Temporal and spatial profiles of chitinase expression by norway spruce in response to bark colonization by Heterobasidion annosum

Abstract

Pathogen colonization and transcript levels of three host chitinases, putatively representing classes I, II, and IV, were monitored with real-time PCR after wounding and bark infection by Heterobasidion annosum in 32-year-old trees of Norway spruce (Picea abies) with low (clone 409) or high (clone 589) resistance to this pathogen. Three days after inoculation, comparable colonization levels were observed in both clones in the area immediately adjacent to inoculation. At 14 days after infection, pathogen colonization was restricted to the area immediately adjacent to the site of inoculation for clone 589 but had progressed further into the host tissue in clone 409. Transcript levels of the class II and IV chitinases increased after wounding or inoculation, but the transcript level of the class I chitinase declined after these treatments. Transcript levels of the class II and class IV chitinases were higher in areas immediately adjacent to the inoculation site in clone 589 than in similar sites in clone 409 3 days after inoculation. This difference was even more pronounced 2 to 6 mm away from the inoculation point, where no infection was yet established, and suggests that the clones differ in the rate of chitinase-related signal perception or transduction. At 14 days after inoculation, these transcript levels were higher in clone 409 than in clone 589, suggesting that the massive upregulation of class II and IV chitinases after the establishment of infection comes too late to reduce or prevent pathogen colonization.

FIG. 1.

Relative transcript levels of PaChi4 and pathogen/host nuclear ratios in the sampled sections of inoculated or wounded 409 ramets (left panels) and 589 ramets (right panels) on days 3 (A and B), 7 (C and D), and 14 (E and F) after treatment. The transcript levels in inoculated (P) and wounded (W) samples are indicated on the left y axis, and the pathogen/host nuclear ratios (+) are indicated on the right y axis. The basal transcript levels in the control tissues at the times of inoculation (— — —) and wounding (- - -) are included. The basal transcript level in the 409A ramet at the time of inoculation was used as a reference for all calculations and is referred to with the value 1. Each datum point represents the mean of two ramets. The error bars (for transcript levels [—] and pathogen/host nuclear ratios [+]) indicate ± 1 standard error derived from the error mean square in an analysis of variance.

FIG. 2.

Relative transcript levels of PaChi1 and pathogen/host nuclear ratios in the sampled sections of inoculated or wounded 409 (A) and 589 (B) ramets on day 3 after treatment. For figure explanation, see the legend for Fig. 1.