The NLR Family of Disease Resistance Genes in Cultivated Watermelon and Other Cucurbits: Opportunities and Challenges

The narrow genetic diversity of cultivated watermelon (Citrullus lanatus) makes it vulnerable to a wide variety of biotic stressors; hence, breeding for resistance has become a research priority. Most resistance (R) genes encode intracellular nucleotide-binding/leucine-rich repeat (NLR) immune receptor proteins with a clear advantage for breeding given their inheritance and manipulability. Several genomes of watermelons have been sequenced, revealing a relatively reduced number of NLRs, a common phenomenon in the Cucurbitaceae. Wild and semi-cultivated Citrullus species have expanded genetic diversity and are potential sources of resistance; however, scarce information is available about their NLR content. This chapter provides a broad perspective of significant watermelon diseases, their management, and sources of resistance for breeding. We explore the application of genomic resources, germplasm screening, bioinformatic tools, and targeted sequence enrichment technologies to survey and characterize the watermelon NLR complement or NLRome. The chapter concludes with some reflection on further steps to speed NLR cloning for breeding and the value of the watermelon NLRome to understand the evolution and regulation of its immune system.