By

Rakesh (2014A42D)

PhD Plant Pathology

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Pathogenesis

Related Proteins

PR protein, are a structurally diverse group of

plant proteins that are toxic to invading fungal

pathogens.

They are widely distributed in plants in trace

amounts, but are produced in much greater

concentration in pathogen attack or stress.

Exist in plant cells both intracellularly and also

in the intercellularly.

INTRODUCTION

PR proteins

SAR

Intra and Intercellular

Different plant organs, e.g., leaves, seeds, and

roots, may produce different sets of PR proteins.

They are either extremely acidic or extremely

basic and therefore are highly soluble and

reactive.

Seventeen families (Van Loon et al., 2006).

Inhibit the invading pathogens by their

enzymatic activities

The signal compounds responsible for induction of PRPs

include salicylic acid, ethylene, xylanase, the polypeptide

systemin and jasmonic acid.

INTRODUCTION

Although healthy plants may contain trace amounts of

several PRPs, attack by pathogens, treatment with elicitors,

wounding, or stress induce transcription of a battery of

genes that code for PRPs.

Thirty-three PR proteins - tobacco, 20 - sugar

beet and more than 30 - Norway spruce

Glucanases and chitinases - potato plants

PR proteins developmentally regulated and are

found in leaves of healthy tobacco plants during

flowering and are constitutively present in bean

leaves

In potato, leaving aside the leaves, PR 10a is

reported to accumulate in tubers, petioles and

stem region after infection

Occurrence and Distribution

S.N. PR Protein

Family

PR Protein activity Targeted pathogen sites

1 PRP-1 Unknown Active against Oomycetes

2 PRP-2 β-1,3 glucanase Cell wall glucan of fungi

3 PRP-3 Endochitinase Cell wall chitin of fungi

4 PRP-4 Endochitinase Cell wall chitin of fungi

5 PRP-5 Thaumatin-like Active against Oomycetes

6 PRP-6 Proteinase inhibitor Active on nematodes and insects

7 PRP-7 Endoproteinase Microbial cell wall dissolution

8 PRP-8 Endochitinase with

lysozyme activity

Cell wall chitin of fungi, and mucopeptide

wall of bacteria

Classification of PR proteins (Van Loon et al., 2006)

S.N. PR Potein

Family

PR Potein activity Targeted pathogen sites

9 PRP-9 Peroxidase *

10 PRP-10 Ribonuclease activity Viral- RNA

11 PRP-11 Endochitinase Cell wall glucan of fungi

12 PRP-12 Defensin Antifungal and antibacterial activity

13 PRP-13 Thionin Antifungal and antibacterial activity

14 PRP-14 Lipid transfer

proteins

Antifungal and antibacterial activity

15 PRP-15 Oxalate-oxidase Produces H2O2 that inhibits microbes and

also stimulates host defense

16 PRP-16 Oxalate-oxidase-like

with superdismutase

activity

Produces H2O2 that inhibits microbes and

also stimulates host defense

17 PRP-17 Uncharacterized Unknown

Peroxidase exert indirect antimicrobial activity by catalyzing oxidative cross-linking of protein and phenolics in the

plant cell wall leading to reinforcement of physical barrier

Pathogen Inducible

Bacteria, fungi, viruses and nematodes induce

PR proteins upon entry into the incompatible host

Rice leaves inoculated with Pseudomonas

syringae enhance the production of PR-1, 2,3,5

and 9

PR-6 proteins showed increasing accumulating

trends in tomato and pepper when inoculated with

Xanthomonas vesicatoria

peroxidases, glucanases and chitinases are

found to increase in rice upon inoculation with

Erwinia caratovora

Association of PRPs with Disease Resistance

PR Proteins in Systemic Acquired Resistance

Association of PRPs with Disease Resistance

Possible function of PRPs

Plays important role in defence not only against pathogen

infection but also in eliciting acquired resistance

Glucanases and the chitinases might be involved in the

recognition process-releasing defence activating signaling

molecules from the invading pathogen

Antimicrobial Properties of PR Proteins

PR-1, 2, 3, 4 and 5 - inhibit growth of fungi

Tobacco gp 35 with glucanase like properties and gp 22

with PR-5 inhibit the viral infection

PR-1 was inhibited oomycetes like Phytophthora parasitica

and Peronospora tabacina and bacteria like Pseudomonas

syringae

Pathogenesis Related Function of PRPs

PR-1 Protein

Detected - Rice, Wheat, Barley, Corn, Tomato,

Tobacco and several other Plant species

belonging to Graminae, Solanacae ,

Amaranthaceae And Chenopodiaceae

(Rodrigues et al., 2005: Makandar et al., 2006).

Soluble in acidic buffers (pH 3.0)

Molecular weight ranges from 14 to 16 kDa

Mode of action

Strengthening of host cell walls to prevent

spread of the Pathogen (santen et al., 2005)

Inhibitory function on growth of the pathogen

(santen et al., 2005)

In tomato leaves inhibit germination of zoospores

and pathogenicity of Phytophthora infestance

PR-2 Proteins

Shows β-1,3-glucanase activity

M.W. of β -1,3-Gs 33 to 44 kDa

In Nicotiana species:

Class I: PR2e subgroup, these are basic protein

localized in cell vacuole

Class II: PR2a ,PR2b and PR2c subgroup, these

are acidic protein

Class III: include - PR2d subgroup it is also

acidic protein but differs in sequence by at

least 43% from class I and class II

Mode of action

β-1,3-glucanases are involves in hydrolytic

cleavage of the 1,3-β-D-glucosidic linkages in β-

1,3-glucans, a major componant of fungi cell

wall. So that cell lysis and cell death occur as

a result of hydrolysis of glucans present in the

cell wall of fungi.

PR-3 Proteins

Shows Chitinase activity

Chitinases are endo β-1,4-glucosaminidases which

hydrolyze the β-glycosidic bond at the reducing end

of lucosaminidinides found in chitin, chitosan or

peptidoglycan (Neuhaus,1999)

Most of Chitinase having molecular mass in the range

of 15 kDa and 43 kDa

Chitinase can be isolated from Chickpea, Cucumber,

barley

Mode of action

Cleaves the cell wall chitin polymers , resulting

in a weakened cell wall and rendering fungal

cells osmotically sensitive

These Chitinases have Significant antifungal

activities against plant pathogenic fungi like

Alternaria sp.

For grain discoloration of rice, Bipolaris oryzae

for brown spot of rice

PR-4 Proteins (Chitin Binding Protein)

Isolate from -sugar beet, tobacco, pepper,

tomato and potato

Molecular weight -9 kDa to 30 kDa

Mode of action

Shows strong inhibitory effect against fungi

Aspergillus species, Cercospora beticola,

Xanthomonas campestris and many more

and several crop fungal pathogen

Enzymatically CBP has not any function but

it binds to insoluble chitin and enhances

hydrolysis of chitin by other enzyme like

Chitinase

PR-5 Proteins (Thaumatin like protein)

Resemblances to sweet- tasting protein ,thaumatin,

which occurs in the fruit of West African shrub

(Pierpoint et al., 1987)

Isolated from barley, kiwifruit, maize

Molecular weight -18 kDa to 25 kDa and pH 4.5 to 5.5

Linusitin is a 25-kDa Thaumatin-like Protein isolated

from flax seeds

Mode of action

Involved in systemetically acquired resistance

and stress response in plants

Thaumatin production is induced in katemfe

fruit -by viroid pathogens

Display significant in vitro inhibition of hyphal

growth and sporulation by various fungi.

PR-6 Proteins (Plant protease inhibitors)

The possible role of protease inhibitors (PIs) in

plant protection was investigated as early as

1947 by, Mickel and Standish.

Highly stable defensive proteins that are

developmentally regulated and induced only in

response to insect and pathogen attack

The proteinase inhibitors are classified into

1. Serine proteinase inhibitors

2. Cysteine proteinase inhibitors

3. Aspartate/metallo proteinase inhibitor

Mode of action

Exhibit a very broad spectrum of activity

including suppression of pathogenic nematodes

like Globodera tabaccum, G. pallida, and

Meloidogyne incognita (Williamson and Hussey,

1996)

Based on the active amino acid in their “reaction

center” (Koiwa et al. 1997), are classified as

serine, cysteine, aspartic and metallo-proteases

PR-10 Proteins (Ribosome inactivating

protein)

RIPs are toxic N-glycosidases that depurinate

the universally conserved sarcin loop of large

rRNAs

This depurination inactivates the ribosome,

thereby blocking its further participation in

protein synthesis

RIPs are widely distributed among different plant

genera and within a variety of different tissues

Mode of action

RIPs are active against a wide variety of both

pathogenic and non-pathogenic fungi including

Fusarium and Trichoderma species

A type 2 RIP from seeds of the camphor tree

Cinnamomum camphora, was toxic to larvae of

mosquito and cotton bollworm.

Pathogenesis related protein-12 (Plant

defensins)

“Plant defensin” was coined in 1995 by Terras

Plant defensins are small (M.W. 5kDa), basic,

cysteine-rich antifungal peptides ranging from

45 to 54 amino acids, and are positively

charged

Isolated from wheat and barley and were

initially classified as a subgroup of the

thionin family called the γ-thionins

Mode of action

In bacteria, permeabilization coincided with the

inhibition of RNA, DNA and protein synthesis

and decreased bacterial viability

Antifungal defensins reduce hyphal elongation

and induce hyperbranching

Pathogenesis related protein-13 (Thionins)

45 to 50 amino acids in length, which include

three or four conserved disulfide linkages

Found in seeds where they may act as a

defence against consumption by animals

A barley leaf thionin that is highly toxic to plant

pathogens and is involved in the mechanism of

plant defence against microbial infections has

also been identified

Mode of action

Presumably attacking the cell membrane

and rendering it permeable, this results in

the inhibition of sugar uptake and allows

potassium and phosphate ions, proteins, and

nucleotides to leak from cells.

γ –hordothionins isolated from sorghum was

the first example of a thionin able to inhibit

insect -amylases

PR protein -14 (Lipid-transfer Proteins In Plants)

LTP are basic, 9-kDa proteins present in high

amounts in higher plants

Located in the cell wall

LTPs: participation in cutin formation,

embryogenesis, defense reactions against

phytopathogens, symbiosis, and the adaptation

of plants to various environmental conditions

Conclusion

PR proteins play important role in disease resistance, seed

germination and also help the plant to adapt to the

environmental stress.

The increasing knowledge about the PR proteins gives

better idea regarding the development and defense system

of plants.

Primary aspects of the gene regulation of the PR proteins

are understood but the study of exact mechanism of gene

regulation and receptor cascade will open new ways for the

plant genetic engineering technology for crop improvement.

Rakesh Punia