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Watermelon (Citrullus lanatus) is an important vegetable fruit crop, widely grown in many countries in the tropical and subtropical regions around the world. The narrow genetic base of cultivated watermelon makes it susceptible to many pests and diseases, especially viruses. However, genetic sources of resistance are available in wild relatives that could potentially be used to incorporate the resistance genes into watermelon cultivars. Ribosome-inactivating proteins (RIPs) are prevalent in many plant species and are known to have an important role in defence against bacteria, fungi, and viruses. The objective of this study was to identify RIP-like sequences in the watermelon genome and to conduct comparative measurement of RIP gene expression between the potyvirus-resistant Citrullus amarus 'PI 244019ʹ versus the susceptible watermelon (C. lanatus) 'Charleston Gray' in response to infection by papaya ringspot virus (PRSV). Taking advantage of the recently available genome sequences of 'PI 244019ʹ and 'Charleston Gray', in this study we mined out watermelon RIP genes and conducted comparative sequence and expression analyses of RIP-I and RIP-II between the genotypes. The RIP gene expression analysis revealed elevated RIP gene expression in resistant PI 244019 plants, but not in susceptible 'Charleston Gray' plants, in response to inoculation with PRSV, indicating a potential association of RIP gene expression and resistance to potyviruses in watermelon. This elevated expression of RIP genes in association with potyvirus resistance opens a new avenue to investigate the underlying mechanism of potyvirus resistance in watermelon. [ABSTRACT FROM AUTHOR]