Molecular, phenotypic and histological analysis reveals a multi-tiered immune response and callose deposition in stone fruit rootstocks (Prunus spp.) against Pseudomonas syringae pv. syringae (Pss) infection

•A novel leaf disk assay discriminates between resistant and sensible stone fruit rootstocks to Pss infection.•A multi-tiered immune response is activated in Prunus rootstocks by Pss infection.•The mechanical wounding promotes the differential accumulation of callose in Prunus rootstocks. Bacterial canker, caused by Pseudomonas syringae pv syringae (Pss), is a serious disease of stone fruit orchards causing severe yield reductions and the death of entire trees. To analyze the response and to identify novel sources of Pss resistance in our rootstock breeding program, we used a novel, rapid and reliable leaf disk pathogenicity test to screen pre-breeding material constituted by interspecific hybrids of plum/peach/almond (‘AGAF’/‘ADAGAF’) or sweet cherry (‘PACIR’) species and commercial rootstocks with two Pss strains. Some ‘AGAF’/ ‘ADAGAF’ and all ‘PACIR’ rootstocks showed lower susceptibility than commercial rootstocks to Pss strains used. The reduced symptoms displayed by the ‘PACIR’ hybrids with Prunus lannesiana and P. incisa as genomic background suggest that these species could be novel sources of genetic resistance to Pss within the Cerasus subgenus. Histological and candidate gene expression analysis suggest a multi-tiered immune response of Prunus species to Pss attack with a higher callose deposition and activation of molecular mechanisms that would restrict the bacteria infection and help to maintain a better redox homeostasis and performance of ‘Marianna 2624′, a plum resistant genotype. Identification of Prunus species with resistance to Pss and the analysis of their immune response are the first step towards characterizing the underlying genetic mechanisms of resistance and introducing these traits into commercial germplasm.