Identification and fine-mapping of Xo2, a novel rice bacterial leaf streak resistance gene

A novel rice resistance gene, Xo2, influencing pathogenesis of the bacterial leaf streak disease, has been identified, and candidate genes for Xo2 in the fine mapping region have been shown to be involved in bacterial leaf streak resistance. Rice (Oryza sativa) bacterial leaf streak, caused by Xanthomonas oryzae pv. oryzicola (Xoc), is one of the most serious rice bacterial diseases. The deployment of host resistance genes is an effective approach for controlling this disease. The cultivar BHADOIA 303 (X455) from Bangladesh is resistant to most of Chinese Xoc races. To identify and map the resistance gene(s) involved in Xoc resistance, we examined the association between phenotypic and genotypic variations in two F₂ populations derived from crosses between X455/Jingang 30 and X455/Wushansimiao. The segregation ratios of the F₂ progeny were consistent with the action of a single dominant resistance gene, which was designated as Xo2. Based on rice SNP chip (GSR40K) assays of X455, Jingang 30, and resistant and susceptible pools thereof, we mapped Xo2 to the region from 10 Mb to 12.5 Mb on chromosome 2. The target gene was further finely mapped between the markers RM12941 and D6-1 within an approximately 110-kb region. The de novo sequencing and gene annotation of X455 and Jingang 30 revealed nineteen predicted genes within the target region. RNA-seq and expression analysis showed that four candidate genes, including Osa002T0115800, encoding an NLR resistance protein, were distinctly upregulated. Differential sequence and synteny analysis between X455 and Jingang 30 suggested that Osa002T0115800 is likely the functional Xo2 gene. This study lays a foundation for marker-assisted selection resistance breeding against rice bacterial leaf streak and the further cloning of Xo2.