Construction of a Spinach Bacterial Artificial Chromosome (BAC) Library as a Resource for Gene Identification and Marker Development

Abstract

Spinach (Spinacae oleracae) is a nutritious vegetable and an economically important crop worldwide. Spinach is threatened by a number of diseases, and the most significant of which is downy mildew, caused by Peronospora farinosa f. sp. spinaciae (Pfs). The spinach locus RPF1 confers resistance to 11 of the 15 known races of Pfs. To study the RPF1 locus in depth, a BAC library was constructed from DNA of a single spinach plant that was homozygous at the RPF1 locus. The BAC library contained 73,728 clones, with an average insert size of 183 kb, which represented approximately 13 spinach genome equivalents (989 Mb). The ends of 1920 BAC clones were sequenced from both directions, and 3405 BAC end sequences (BESs) ranging from 100 to 935 bp were obtained. About 38.59 % of the BESs contained repetitive sequences, including 363 copies of retrotransposable elements and 32 copies of transposable elements. Less than 3 % of the clones originated from chloroplasts or mitochondria. Various categories of protein-encoding genes, including transcription factors, stress responsive genes, and reproduction-associated genes, were identified. Putative disease resistance genes containing nucleotide binding site-leucine-rich repeat (NBS-LRR) domains were also identified, one of which was closely linked to the RPF1 locus. Simple sequence repeat (SSR) motifs were identified from 154 BESs, including various di-, tri-, tetra-, and penta-nucleotide repeats. One hundred pairs of SSR primers and 24 retrotransposon primers were designed from BESs, all of which successfully amplified spinach DNA. This spinach BAC library will provide a useful resource for spinach genetic studies and breeding efforts.