Bigleaf hydrangea (Hydrangea macrophylla) is an economically important ornamental shrub produced worldwide for the floral trade, as a container crop, and as a landscape plant. Powdery mildew (PM), caused by Golovinomyces orontii, is a widespread disease of bigleaf hydrangea impacting production and salability of plants. However, mechanisms of resistance to PM of bigleaf hydrangea are still largely unexplored. The purpose of this study was to investigate whole-plant response to PM infection and identify differentially expressed genes (DEGs) that contribute to the PM disease response in bigleaf hydrangea. Mature plants of four cultivars (‘Blushing Bride’, ‘Endless Summer’, ‘Nigra’, and ‘Veitchii’) were chosen based on their variable responses to natural PM infection. Powdery mildew was collected by harvesting naturally infected leaves from field plants and applied via spray inoculation averaging ~20ml per plant, with inoculum rate being 1 x 104 CFUmL-1, to six replicate plants per cultivar; one plant per cultivar was sprayed with water as a control. Whole plant response (% of plant tissue infected) was measured visually on a scale of 0-100% disease severity weekly starting from 29 Nov 2023 to 14 Feb 2024 and used to calculate Area Under the Disease Progress Curve (AUDPC). Plant tissue was sampled at 12 different time points, from 1 hour after inoculation (HAI) to 5 days after inoculation (DAI) using a leaf disc puncher and immediately flash frozen in liquid nitrogen. RNA was extracted using a Qiagen RNeasy Plant Mini kit and sequenced using NovaSeq. Adapters were removed from raw reads using fastp (0.23.4) and trimmed reads were aligned to the ‘Endless Summer’ reference genome using STAR (2.7.11b). STAR bam files were sorted with samtools (1.21) and featurecounts (2.0.6) was used for the gene model counting. DESeq was used to identify DEGs between ‘Veitchii’ and ‘Nigra’. There were significant differences among cultivars for AUDPC, with disease severity ranging between 7.7 and 19.2%. Bigleaf hydrangea ‘Nigra’ and ‘Endless Summer’ were the most susceptible to PM infection and ‘Veitchii’ the most tolerant. There were 11,629 DEGs total with 6,145 upregulated compared to ‘Veitchii’ and 5,484 downregulated compared to ‘Nigra’. DEGs were sorted by their P-adjust value followed by the Log 2-fold change. Many of the top 25 strongest DEGs include genes for plant stress such as serine threonine-protein kinase, PAN_AP, and leucine-rich repeat family proteins. These genes are currently being tested for expression levels among bigleaf hydrangea cultivars.
