| Summary: | Allene oxide cyclase (AOC, EC 5.3.99.6) catalyzes the most important step in the jasmonic acid (JA) biosynthetic pathway and mediates plant defense response to a wide range of biotic and abiotic stresses. In this study, two <i>AOC</i> genes were identified from watermelon. Sequence analysis revealed that each of <i>ClAOC1</i> and <i>ClAOC2</i> contained an allene oxide cyclase domain and comprised eight highly conserved β-strands, which are the typical characteristics of AOC proteins. Phylogenetic analysis showed that <i>ClAOC1</i> and <i>ClAOC2</i> were clustered together with AOCs from dicotyledon, with the closest relationships with <i>JcAOC</i> from <i>Jatropha curcas</i> and Ljaoc1 from <i>Lotus japonicus</i>. Different intron numbers were observed in <i>ClAOC1</i> and <i>ClAOC2</i>, which may result in their functional divergence. qRT-PCR analysis revealed that <i>ClAOC1</i> and <i>ClAOC2</i> have specific and complex expression patterns in multiple organs and under hormone treatments. Both <i>ClAOC1</i> and <i>ClAOC2</i> displayed the highest transcriptional levels in stem apex and fruit and exhibited relatively lower expression in stem. JA, salicylic acid (SA), and ethylene (ET) could enhance the expression of <i>ClAOC1</i> and <i>ClAOC2</i>, particularly that of <i>ClAOC2</i>. Red light could induce the expression of <i>ClAOC2</i> in root-knot nematode infected leaf and root of watermelon, indicating that <i>ClAOC2</i> might play a primary role in red light-induced resistance against root-knot nematodes through JA signal pathway. These findings provide important information for further research on <i>AOC</i> genes in watermelon.
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