BYK.T. RAVI KIRAN

TAD/13-20

SIGNAL TRANSDUCTION IN PLANT DEFENSE RESPONSES

SIGNAL TRANSDUCTION

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.

SIGNAL TRANSDUCTION

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.

Components of signal transduction 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

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 - bio chemical function of their products remains unknown

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.

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

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.

Molecular recognition in gene for gene interaction

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

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

G proteins

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

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

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 .

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

interactions

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

Endogenous secondary signals in plant disease resistance

Salicylic acid: Inhibit the activity of catalase and ascorbate peroxidase By serving as a one-electron-donat ing substrate – SA free

radicles Phenolic free radi cals 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

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 Arabidop sis.

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 poly some dissociation.

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 be tween activated transcription factors and pathogen-re sponsive cis elements in the promoters of defense genes.

A single pathogen elicitor may activate multiple tran scription factors that interact with different cis elements in the same or different promoters, leading to induction of many defense genes

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 pars ley 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.

SUMMARY

SUMMARY

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.