Molecular insights into the origin of the brown rust resistance gene Bru 1 among Saccharum species

Key message Analysis of 387 sugarcane clones using Bru 1 diagnostic markers revealed two possible sources of Bru 1 in Chinese cultivars: one from Saccharum spontaneum and another from Saccharum robustum of New Guinea. Abstract Sugarcane brown rust (SBR) is an important fungal disease in many sugarcane production areas around the world, and can cause considerable yield losses in susceptible sugarcane cultivars. One major SBR resistance gene, named Bru 1, initially identified from cultivar R570, was shown to be a major SBR resistance source in most of the sugarcane producing areas of the world. In this study, by using the two Bru 1-associated markers, R12H16 and 9O20-F4, we surveyed the presence of Bru 1 in a Chinese sugarcane germplasm collection of 387 clones, consisting of 228 hybrid cultivars bred by different Chinese sugarcane breeding establishments, 54 exotic hybrid cultivars introduced from other countries and 105 clones of sugarcane ancestral species. The Bru 1-bearing haplotype was detected in 43.4% of Chinese sugarcane cultivars, 20.4% of exotic hybrid cultivars, and only 3.8% of ancestral species. Among the 33 Chinese cultivars for which phenotypes of resistance to SBR were available, Bru 1 was present in 69.2% (18/26) of the resistant clones. Analyses of the allelic sequence variations of R12H16 and 9O20-F4 suggested two possible sources of Bru 1 in Chinese cultivars: one from S. spontaneum and another from S. robustum of New Guinea. In addition, we developed an improved Bru 1 diagnostic marker, 9O20-F4- Hae III, which can eliminate all the false results of 9O20-F4- Rsa I observed among S. spontaneum, as well as a new dominant Bru 1 diagnostic marker, R12E03-2, from the BAC ShCIR12E03. Our results provide valuable information for further efforts of breeding SBR-resistant varieties, searching new SBR resistance sources and cloning of Bru 1 in sugarcane.