Page 7 - Volume 5, Issue 2, July 2012

© Benaki Phytopathological Institute
Phytobacterial type III secretion systems in the era of biotechnology 35
surface, prevent stomata closure and inhib-
it enzymatic degradation of bacterial pep-
tidoglycans in the leaf apoplast. Moreover,
they mask elicitor recognition and block sig-
naling receptors. They are able to down-
regulate, suppress and/or degrade defense
mechanisms such as callose deposition and
papillae formation, kinase phosphoryla-
tion and dephosphorylation, resistance pro-
tein activation, and block the hypersensi-
tive response triggered by other effectors
(reviewed in Jones and Dangl, 2006; Göhre
and Robatzek, 2008). Moreover, they mod-
ulate plant transcription, proteosomal deg-
radation machinery and hormone-signaling
networks (involved in defense signaling) by
down regulation of microRNAs in the RNA
silencing mechanism (Navarro et al., 2008;
Zhang et al., 2011). The plant defense-sup-
pression functions of P. syringae effectors
are summarized in Table 1.
Large scale genome sequencing and
subsequent effector prediction, answered
the riddle of effector redundancy and re-
vealed the rich effector repertoire of dif-
ferent pathogens: Studies by Sharkar et al.
(2006) on host specificity showed that low
conservation of T3SS effector repertoires
among different bacterial pathovars and
species underlies their differences on host
specificity, while high conservation of T3SS
effectors can explain pathogenicity and a
broad host range. Further on, acquisition of
new T3SS effectors by HGT or other genet-
ic mechanisms can widen a pathogen’s host
range (Jones and Dangl, 2006). It is there-
fore evident that the effector repertoire of
a pathogen largely determines its infection
strategy as well as its host range.
Biotechnological applications
Mining for effectors reveals new poten-
tial tools in plant breeding
Following Flor’s hypothesis and using
Klement’s technique (see introduction), dif-
ferent panels of scientists determined that
plant resistance was associated with HR elic-
itation to specific pathogen races. This led
to the characterization of many avirulence
(effector) genes and their matching plant re-
sistance (R) genes, which in some cases en-
abled the full characterization of resistance
due to avr-R interacting gene pairs in sever-
al pathosystems at the race/cultivar or spe-
cies level. One of the first resistance genes
to be cloned was Bs1 from pepper accession
PI163192, which interacted with the protein
effector encoded by the avr gene avrBs1
from X. campestris pv. vesicatoria (Minsav-
age et al., 1990). Backcross programs were
initiated to transfer the resistance gene into
commercially valuable cultivars (Early Cal-
wonder). Similar efforts will help the devel-
opment and/or identification of several cul-
tivars of pepper (Bs1-4) and tomato (rx1-3,
Xv3, Xv4) with resistance towards bacteri-
al spots caused by Xanthomonas spp. (re-
viewed in Stall et al., 2009). Indeed, several
pepper varieties resistant to three or more
of the six bacterial leaf spot races are now
commercially available (Sweet Bell, Sweet
Italian, Hot peppers etc.). Also in the ear-
ly nineties, Martin et al. (1991) isolated the
Pto kinase R gene, which confers resistance
to bacterial speck disease in tomato by rec-
ognition of the corresponding avrPto aviru-
lence gene, in the pathogen P. syringae pv.
tomato. Similarly, breeding efforts with rice
R genes such as Xa1-26 that confer resistance
to bacterial blast caused by Xanthomonas
oryzae pathovars, are now implemented in
control measures along with cultural prac-
tices, chemical and biological control, and
disease forecasting (Niño-Liu et al. , 2006).
Several other efforts have been reported on
different pathosystems. Genome sequenc-
ing programs and protein motif comparison
and function prediction analysis facilitate
similar efforts and will provide breeders and
geneticists with a significant number of re-
sistance genes in the near future.
TAL Effectors: Novel tools for gene tar-
geting and genome engineering
Transcription activator–like effectors
(TALEs) are T3SS-secreted avirulence proteins
found in Xanthomonas. These proteins can
bind promoter sequences in the host plant