S05 - Session O2 - Understanding impatiens susceptibility and resistance to downy mildew caused by Plasmopara destructor

Authors: Zhanao Deng *, Ze Peng, Yanhong He, Saroj Parajuli, Qian You, Weining Wang, Krishna Bhattarai, Aaron Palmateer

Impatiens are popular annual bedding plants in many countries. In recent years, impatiens downy mildew (IDM), incited by Plasmopara destructor , has become a very important disease of impatiens; it has caused multiple outbreaks in Europe, Australia, and North America, resulted in huge significant economic losses to the floriculture industry. Improving impatiens resistance to IDM has become a top priority in impatiens breeding. Impatiens walleriana and Impatiens hawkeri are the two most commonly cultivated impatiens species, and they have different responses to IDM, with the former being susceptible and the latter resistant, respectively. To understand the differences, plants of I . walleriana and I . hawkeri were inoculated with P . destructor spores at three stages (cotyledon, 1 st /2 nd pair of true leaf, and mature plant), observed for pathogen development and sporulation, and sampled for transcriptome analysis. Most I . hawkeri cultivars were susceptible to IDM at the cotyledon stage, became resistant at the 1 st true leaf stage, and remained resistant thereafter. Two I . hawkeri cultivars ('Divine Lavender' and 'Florific Lavender') showed strong resistance to IDM even at the cotyledon stage. These cultivars can be good sources of IDM resistance for future impatiens breeding. Pathogen hyphae growth, haustoria development, and sporulation were suppressed in IDM-resistant cotyledons and true leaves. A leaf transcriptome was assembled using long and short cDNA reads from three cultivars and two tissue types with contrasting IDM responses. The impatiens transcriptome comprises of 48,758 reference transcript sequences. Forty-five NBS-containing genes and 246 LRR-RLK genes were expressed in the sampled impatiens leaves. Comparative transcriptome and qRT-PCR analyses revealed strong candidate genes for IDM resistance, including three resistance genes orthologous to the sunflower gene RGC203 , a potential candidate associated with downy mildew resistance. These results may facilitate further research to understand and improve IDM resistance in impatiens.