Page 4 - Volume 5, Issue 2, July 2012

Skandalis

et al.

32

tary genetic systems of the flax rust fungus

(

Melampsora lini)

and its host, uncovered

the complementary genetic systems of the

pathogen and the plant, determining the

outcome of race-cultivar interaction (either

compatible or incompatible) and led him to

formulate the “gene-for-gene hypothesis”,

which explained in genetic terms the basis

of race-cultivar specificity (Flor, 1971).

In the early 60s, Klement and his associ-

ates discovered that phytopathogenic bac-

teria, like fungi, were able to induce a rap-

id plant cell death at the infection site, also

known as the plant hypersensitive response

(HR), which was associated with restriction of

pathogen proliferation (Klement and Good-

man, 1967). Against this background, several

parallel discoveries in the mid-1980s and the

decade that followed led to the unified con-

cept that bacterial virulence on plants relies

heavily on sophisticatedmolecular machines,

known as bacterial secretion systems or in-

jectisomes, which enable them to target host

defense systems at various levels. Key dis-

coveries included the cloning of phytobacte-

rial avirulence (

avr

) genes (Staskawicz

et al.

,

1984), the discovery of gene clusters includ-

ing the

hrp

genes (phonetic “Harp”) (Panop-

oulos

et al.

, 1984; Panopoulos and Peet, 1985;

Lindgren

et al.

, 1986), and the identification of

virulence genes in animal pathogens (Isberg

and Falkow, 1985). These were later shown

to share homologies with

hrp

genes and to

mediate secretion of virulence-related pro-

teins lacking a canonical signal peptide (rev.

in Tampakaki

et al.

, 2010). The boost in molec-

ular functional genetics of bacteria that fol-

lowed confirmed this concept and revealed

the injectisomes’ organization and function.

The latter is to help pathogens transfer (in-

ject) directly into the plant cytosol proteins

that affect immunity and are known as effec-

tor proteins. Today, the products of pathogen

avr

genes are collectively referred to as effec-

tors, since in the absence of cognate plant

immunity receptors they promote patho-

gen virulence by directly or indirectly inter-

fering with plant defense mechanisms (Ritter

and Dangl, 1996, Grant

et al.

, 2006; Jones and

Dangl, 2006; Guo

et al.

, 2009). These findings

helped explain the enigma why phytopatho-

gens carry avirulence genes (Dangl, 1994; Ga-

briel, 1999)

.

Plant immunity consists of different lay-

ers of defense, including a series of immune

responses that are triggered post-infection

by a variety of elicitors. One group of elicitors

that are conserved among different bacteri-

al pathogens includes the so-called patho-

gen-associated molecular patterns (PAMPs),

which elicit PAMP-triggered immunity (PTI).

Another group includes effectors, which as

previously mentioned are often coded by

classical avirulence genes. Molecular recog-

nition of PAMPs by the host is mediated by

proteins that recognize molecular structures

conserved across a broad range of pathogen-

ic species and are known as pattern recogni-

tion receptors (PPRs). Recognition of effectors

on the other hand by the host is mediated by

resistance proteins that are plasma mem-

brane or intracellularly located.

It is now widely recognized that in their

interactions with eukaryotes, Gram-nega-

tive bacteria use a variety of molecular de-

vices, including extracellular appendages,

to deliver diverse proteins and other mole-

cules to the host cell interior (Charova

et al.

,

2012). With the rapid accumulation of bac-

terial genome sequences, our knowledge

of the complexity of bacterial protein secre-

tion systems has expanded and numerous

biochemical studies have revealed the ex-

istence of at least six major mechanisms of

protein secretion (Type I, II, III, IV, V and VI),

which are often highly conserved among the

Gram-negative bacteria species. This review

focuses on the type III and type VI protein se-

cretion systems (T3SS, T6SS) that mediate se-

cretion and/or translocation of bacterial vir-

ulence-related proteins inside the host cell

in a host contact dependent manner and

under specific

in vitro

conditions (Hayes

et al.

2010). Emphasis in this review is given on the

biotechnological applications that emerged

from the study of these systems, and range

from plant pathogen diagnostics to antibiot-

ic development. The review does not cover

the Agrobacterial type IV secretion system,

which has provided a basis for biotechnolog-