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The OPR gene family in watermelon: Genome-wide identification and expression profiling under hormonal treatments and root-knot nematode infection.

12-oxo-phytodienoic acid reductase (OPR) is an important enzyme in the jasmonic acid (JA) biosynthesis pathway and thus plays a vital role in plant defense. However, systematic and comprehensive analyses of OPR genes in watermelon and their roles in defense responses are extremely limited. The physicochemical properties, phylogenetic tree, gene structure, and cis-acting elements of watermelon OPR genes were analyzed by bioinformatics, and qRT-PCR and RNA-Seq were applied to assay the expression of OPR genes in watermelon. A total of five OPR family genes were identified in watermelon, which were unevenly distributed across the four chromosomes. Phylogenetic analysis assigned OPR members from different plant species to five subfamilies (OPRI-OPRV). The motif compositions of OPR members were relatively conserved. Expression analysis using qRT-PCR revealed that ClOPR genes, except for ClOPR5, were highly expressed in the flower and fruits. RNA-seq analysis showed that the ClOPR genes had different expression patterns during flesh and rind development. Furthermore, the ClOPR genes, particularly ClOPR2 and ClOPR4, were significantly upregulated by exogenous JA, salicylic acid (SA), and ethylene (ET) treatments. In addition, red light induced the expression of ClOPR2 and ClOPR4 in leaves and roots of the root-knot nematode (RKN)-infected watermelon plants, suggesting their involvement in red-light-induced defense against RKN. These results provide a theoretical basis for elucidating the diverse functions of OPR family genes in watermelon.