Elucidation of resistance signaling and identification of powdery mildew resistant mapping loci (ClaPMR2) during watermelon-podosphaera xanthii interaction using RNA-seq and whole-genome resequencing approach

Authors: MIHIR, MANDAL - Claflin University; HAKTAN, SUREN - Virginia Tech; Kousik, Chandrasekar - Shaker

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2020
Publication Date: 8/20/2020
Citation: Mihir, M., Haktan, S., Kousik, C.S. 2020. Elucidation of resistance signaling and identification of powdery mildew resistant mapping loci (ClaPMR2) during watermelon-podosphaera xanthii interaction using RNA-seq and whole-genome resequencing approach. Scientific Reports. https://doi.org/10.1038/s41598-020-70932-z.
DOI: https://doi.org/10.1038/s41598-020-70932-z

Interpretive Summary: Watermelon is an important vegetable crop grown in 44 states in the U.S.A. Many diseases and pests attack watermelon seedlings and plants and reduce their yield resulting in monetary loss for growers. One such disease, known as powdery mildew can infest watermelon seedlings and can cause reduced vigor or death of seedlings. Growers routinely spray pesticides to manage powdery mildew of watermelon. ARS scientists in Charleston, SC developed one excellent source of resistance to this disease and characterized the mechanisms of resistance in this line called USVL531-MDR. The developed a potential marker to be used in breeding program to incorporate this resistance into commercial cultivar. The results of this study will be useful to seed industry and University vegetable breeders to develop new watermelon varieties with resistance to powdery mildew. The results will also be useful to watermelon growers, extension workers, seed industry and University researchers to help manage powdery mildew in the greenhouse where watermelon seedlings are generally grown.

Technical Abstract: Watermelon is an important vegetable crop and is widely cultivated in USA with an approximate global production of >100 million tons. Powdery mildew (PM) caused by Podosphaera xanthii is a major production-limiting factor on watermelon and other cucurbits. Numerous PM/ multiple disease resistant (MDR) watermelon germplasm lines have been developed by the USDA in Charleston, SC. To gain a better understanding of the innate and activated molecular defense mechanisms involved during compatible and incompatible PM-watermelon interactions, we inoculated PM susceptible (USVL677-PMS) and resistant (USVL531-MDR) watermelon plants with 105 conidia ml-1 of P.xanthii. RNA-seq profiling was done on leaf samples collected at 0, 1, 3, and 8 days post inoculation (DPI). A total of 2566 unique differentially expressed genes (DEGs) were identified between compatible and incompatible interactions with P. xanthii. The compatible interactions resulted in distinct plant gene activation (>2 fold unique transcripts, 335:191:1762 :: 1:3:8 DPI) as compared to incompatible interaction (>2 fold unique transcripts, 314:681:487 :: 1:3:8 DPI). Further, comparative whole-genome resequencing analysis of USVL531-PMR, USVL677-PMS and four introgressed PM resistant recombinant inbred lines (RIL, USVL531-PMR x USVL677-PMS) were performed to identify the region of PM resistance introgressed break points along with other traits inherent by USVL531-PMR by comparing the SNPs and InDels. Based on SNPs identification and CAPS markers, the resistance gene was identified as ClaPMR2, Citrullus lanatus PM Resistance gene 2 {Chr2 : 26750001 .. 26753327 (-)}, a NBS-LRR resistance protein (R) with homology to the Arabidopsis thaliana powdery mildew resistance protein, RPW8. The transcriptome data also revealed a complex regulatory network associated with the introgressed junctions mediated by PM resistance R proteins (R genes) that may involve multiple signal regulators and transducers, carbohydrate metabolism, cell wall modifications and the hormone-signaling pathway.