Transcription factors control apple pathogen response

Abstract

The apple pathogen Venturia inaequalis can infect Malus x domestica, causing an economically damaging disease, scab. Knowledge of how the recognition of a potential pathogen by an R gene product is translated into an effective resistance response in apple is limited. One apple accession, carrying the Rvi5 R gene (Rvi5), is resistant to V. inaequalis. In contrast, the commercial variety Royal Gala is highly susceptible while other accessions, such as those carrying the R gene Rvi8 (Rvi8) are partly resistant to infection caused by V. inaequalis. Transcription factors (TFs) are a vital class of proteins which interact with DNA binding domains to manipulate gene transcription and, therefore, affect gene expression. Some TF families, such as WRKYs, MYB, bHLHs have important functions in plant immunity. One of the functions of TFs is triggering expression of pathogenesis-related (PR) genes in response to attempted infection. This thesis describes the generation of NGS data based on a V. inaequalis inoculation experiment on the resistant accession Rvi5. This shows the global gene expression during resistance including the expression of specific TFs which were dramatically activated by attempted infection. Based on these data and the construction of phylogenetic trees, candidates from the WRKY family of plant TFs were selected for further analysis including WRKY7, WRKY9, WRKY11, WRKY18, WRKY40, WRKY75a and WRKY75b. PR1a, PR2, PR4, PR5 and PR8 were selected as the PR gene candidates. qPCR was employed to validate the NGS results. Functional experiments of TFs and PR genes proved there was a direct link between specific combinations and that there was evidence for interaction between individual WRKY TFs. In contrast to the NGS and qPCR data from Rvi5, qPCR expression profiles of TFs and PR genes in either the partly resistant Rvi8 or the susceptible ‘Royal Gala’ showed a very different expression profile. Expression of the selected TF and PR gene candidates was not activated by infection of V.inaequalis. From this work, it appears that expression of these TFs is vital in apple immunity and that they activate PR genes during the resistance response.