Biomedical Engineering Reference
In-Depth Information
Kamoun et al. (1999) used potato virus X (PVX), a pathogenic virus on
many solanaceous plants, as a gene expression vector to test whether the
avirulence genes from fungal pathogens could confer resistance to untreated
pathogens in a different host. They used
Avr9
and
inf1
from the fungal
pathogen
Cladosporium fulvum
and the oomycete pathogen
Phytophthora
infestans,
respectively, to see whether they could confer avirulence to PVX
on tobacco. The gene
Avr9
encodes the elicitor peptide AVR9 which is
race-specifi c (van den Ackerveken et al. 1992) and present in all
C. fulvum
races that are avirulent on Cf9 tomato genotypes. The elicitor peptide AVR9
induces HR in leaves of Cf9 tomato genotypes upon infection. Similarly,
the
inf1
elicitin gene of
P. infestans
encodes the species-specifi c elicitor
INF1 and injection of INF1 into tobacco also results in HR (Kamoun et al.
1997). Earlier the role of INF1 was confi rmed by Kamoun et al. (1998) by
engineering
P. infestans
strains defi cient in the production of INF1. They
observed that it induces disease lesions when inoculated onto
Nicotiana
benthamiana,
suggesting that INF1 elicitin is a species- specifi c avirulence
factor. Further, they examined whether the
Avr9
and
inf1
genes could confer
avirulence to PVX on tobacco (
Nicotiana tabacum
) and whether mutant
alleles of the genes encoding less active elicitors would be less potent
in inhibiting PVX spread. The PVX expression system was designed in
such a way that it allows transient expression of heterologous genes in
solanaceous plants, as has been shown previously for the expression of the
Avr9
gene of
C. fulvum
in Cf9 tomato genotypes (Hammond-Kosack et al.
1995; Kooman-Gersmann et al. 1997). Similar to
Cf-9
tomato genotypes
Cf-9
transgenic tobacco also responds with HR to AVR9 injection (Hammond-
Kosack et al. 1998). Tobacco plants with transformed
Cf-
9 upon inoculation
with PVX::
Avr9
produces only small necrotic lesions compared to tobacco
plants without the
Cf-9
transgene which are readily infected by PVX::
Avr9
and shows typical PVX infection through systemic development of mosaic
symptoms. In a similar fashion Kamoun et al. (1997, 1998) also constructed
a PVX::
inf1
derivative containing a chimeric gene encoding the mature
98-amino-acid INF1 elicitin of
P. infestans
and fused to the
PR-1a
signal
peptide for extracellular targeting in a manner similar to that described for
PVX::
Avr9
for testing the effi cacy of other protein elicitors on PVX infection
of tobacco (Kooman-Gersmann et al. 1997). Localized necrotic lesions were
observed on tobacco leaves after inoculation of PVX::
inf1
on tobacco leaves
and no systemic symptoms developed even in the later stage of infection.
Further studies showed that PVX::
Avr9
and PVX::
inf1
remained localized to
the inoculated leaves of Cf9 tobacco plants inoculated with PVX::
Avr9
and
tobacco plants inoculated with PVX::
inf1
,
respectively (Kooman-Gersmann
et al. 1997). The observed local lesions and the absence of systemic spread of
PVX derivatives encoding the two active elicitors on tobacco are reminiscent
of HR-mediated resistances induced in plants against viruses, such as the
