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Abstract
Downy mildew (DM), caused by obligate parasitic oomycetes, is a destructive disease for a wide range of crops worldwide. Recent outbreaks of impatiens downy mildew (IDM) in many countries have caused huge economic losses. A system to reveal plant–pathogen interactions in the early stage of infection and quickly assess resistance/susceptibility of plants to DM is desired. In this study, we established an early and rapid system to achieve these goals using impatiens as a model. Thirty-two cultivars of Impatiens walleriana and I. hawkeri were evaluated for their responses to IDM at cotyledon, first/second pair of true leaf, and mature plant stages. All I. walleriana cultivars were highly susceptible to IDM. While all I. hawkeri cultivars were resistant to IDM starting at the first true leaf stage, many (14/16) were susceptible to IDM at the cotyledon stage. Two cultivars showed resistance even at the cotyledon stage. Histological characterization showed that the resistance mechanism of the I. hawkeri cultivars resembles that in grapevine and type II resistance in sunflower. By integrating full-length transcriptome sequencing (Iso-Seq) and RNA-Seq, we constructed the first reference transcriptome for Impatiens comprised of 48,758 sequences with an N50 length of 2060 bp. 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 DM resistance. Our approach of integrating early disease-resistance phenotyping, histological characterization, and transcriptome analysis lay a solid foundation to improve DM resistance in impatiens and may provide a model for other crops.
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1 Gulf Coast Research and Education Center, University of Florida, IFAS, Department of Environmental Horticulture, Wimauma, USA (GRID:grid.15276.37) (ISNI:0000 0004 1936 8091); South China Agricultural University, State Key Laboratory for Conservation and Utilization of Subtropical Agro-Bioresources, Guangzhou, China (GRID:grid.20561.30) (ISNI:0000 0000 9546 5767)
2 Gulf Coast Research and Education Center, Visiting scholar at University of Florida, IFAS, Department of Environmental Horticulture, Wimauma, USA (GRID:grid.15276.37) (ISNI:0000 0004 1936 8091); Huazhong Agricultural University, Key Laboratory of Horticultural Plant Biology, Ministry of Education, College of Horticulture and Forestry Sciences, Wuhan, China (GRID:grid.35155.37) (ISNI:0000 0004 1790 4137)
3 Gulf Coast Research and Education Center, University of Florida, IFAS, Department of Environmental Horticulture, Wimauma, USA (GRID:grid.15276.37) (ISNI:0000 0004 1936 8091)
4 Tropical Research and Education Center, University of Florida, IFAS, Department of Plant Pathology, Homestead, USA (GRID:grid.15276.37) (ISNI:0000 0004 1936 8091); Bayer Environmental Science US, 5000 Centregreen Way, Cary, USA (GRID:grid.15276.37)