively termed jasmonates.
 Wounding, systemin, and jasmonates induce proteinase
inhibitors (PI) I and II, PAL, and JIP60.
 The PI proteins reduce herbivory and insect attack by
inhibiting key degradative enzymes, whereas JIP60 has
been proposed to reduce pathogen attack by mediating
polysome dissociation.

22. Integration of signaling pathways and
activation of plant defense responses
 Many of the signals are probably integrated into one of a
few terminal pathways that lead to the transactivation steps
involved in the interaction between activated transcription
factors and pathogen-responsive cis elements in the
promoters of defense genes.
 A single pathogen elicitor may activate multiple transcription
factors that interact with different cis elements in the same
or different promoters, leading to induction of many defense
genes

23. Integration of signaling pathways and
activation of plant defense responses
 A number of plant defense genes contain an elicitor-
responsive TTGACC element. This element is also present
as the W boxes in the parsley PR-1 gene promoter
 Transcription of the parsley PR-2 gene is stimulated rapidly
by fungal or bacterial elicitors and mediated by an 11-bp cis
element (CTAATTGTTTA) present in its promoter.
 A 10-bp TCA (TCATCTTCTT) element is present in the
promoters of many stress-inducible genes, including
tobacco PR genes. A 40-kD tobacco nuclear protein binds
to this TCA element in an SA-dependent manner.

24. SUMMARY

25. SUMMARY

26. REFERENCES
 Blumwald, E., Aharon, G.S and Lam, B.C.H. 1996. Early
signal transduction pathways in plant-pathogen interactions.
Elsevier Reviews. 3 (9): 342-346
 Scheel, D. 1998. Resistance response physiology and signal
transduction. Current Opinion in Plant Biology. 1: 305-310.
 McDowell, J.M and Dangl, J.L. 2000. Signal transduction in
the plant immune response. TIBS. 99: 79-87.
 Yang, Y., Shah, J and Klessing, D.F. 2014. Signal perception
and transduction in plant defense responses. Genes and
Development. 11: 1621-1639.