ABSTRACT
Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, is one of the most serious soybean (Glycine max) diseases in tropical and subtropical regions. To facilitate the development of resistant varieties using gene pyramiding, DNA markers closely linked to seven resistance genes, namely, Rpp1, Rpp1-b, Rpp2, Rpp3, Rpp4, Rpp5, and Rpp6, were identified. Linkage analysis of resistance-related traits and marker genotypes using 13 segregating populations of ASR resistance, including eight previously published by our group and five newly developed populations, identified the resistance loci with markers at intervals of less than 2.0 cM for all seven resistance genes. Inoculation was conducted of the same population with two P. pachyrhizi isolates of different virulence, and two resistant varieties, 'Kinoshita' and 'Shiranui,' previously thought to only harbor Rpp5, was found to also harbor Rpp3. Markers closely linked to the resistance loci identified in this study will be used for ASR-resistance breeding and the identification of the genes responsible for resistance.
ABSTRACT
Asian soybean rust caused by the fungal pathogen Phakopsora pachyrhizi is the most devastating disease of soybean. The host cultivar specificity of the pathogen shows considerable differentiation depending on the area and season of its emergence. Although resistance genes for P. pachyrhizi (Rpp) have been reported in several soybean varieties, the genetic background of these varieties is highly differentiated. Furthermore, some of the varieties harbor unknown genetic factors in addition to Rpp that could influence resistance reactions against the pathogen. In order to gain a comprehensive understanding of Rpp-P. pachyrhizi interactions, homogenous plant material harboring Rpp genes is necessary. In this study, we bred Rpp-near isogenic lines (Rpp-NILs), which retained identical plant characters originating from a single genetic background, and accordingly showed low-variant compatible/incompatible reactions against the pathogen. These Rpp-NILs can be used as genetic resources for studying P. pachyrhizi epidemiology and elucidating resistance mechanisms. Compatible/incompatible relationships between the soybean rust resistance gene Rpp and isolates of the pathogen P. pachyrhizi are clearly distinguishable using the Rpp-NILs bred in this study.
