Fine mapping of ClLOX, a QTL for powdery mildew resistance in watermelon ( Citrullus lanatus L.)

Key message ClLOX , is located on chromosome 2 and encodes a lipoxygenase gene, which induced watermelon powdery mildew resistance by inhibiting pathogen spread. Abstract Powdery mildew is one of the most severe fungal diseases reducing yield and quality of watermelon ( Citrullus lanatus L.) and other cucurbit crops. Genes responsible for powdery mildew resistance in watermelon are highly valuable. In this study, we first identified the QTL pm-lox for powdery mildew resistance in watermelon, located within a 0.93 Mb interval of chromosome 2, via XP-GWAS method using two F 2 populations. The F 2:3 families from one of the F 2 populations were then used for fine-mapping the pm-lox locus into a 9,883 bp physical region between 29,581,906 and 29,591,789, containing only two annotated genes. Of these, only ClG42_02g0161300 showed a significant differential expression between the resistant and susceptible lines after powdery mildew inoculation based on RNA sequencing (RNA-seq) and qRT-PCR analysis, and is designated ClLOX . Derived Cleaved Amplified Polymorphic Sequence (dCAPs) markers were developed and validated. In addition, our tests showed that the resistance was anti-spread rather than anti-infection of the pathogen. This study identified a new resistance gene ( ClLOX ), provided insights into the mechanism of powdery mildew resistance, and developed a molecular marker for watermelon breeding.