aphid infestation. All results concerning gene expression are correlative
and further studies are needed to find out whether they
are related to the effects on aphid reproduction and behaviour.
As yet, no resistance factor towards R. padi has been identified in
barley. Five of the six genes that we studied here have before been
shown to be up-regulated by R. padi in barley after 48 h of aphid
infestation, and one of them (thionin) was constitutively expressed
only in a resistant genotype. The fact that they were up-regulated
suggests that they are part of the basal immune response to aphids,
but there is no further evidence for any biological effect of the gene
products against aphids. One of the up-regulated genes, BCI-4, was
earlier found up-regulated only in resistant genotypes and has
therefore been suggested to be a candidate as aphid resistance factor
(Delp et al., 2009), but this remains to be confirmed.