Identification of the BrRHP1 locus that confers resistance to downy mildew in Chinese cabbage (Brassica rapa ssp. pekinensis) and development of linked molecular markers.

Inheritance of resistance to downy mildew (Hyaloperonospora parasitica) in Chinese cabbage (Brassica rapa ssp. pekinensis) was studied using inbred parental lines RS1 and SS1 that display strong resistance and severe susceptibility, respectively. F(1), F(2), and BC(1)F(1) populations were evaluated for their responses to downy mildew infection. Resistance to downy mildew was conditioned by a single dominant locus designated BrRHP1. A random amplified polymorphic DNA (RAPD) marker linked to BrRHP1 was identified using bulked segregant analysis and two molecular markers designated BrPERK15A and BrPERK15B were developed. BrPERK15B was polymorphic between the parental lines used to construct the reference linkage map of B. rapa, allowing the mapping of the BrRHP1 locus to the A1 linkage group. Using bacterial artificial chromosome clone sequences anchored to the A1 linkage group, six simple polymerase chain reaction (PCR) markers were developed for use in marker-assisted breeding of downy mildew resistance in Chinese cabbage. Four simple PCR markers flanking the BrRHP1 locus were shown to be collinear with the long-arm region of Arabidopsis chromosome 3. The two closely linked flanking markers delimit the BrRHP1 locus within a 2.2-Mb interval of this Arabidopsis syntenic region.