The ability of bacteria to cause disease is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms, and intrinsic characteristics of the bacteria called virulence factors. Many virulence factors are so-called effector proteins that are injected into the host cells by specialized secretion apparati, such as the type three secretion system. Host-mediated pathogenesis is often important because the host can respond aggressively to infection with the result that host defense mechanisms do damage to host tissues while the infection is being countered

1. UNIVERSITY OF AGRICULTURAL
SCIENCES, BANGALORE
COLLEGE OF AGRICULTURE V. C. FARM
MANDYA
ASSIGNMENT TOPIC :
MOLECULAR MECHANISMS OF PATHOGENESIS AND VIRULENCE OF
PLANT PATHOGENEIC BACTERIA
REDDY KUMAR A V
PAMM3005
DEPT. OF PLANT PATHOLOGY

3. The terms virulence and pathogenicity, which are often erroneously
considered synonyms. Shurtleff and averre defined that pathogenicity is
the ability of a pathogen to cause disease, whereas virulence is the degree
of pathogenicity of a given pathogen.
Most phytopathogens must evolve strategies to survive in different
environmental conditions to invade and colonize their host known as
virulence factors (Toth et al., 2003).
Bacteria evade, overcome or suppress antimicrobial plant defences using
these virulence factors, which elicit release of water and nutrients from
host cells to colonize in the apoplast successfully.

4. Virulence factors: The molecules that assists the bacteria
to colonize the host at the cellular level.

5. Production of
extracellular
polysaccharides
Virulence
factors
Adhesion of
bacteria to
plant surfaces
plant cell wall
degrading
enzymes
Production of
phytohormones
Production of
bacterial
toxins
Secretion
system of
bacteria

6.  The first step in a successful colonization by pathogenic bacteria is their
ability to maintain close proximity to a mucosal surface by adhesion.
 Adhesins are considered as biomolecules such as proteins and
glycoproteins that mediate the binding of the bacteria to the host cell
(Coa et al., 2001).
 By irreversible binding with host cell receptors the appropriate
organisms avoid being washed away by the fluids or being thrust out by
ciliated cells.
 Bacterial cell adhesion to their host cell depends on the specific binding
to carbohydrates presented at the cell surface which is mediated by
adhesive organelles of bacteria, called fimbria.
Adhesion of bacteria to plant surfaces

7.  Plant pathogenic bacteria have evolved numerous sophisticated
strategies for selective transport of proteins and nucleoproteins
involved in the virulence across the cell membrane.
 Six major classes of systems implicated in the virulence have been
identified in plant pathogenic bacteria from type I to type VI or
T1SS to T6SS.
 In plant pathogenic Gram-negative bacteria, two major systems:
* Single step process in which the secretion proteins are exported
inner and outer membrane without periplasmic step.
* The two steps process namely Sec and the Tat secretion system
are first exported in periplasmic and then transported across the
external membrane to the exterior of bacteria cell.
Secretion system of bacteria

8. Secretion system of bacteria

9.  Type I secretion system also known as the ATP binding cassette (ABC) transporters.
 ABC are involved in the export of various molecules from the cytosol to the
external environment without periplasmic step (Delepelaire et al., 2004).
 The type I secretion system have three different proteins that composed of
continuous channel.
 ABC proteins transporters is specific outer membrane known as outer membrane
protein (OMP) and also called as membrane fusion protein (MFP) which is
connected to the inner membrane and spans the periplasmic space and extends to
the outer membrane.
 Many proteins have great importance in pathogenesis are transferred by ABC
secretion system in plant pathogenic bacteria including proteases, lipases or
performing toxins.
E.g. T1SS required bacteria are Erwinia amylovora and Dickeya chrysanthemi.
Type 1 secretion system

11.  T2SS uses a two-step process in which proteins transit the inner membrane
in a Sec- or Tat-dependent process.
 The secreted proteins fold in the periplasmic space prior to passage through
an outer membrane secretin pore (Korotkov et al. 2012).
 T2SSs are used for the transport of many exoproteins, including proteases,
lipases, and phosphatases.
 Examples of T2SS substrates include V. cholerae cholera toxin,
enterotoxigenic E. coli (ETEC) LT toxin, and the P. aeruginosa virulence
factors ExoA (exotoxin A), PlcH (hemolytic phospholipase C), LasA
(staphylolysin), LasB (pseudolysin elastase), PrpL (protease IV), AprA
(aeruginolysin), ChiA (chitinase), and NanH (neuraminidase).
Type 2 secretion system

12. Plant bacterial pathogens have evolved a strategy of delivering an
array of effectors and toxins proteins directly into the cytoplasm
of host cell.
The type III secretion system apparatus is composed of more
than of 20 proteins consisting of basal body spanning both inner
and outer membrane of bacterial cells, and extra needle with the
tip complex extending into the host cell.
T3SS is encoded by hypersensitive response and pathogenicity
(hrp) gene involved in the transfer of Avr proteins in the host cell
(Galan and Collmer. 1999).
Type 3 secretion system

13. • Actin dynamics, mitogen-activated protein kinase (MAPK)
signaling, and nuclear factor-κb (nf-κb)-based inflammasome
activation are modulated by enteropathogenic E. Coli (EPEC),
enterohemorrhagic E. Coli (EHEC), P. Aeruginosa, and V.
Cholerae T3SS effectors.
• Y. Pestis yop effectors also affect these signaling pathways as well
as facilitating intracellular persistence within macrophage.
• The intracellular pathogens C. Trachomatis, S. Enterica, and S.
Flexneri require T3SS effectors to invade and persist within the
vacuolar system of host cells.

15. The type IV secretion system is present in both the Gram-
negative and positive plant pathogenic bacteria (Wallden et al.,
2010).
This translocation system that deploy the sec gene to transport
the pathogenicity factors from the inner bacterial cell or into the
extracellular environment or directly into the host cell.
T4SS, which are structurally related to DNA conjugation
systems, have the ability to transport DNA, DNA-protein
complexes, and protein effectors across membranes.
Type 4 secretion system

16.  Agrobacterium tumefaciens that target the oncogenic
dna-protein complex in the plant cell.
N. Gonorrhoeae uses a T4SS to acquire virulence genes
through horizontal gene transfer mechanisms.
L. Pneumophila inject ~330 effector proteins that affect
multiple host processes, including vesicle trafficking,
autophagy, host protein synthesis, host inflammatory
response, macrophage apoptosis, and host cell egress.
 H. Pylori uses the T4SS to insert effectors that modulate
the host immune response.
Examples for T4SS

17. Type 5
secretion
system
 This type V secretion system is present in Gram-negative bacteria (Tseng et al.,
2009). It is one of the simplest secretion pathway.
 The T5SS translocation system is dedicated to transfer a single specific
polypeptide known as the passenger domain in two step process:
1. Sec translocator across the inner membrane.
2. The transportation of the passenger through the outer membrane by forming a
outer memebrane pore.
 The virulence factors associated with T5SS passengers includes biofilm
formation, adhesions, toxins, enzymes productions and cytotoxic activity
(Huang and Allen. 1997).

18. Examples of T5SS effectors include:
Adhesins (B. Pertussis FHA, pertactin and Bapc, E. Coli
AIDA-I and ag43, H. Influenzae hia, HWM1, and HWM2,
Shigella flexneri Icsa, Y. Enterocolitica yada)
Proteases (N. Gonorrhoeae and N. Meningitidis iga
Protease, S. Flexneri sepa)
Toxins (H. Pylori vaca).

19. Type 6
secretion
system
The contact-dependent T6SS uses a phage-tail-spike-like
injectisome structure to deliver effectors not only to host
cells but also to competitor bacterial species, thereby
giving pathogens competitive advantages within certain
host growth niches.
This system also includes in formation of biofilm, the
quorum sensing and antibacterial toxins (Benali et al.,
2014).

20. T6SS effector
Functions Bacteria
Adherence E. coli, C. jejuni, V.
parahaemolyticus
Host cell invasion E. coli, C. jejuni, S.
enterica, P.
aeruginosa, Y.
pseudotuberculosis
Actin dynamics E. coli, V. cholerae
Host immune
responses
K. pneumoniae,
V .Parahaemolyticus

21. Production of plant cell
wall degrading enzymes
 Plant cell walls consist of three major polysaccharides such as cellulose,
hemicellulose and pectin, in woody and some other plants, lignin.
 The number of genes coding cell wall degrading enzymes varies include
pectinases, proteases, cellulases and xylanases. Proteases are secreted by the
T1SS, whereas the rest of the above said enzymes by the T2SS (Preston et al.,
2005).
 Pectinases to be most important in pathogenesis, because they are responsible for
tissue maceration by degenerating the pectic substances in the middle lamella and
eventually, for cell death.
 Four major types of pectin degrading enzymes are secreted viz. pectate lyase,
pectin lyase, pectin methyl esterase and polygalacturonase.
 Among these pectinase enzymes, pectate lyases (Pels) are largely involved in the
virulence of soft rot Pectobacterium species.

22.  Cell wall degrading enzymes are believed to play a role in
pathogenesis by facilitating penetration and tissue
colonization, but they are also virulence determinants
responsible for development of symptom once growth of the
bacteria has been started.
 A few Xanthomonads, e.g., X. campestris pv. campestris, the
causal agent of black rot of crucifers, have genes for two
pectin esterases and polygalacturonases, four pectate lyases,
five xylanases and nine cellulases.
 Other deprived pectinolytic bacteria include A. tumefaciens,
which has only four genes encoding pectinases of any form
and Xylella, which has only one gene coding for a
polygalacturonase.

23. Productions of bacterial
toxins
 Toxins play a vital role in pathogenesis and parasitism of plants by several plant
pathogenic bacteria.
 Plant pathogenic bacteria are known to produce a wide range of both specific and
nonspecific host phytotoxins. Some are polypeptides, glycoproteins others are
secondary metabolites.
 These toxins acts by using diverse mechanisms from modulating and suppressing
plant defence response to alternation and inhibition of normal host cellular metabolic
process.
 P. syringae pv. syringae, the cause of many diseases and kinds of symptoms in
herbaceous and woody plants, generates necrosis-inducing phytotoxins,
lipodepsipeptides, which are generally categorized into two groups, such as mycins
and peptins (Melotto et al., 2006).
 Chlorosis inducing phytotoxins include coronatine formed by P. syringae pv.
atropurpurea, glycinea.

24.  Coronatine biosynthesis plays an important role in virulence of toxin-
producing P. syringae strains.
 Coronatine is also believed to induce hypertrophy of storage tissue, thickening
of plant cell walls, accumulation of protease inhibitors, compression of
thylakoids, inhibition of root elongation and stimulation of ethylene
production (Alarcon-Chaidez et al., 1999).

25.  Extracellular polysaccharides (EPS) may be connected to the bacterial cell
as a capsule, be produced as fluidal slime, or be present in both forms.
 EPS play a significant role in pathogenesis of many bacteria by both direct
interference with host cells and by providing resistance to oxidative stress.
 EPS1 is the chief virulence factor of the bacterial wilt disease caused by R.
solanacearum in solanaceous crops (Milling, et al., 2011).
 EPS1 is a polymer made of a trimeric repeat unit consisting of N-acetyl
galactosamine, deoxyl-galacturonic acid and trideoxy-d-glucose, where it is
produced by the bacterium in huge quantity and constitutes more than 90%
of the total polysaccharides.
 Xanthan, the major exopolysaccharide secreted by Xanthomonas spp., plays
a key role in X. campestris pv. campestris pathogenesis.
Production of extracellular
polysaccharides

26. Production of phytohormones
 Biosynthesis of the phytohormones, auxins (e.g. indole-3-
acetic acid-IAA) and cytokinins are major virulence factors
for the gall-forming plant pathogenic bacteria, Pantoea
agglomerans pv. Gypsophilae.
 Ethylene, the gaseous phytohormone formed by several
microbes including plant pathogenic bacteria, can also be
considered a virulence factor for some of them
P. savastanoi pv. Phaseolicola (Weingart et al., 2001).

27. QUORUM SENSING AND BIOFILM PRODUCTION:
 It is a bacterial communication mechanism that regulates the density of
microbial population using the gene expression in response to the
environment (Kanda et al., 2011).
 This molecules are also known as autoinducers. Quorum-sensing signal
N-acyl homoserine lactones are known to regulate numerous virulence
factors including enzymes production and exopolysaccharides in many
plant pathogenic bacteria.
E. g. in E. amylovora a series of regulators namely MqsR, QseBC and
exporter TqsA.
 Biofilm is a complex multilayer cellular structure attached to a tissues and
embedded with an exopolysaccharide. Several plant pathogenic bacteria
have been considered as biofilm producer as virulence factors including
X. compestris and P. syringae (Keith et al., 2003)

28. Biofilm formation process

29. Bacterial virulence factors and the respective phytopathogens and
diseases
Phytopathogen Disease Virulence Factors
Erwinia amylovora Fire Blight Extracellular
polysaccharides (EPS) –
amylovoran and levan
6-Thioguanine (6-TG)
Desferrioxamine (DFO)
Pseudomonas syringae pv
syringae
Bacterial-LeafSpot,
Bacterial Speck, Bacterial
Blight
Coronatine (COR)
Phaseolotoxin Tabtoxin
Syringopeptin
Syringomycin
Xanthomonas spp. Cassava Bacterial Blight,
Black Rot Disease,
Bacterial Leaf streak
Xanthan, Extracellular
polysaccharides,
Lipopolysaccharide,
Xanthoferrin,
Cyclic β-(1,2)-glucans

30. Disease symptoms caused by some bacterial pathogens of plants and
representative virulence mechanisms used by these pathogens

31.  Plant pathogenic bacteria employ a sophisticated array of molecular
mechanisms to cause disease in plants. Key virulence factors include
toxins, cell wall-degrading enzymes, and secretion systems.
 Secretion systems (T3SS) are crucial virulence determinants, injecting
effector proteins directly into plant cells to modulate host physiology.
 Quorum sensing enables coordinated gene expression, facilitating
virulence factor production and biofilm formation. Furthermore, some
bacteria produce phytotoxins like phaseolotoxin, disrupting cellular
processes and causing disease symptoms.
 Understanding these molecular mechanisms is crucial for developing
strategies to combat plant pathogens. Targeting virulence factors,
disrupting communication systems, and enhancing plant immunity are
promising approaches. Future research aims to unravel complex
interaction networks and develop sustainable solutions for plant disease
management.
CONCLUSION

32. THANK YOU