Agriculture Reference
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disease, resulting in further protein
misfolding. HLB strongly affected
pathways involved in source-sink com-
munications such as sucrose and starch
metabolism and hormone biosynthesis and
signalling. Transcription of several genes
for synthesis and signal transduction of
cytokinins and gibberellins was down-
regulated, but ethylene pathways were
induced. CaLas infection seemed to cause
an induction of salicylic acid and jasmonic
acid pathways and to enhance the
transcript levels of several members of the
WRKY family of transcription factors. A
picture of the main changes in gene
regulatory networks in response to HLB in
the fruit was constructed (see Plate 12).
This study identifi ed several genes
differentially expressed before symptoms
appear that could help disease detection
at the primary stages of infection.
Obviously, it will be important to
determine that these genes are not also
induced by
Citrus tristeza
virus,
Xylella
fastidiosa
or
Xanthomonas axonopodis
infections, other diseases of citrus. In fruit
peel, HLB induced altered levels of
transcript abundance in hormone and
isoprenoid pathways and in sucrose and
starch metabolism. WRKY transcription
factors seemed to regulate the defence
responses to CaLas in the fruit. Treatments
with small-molecule hormones could
represent a short-term strategy to reduce
the enormous negative effect of this
disease.
while domesticated European and Asian
pears are derived from different species.
Rosaceae species have been studied
using molecular-assisted breeding, genetic
engineering, and functional and structural
genomics.
18.10.1 RNA interference (RNAi)
Computational analysis of ESTs in public
databases identifi ed seven conserved
plant microRNA (miRNA) families and
structures of precursor miRNAs (Schaffer
et al.
, 2007). Ten distinct sequences were
classifi ed into seven conserved plant
miRNA families (Gleave
et al.
, 2008). A
candidate gene approach has been linked
with RNA RNAi silencing to elucidate the
role of some genes in resistance to bacterial
fi re blight (Norelli
et al.
, 2007).
Bioinformatics identifi ed ESTs either
specifi cally linked to fi re blight or to
Pseudomonas syringae
pv.
tomato
in-
fection. Genetic engineering was employed
to upregulate a single EST-silencing gene
and select apple RNAi mutants. Additional
candidate ESTs are currently being
identifi ed using different biotechnological
techniques (suppression subtractive
hybridization and cDNA-amplifi ed frag-
ment length polymorphism analyses).
Neither a mutant phenotype nor a gene
sequence by itself explains the molecular
function of a gene. Therefore, modern
functional genomics consists of high-
throughput methods of different 'omics'
technologies followed by bioinformatics
analysis for detailed functional genomics,
while genetic engineering provides the
means to validate gene function
in vivo
.
18.10 Functional Genomics for
Qualitative Improvement of Rosaceous
Crops
The Rosaceae family contains some 3000
species in more than 100 genera and
includes economically important crops
grown in temperate environment (Dirle-
wanger
et al.
, 2002). The Rosaceous tree
genera, including
Malus
,
Pyrus
and
Prunus
, derive mainly from an ancient
Malus
progenitor that gave rise to the
cultivated apple (
Malus
×
domestica
),
18.10.2 Functional genomics approaches
Apple is a food crop and a source of pectin,
used to thicken jams and laboratory culture
media. The fruits are processed into
sauces, slices, sweets, alcoholic beverages,
vinegar and juice. A large collection of
apple ESTs has been used to produce
microarrays for gene expression analyses
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