Analysis of organ-specific responses of Pinus sylvestris to shoot (Gremmeniella abietina) and root (Heterobasidion annosum) pathogens

Using root- and shoot-specific necrotrophic pathogens Heterobasidion annosum (Fr.) Bref. and Gremmeniella abietina (Lagerb.) Morelet we compared host gene expression in root tissues with aerial parts of conifers. Inoculations with the shoot pathogen (G. abietina) on its preferred host tissue (pine needles) led to a necrotic browning reaction but corresponding infection of the root with the same pathogen did not provoke any macroscopically visible host reaction. Interestingly, both shoot and root tissues infected with the root pathogen H. annosum responded with strong necrosis. Host responses were further characterized using mRNA profiling technology for identification of differentially expressed genes. A macroarray containing 384 individual pine cDNAs, representing a range of transcripts expressed during different stages of development, was examined. Hierarchical clustering of the differentially expressed genes from pine tissues challenged with H. annosum or G. abietina suggests that the responses were more organ-specific than pathogen-specific: transcript profile of roots infected with H. annosum was more similar to roots challenged with G. abietina than to the shoot response to H. annosum. The results of the macroarray analysis were further verified and confirmed by virtual northern blot analysis. Overall, many transcripts of defence related genes preferentially accumulated in the infected roots in comparison to the shoots. The differences in defence strategies employed by the different plant organs are discussed with reference to Arabidopsis model and crop plants.