Genotypic variation in genome-wide transcription profiles induced by insect feeding: Brassica oleracea--Pieris rapae interactions

Abstract

Background: Transcriptional profiling after herbivore attack reveals, at the molecular level, how plants respond to this type of biotic stress. Comparing herbivore-induced transcriptional responses of plants with different phenotypes provides insight into plant defense mechanisms. Here, we compare the global gene expression patterns induced by Pieris rapae caterpillar attack in two white cabbage (Brassica oleracea var. capitata) cultivars. The two cultivars are shown to differ in their level of direct defense against caterpillar feeding. Because Brassica full genome microarrays are not yet available, 70-mer oligonucleotide microarrays based on the Arabidopsis thaliana genome were used for this non-model plant.

Results: The transcriptional responses of the two cultivars differed in timing as characterized by changes in their expression pattern after 24, 48 and 72 hours of caterpillar feeding. In addition, they also differed qualitatively. Surprisingly, of all genes induced at any time point, only one third was induced in both cultivars. Analyses of transcriptional responses after jasmonate treatment revealed that the difference in timing did not hold for the response to this phytohormone. Additionally, comparisons between Pieris rapae- and jasmonate-induced transcriptional responses showed that Pieris rapae induced more jasmonate-independent than jasmonate-dependent genes.

Conclusion: The present study clearly shows that global transcriptional responses in two cultivars of the same plant species in response to insect feeding can differ dramatically. Several of these differences involve genes that are known to have an impact on Pieris rapae performance and probably underlie different mechanisms of direct defense, present in the cultivars.

Figure 1

Performance of P. rapae larvae on two B. oleracea cultivars. A, Larval weight (mean + SE) after 6 days of feeding. B, Time to reach pupation (mean + SE). C, Pupal weight (mean + SE) just after pupation.

Figure 2

Gene expression changes in cultivars Rivera and Christmas Drumhead after P. rapae feeding. Number of expressed genes induced (closed symbols and solid line) and repressed (open symbols and dashed line) more than twofold and with P < 0.05 at the time points tested.

Figure 3

Comparison of gene induction over time after P. rapae feeding in cultivars Rivera and Christmas Drumhead. A, Venn diagram representing the distribution in Rivera of transcripts activated after 48 and 72 h of P. rapae challenge. B, Venn diagram representing the distribution in Christmas Drumhead of transcripts activated after 24, 48, and 72 h of P. rapae challenge. The numbers in the overlapping area indicate the shared number of genes in the comparisons and include genes with an average expression ratio ≥ 2-fold and a P value < 0.05 in both experiments. Numbers outside the overlapping area represent genes specifically induced at one time point.

Figure 4

Comparison of microarray and qRT-PCR analysis of five genes. Log₂ ratios of five selected genes (At3g45140, At1g72290, At4g31500, At1g47540, and At1g27130) after infestation of Rivera and Christmas Drumhead by P. rapae. On the left, the log₂ ratio patterns from the microarray analysis. On the right, the log₂ ratio patterns from the qRT-PCR analysis. Black, gray and white bars represent log₂ ratios after 24, 48, and 72 h of P. rapae feeding, respectively. All bars contain their corresponding standard deviation.

Figure 5

Gene expression in cultivars Rivera and Christmas Drumhead after P. rapae feeding. Venn diagrams representing the distribution of induced and repressed genes after 48 and 72 h of P. rapae feeding. The numbers in the overlapping areas indicate the shared number of genes in the comparisons and include genes with an average expression ratio ≥ 2-fold or ≤ 0.5-fold and a P value < 0.05 in both experiments. Numbers outside the overlapping area represent genes specifically induced or repressed in one cultivar.

Figure 6

Gene induction in cultivars Rivera and Christmas Drumhead after P. rapae feeding. Venn diagram representing the distribution of induced genes when combining all time points tested. The number in the overlapping area indicate the shared number of genes in the comparisons and include genes with an average expression ratio ≥ 2-fold and a P value < 0.05 in both experiments. Numbers outside the overlapping area represent genes specifically induced in one cultivar.