Chrysanthemum WRKY15-1 promotes resistance to Puccinia horiana Henn. via the salicylic acid signaling pathway

Chrysanthemum white rust disease, which is caused by the fungus Puccinia horiana Henn., severely reduces the ornamental quality and yield chrysanthemum. WRKY transcription factors function in the disease-resistance response in a variety of plants; however, it is unclear whether members of this family improve resistance to white rust disease in chrysanthemum. In this study, using PCR, we isolated a WRKY15 homologous gene, CmWRKY15-1, from the resistant chrysanthemum cultivar C029. Real-time quantitative PCR (RT-qPCR) revealed that CmWRKY15-1 exhibited differential expression patterns between the immune cultivar C029 and the susceptible cultivar Jinba upon P. horiana infection. In addition, salicylic acid (SA) treatment strongly induced CmWRKY15-1 expression. Overexpression of CmWRKY15-1 in the chrysanthemum-susceptible cultivar Jinba increased tolerance to P. horiana infection. Conversely, silencing CmWRKY15-1 via RNA interference (RNAi) in C029 increased sensitivity to P. horiana infection. We also determined that P. horiana infection increased both the endogenous SA content and the expression of salicylic acid biosynthesis genes in CmWRKY15-1-overexpressing plants, whereas CmWRKY15-1 RNAi plants exhibited the opposite effects under the same conditions. Finally, the transcript levels of pathogenesis-related (PR) genes involved in the SA pathway were positively associated with CmWRKY15-1 expression levels. Our results demonstrated that CmWRKY15-1 plays an important role in the resistance of chrysanthemum to P. horiana by influencing SA signaling.

USE OF INTERSPECIFIC HYBRIDIZATION IN THE BREEDING OF ADAPTIVE HYBRIDS AND SORTS OF GARDEN CHRYSANTHEMUM (Chrysanthemum morifolium Ramat.)

Many garden chrysanthemums bred across the world are not fully winter hardy. Many are damaged by fungal diseases due to a high humidity and are late flowering. This makes them unsuitable for general commercial marketing. Since 2000 we have been conducting a breeding program using natural species of the genus Chrysanthemum that are adapted to the local conditions. The strategy of breeding adaptive hybrids and varieties of chrysanthemum native to Russia was proposed based on their biological, genetic peculiarities and natural resources of Chrysanthemum with the use of interspecific hybridization. Research objects are the first generations of interspecific hyb rids of F1obtained previously by the author as a result of the hybridization of varieties and wild species of Chrysanthemum. Derived from different species and varieties, F1hybrids were crossed among themselves to obtain the multicomponent F2progeny. F2seedlings with winter hardiness, resistance to Puccinia horiana Henn. and early flowering were used in closely related crosses. The offspring of F3from closely related crosses were also assessed and selected according to adaptive and decorative characteristics. Inclusion in the selection process of various sources of winter hardiness and resistance to P. horiana allowed positive characteristics to be increased in F2and to be revealed in F3. Adaptive signs of the wild species Chrysanthemum naktongense Nakai, C. coreanum (H. Lév. et Vaniot) Nakai, C. zawadzkii var. tenuisectum Kitag., C. leiophyllum Nakai, and C. zawadzkii subsp. acutilobum (DC.) Kitag., which have formed and fixed during evolution, were inherited and manifested in offspring of the multicomponent hybrids and the closely related crosses. Promising interspecific forms with biological signs determining the possibility of growing in extreme conditions of the subregion were selected.