Biomedical Engineering Reference
In-Depth Information
2011). This is another milestone achieved recently in developing transgenic
plants by incorporation of fungal genes for conferring resistance against
phytopathogens. Similarly Oldach et al. (2001) combined three antifungal
genes, viz., the antifungal protein Ag-AFP from the fungus
Aspergillus
giganteus
, a barley class II chitinase and a barley type I RIP, all regulated
by the constitutive Ubiquitin1 promoter from maize and expressed in
transgenic wheat. In 17 wheat lines, stable integration and inheritance of
one of the three transgenes has been demonstrated over four generations.
The formation of powdery mildew (
Erysiphe graminis
f. sp.
tritici
) or leaf rust
(
Puccinia recondita
f. sp.
tritici
) colonies was signifi cantly reduced on leaves
from afp or chitinase II- but not from rip I-expressing wheat lines compared
with non-transgenic controls. The increased resistance of afp and chitinase
II lines was dependent on the dose of fungal spores used for inoculation.
Heterologous expression of the fungal
afp
gene and the barley
chitinase
II
gene in wheat demonstrated that colony formation and, thereby, spreading
of two important biotrophic fungal diseases is inhibited approximately 40
to 50% at an inoculum density of 80 to 100 spores per cm
2
.
AVIRULENCE GENES
Interactions of transgenic plants resistant to pathogen toxins with plant
pathogen are complex processes which initially depend on the interaction
between resistance/susceptibility genes present in the plant and
avirulence/virulence genes present in the pathogen. When the host plant
lacks the gene encoding resistance against a particular race of a pathogen
then the outcome of the host-pathogen interaction is a compatible
interaction. Once a compatible interaction has been established, the
disease symptoms are developed or biological damage is caused by the
pathogen according to its ability to invade the host plant. The inheritance
of the interacting major resistance genes in the host and the avirulence
genes in the pathogen has been well described in many gene-for-gene
systems. Avirulence genes from several phytopathogenic bacteria and
some fungi have been cloned and characterized, although their subcellular
location and mode of action has not been well characterized in most
cases. Conceptually, the gene product or the product(s) of the enzymatic
activity encoded by the avirulence genes should trigger the hypersensitive
response (HR) in host plants harbouring the corresponding resistance
gene. This has been shown to be correct in practice, at least for the
Avr9
gene of
Cladosporium fulvum
and tomato plants carrying the
Cf9
resistance
gene, the tobacco-mosaic-virus (TMV) coat protein and tobacco plants
carrying the
N'
resistance gene and the
avrD
gene of
Pseudomonas syringae
pv.
tomato
(Herrera-Estrella and Simpson 1995).
