Abstract
Bacterial leaf pustule (BLP), caused by Xanthornonas axonopodis pv. glycines (Xag), is a worldwide disease of soybean, particularly in warm and humid regions. To date, little is known about the underlying molecular mechanisms of BLP resistance. The only single recessive resistance gene rxp has not been functionally identified yet, even though the genotypes carrying the gene have been widely used for BLP resistance breeding. Using a linkage mapping in a recombinant inbred line (RIL) population against the Xag strain Chinese C5, we identified that quantitative trait locus (QTL) qrxp–17–2 accounted for 74.33% of the total phenotypic variations. We also identified two minor QTLs, qrxp–05–1 and qrxp–17–1, that accounted for 7.26% and 22.26% of the total phenotypic variations, respectively, for the first time. Using a genome-wide association study (GWAS) in 476 cultivars of a soybean breeding germplasm population, we identified a total of 38 quantitative trait nucleotides (QTNs) on chromosomes (Chr) 5, 7, 8, 9,15, 17, 19, and 20 under artificial infection with C5, and 34 QTNs on Chr 4, 5, 6, 9, 13, 16, 17, 18, and 20 under natural morbidity condition. Taken together, three QTLs and 11 stable QTNs were detected in both linkage mapping and GWAS analysis, and located in three genomic regions with the major genomic region containing qrxp_17_2. Real-time RT-PCR analysis of the relative expression levels of five potential candidate genes in the resistant soybean cultivar W82 following Xag treatment showed that of Glyma.17G086300, which is located in qrxp–17–2, significantly increased in W82 at 24 and 72 h post-inoculation (hpi) when compared to that in the susceptible cultivar Jack. These results indicate that Glyma.17G086300 is a potential candidate gene for rxp and the QTLs and QTNs identified in this study will be useful for marker development for the breeding of Xag-resistant soybean cultivars.
Highlights
Bacterial leaf pustule (BLP), caused by the causal agent Xanthomonas axonopodis pv.glycines (Xag), is one of the most destructive foliar diseases in susceptible soybean cultivars.BLP causes a significant reduction in soybean yield and quality worldwide, especially in the soybean production areas in China [1], Korea [2], Thailand [3,4], the USA [5,6], and Benin [7], where seasonal high temperature and humidity favor Xanthornonas axonopodis pv. glycines (Xag) growth and development
BLP resistance was assessed in the soybean recombinant inbred line (RIL) population under the artificial inoculation condition and in the soybean association panel under both the artificial inoculation and natural morbidity conditions
Only the soybean genotypes containing the unidentified recessive resistance gene rxp have been used in BLP resistance breeding, even though soybean BLP resistance is a quantitative trait that is regulated by multiple genes and affected by the interaction between genotypes and environment in the field
Summary
Bacterial leaf pustule (BLP), caused by the causal agent Xanthomonas axonopodis pv. glycines (Xag), is one of the most destructive foliar diseases in susceptible soybean cultivars. The tropical storm ‘Rumbia’ caused an outbreak of BLP disease in the soybean growing area of Jianghuai River Basin in China in August 2018. An RNA-Seq analysis identified the differentially expressed genes (DEGs) between resistant and susceptible soybean varieties under the infection of Xag strain 8ra at 0, 6, and 12 h post-inoculation (hpi) [18]. Several minor QTLs conferring resistance to various Xag isolates were identified on Chr 18, 05, 04, 13, 19, and 10 in the recombinant inbred lines (RILs) derived from a cross between susceptible parent ‘Suwonl57’ and resistance parent ‘Danbaekkong’ using 76 SSR markers [14]. (GmLob; Glyma.05g040500) was predicted to be targeted by the virulence factor (tal2b) of Xag as its homolog CsLOB1 in Citrus sinensis has been reported as a resistance gene to citrus canker disease caused by X. citri subsp. Jack following Xag treatments showed that Glyma.17G086300, which is located in qrxp–17–2, is a candidate gene for rxp
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