Histological and transcriptomic characterization of a partial resistance in the rose-Diplocarpon rosae interaction

Black spot, caused by the fungus Diplocarpon rosae, is the most important foliar disease in cultivated garden roses. With the reduction and in some cases the prohibition of agrochemical use (EU policy), the development of resistant roses is essential. So far, research has mainly focused on the study of major genes, but resistances based on these major genes are known to be less durable. Partial resistance to black spot disease was described in Rosa wichurana genotypes. We had previously shown that R. wichurana partial resistance is mainly controlled by two quantitative trait loci located on linkage groups B3 and B5. To dissect the mechanisms underlying such resistance, two complementary approaches were taken. Artificial inoculation on whole plants in the greenhouse allowed us to undertake a detailed study of R. wichurana resistance at macroscopic and microscopic levels as well as to investigate the importance of leaf maturity for partial resistance to the black spot disease. Besides, we proceeded to a comparative study of the transcriptomic changes that happen during the infection process using a time course approach on whole plants under greenhouse conditions. The R. wichurana defense response was characterized as follows: first, a quick PAMP-triggered immune response (PTI) leads to the activation of genes involved in callose deposition in papillae, and then an effector specific response (ETI) results in a hypersensitive response (HR). These results pave the way for marker-assisted selection and for gene cloning.