An array of responses to insect feeding in Brassica

Plants have developed defence mechanisms to deal with attacks from herbivorous insects.
Transcriptional profiling after herbivore feeding reveals, at the molecular level, how plants respond
to this type of stress. Differences in transcriptional profiles often underlie phenotypic variation
among plants from the same as well as different, related species. Studying intra- and interspecific
plant variation on the molecular and the ecological level in an integrated way provides insight into plant
defence mechanisms. Intra- and interspecific variation in resistance or susceptibility to herbivores
has been widely studied through bioassays. However, few studies link this with a genome-wide
transcriptional analysis. Here we take such an approach to study the interaction between cultivated
as well as naturally occurring Brassica species and two specialist herbivores. Because Brassica full
genome microarrays are not available, 70-mer oligonucleotide microarrays based on the Arabidopsis
thaliana genome were used. We analyzed the transcriptional responses of white cabbage cultivars
(Brassica oleracea var. capitata) and the wild black mustard (Brassica nigra) after feeding by either
the caterpillar Pieris rapae or the aphid Brevicoryne brassicae.
We show that there is intraspecific variation among B. oleracea cultivars with respect to herbivore
performance of both P. rapae and B. brassicae. Relative performance of the latter herbivore on the
cultivars was similar in glasshouse and field experiments, suggesting aphid performance to be largely
independent of environmental conditions. The transcriptional responses after 24, 48, and 72 hours of
P. rapae feeding on two white cabbage cultivars that supported different insect performance showed
variation in timing and regulation of individual genes. The majority of P. rapae-induced genes in both
cultivars were jasmonate-dependent. In contrast to P. rapae-induced plant responses, B. brassicae
feeding resulted in the differential regulation of only a small number of genes in the two B. oleracea
cultivars that supported different insect performance. The genes that were regulated in response to
aphid infestation were highly cultivar-specific. We also observed interspecific variation in B. brassicae
performance as well as in transcriptional responses to feeding by P. rapae or B. brassicae when comparing
B. oleracea and B. nigra. Temporal patterns of expression of herbivore-responsive genes in the
Brassica species, together with targeted studies employing A. thaliana knock-out mutants revealed a
role for a trypsin-and-protease inhibitor in resistance against P. rapae as well as B. brassicae.
All transcriptomic experiments mentioned above as well as most microarray studies on
A. thaliana have been performed under carefully controlled environmental conditions in which
plants were exposed to a single herbivore. However, it was unclear whether the observed
intraspecific variation in transcriptional profiles and herbivore performance in the glasshouse sustain
in the field. Therefore, I analysed herbivore occurrence and distribution together with transcriptional
profiles of two B. oleracea cultivars in the field. Early in the season, no clear differences in herbivore
communities and transcriptional profiles were found. Conversely, later in the season
herbivore abundance, species richness, and biodiversity differed greatly between the cultivars. These
differences can, at least partly, be explained by differences in expression levels of particular genes.
In conclusion, the data in this thesis show that inter- and intraspecific variation among plants have a
strong impact on their interaction with herbivores both at the molecular and ecological level. This was
true under glasshouse as well as field conditions. This thesis forms the basis for further unraveling
direct defence mechanisms of white cabbage.