| Summary: | The 12-oxo-phytodienoic acid reductases (<i>OPRs</i>) have been proven to play a major role in plant development and growth. Although the classification and functions of <i>OPRs</i> have been well understood in Arabidopsis, tomato, rice, maize, and wheat, the information of <i>OPR</i> genes in cotton genome and their responses to biotic and abiotic stresses have not been reported. In this study, we found 10 and 9 <i>OPR</i> genes in <i>Gossypium hirsutum</i> and <i>Gossypium barbadense</i>, respectively. They were classified into three groups, based on the similar gene structure and conserved protein motifs. These <i>OPR</i> genes just located on chromosome 01, chromosome 05, and chromosome 06. In addition, the whole genome duplication (WGD) or segmental duplication events contributed to the evolution of the <i>OPR</i> gene family. The analyses of cis-acting regulatory elements of <i>GhOPRs</i> showed that the functions of <i>OPR</i> genes in cotton might be related to growth, development, hormone, and stresses. Expression patterns showed that <i>GhOPRs</i> were upregulated under salt treatment and repressed by polyethylene glycol 6000 (PEG6000). The expression patterns of <i>GhOPRs</i> were different in leaf, root, and stem under <i>V. dahliae</i> infection. <i>GhOPR9</i> showed a higher expression level than other <i>OPR</i> genes in cotton root. The virus-induced gene silencing (VIGS) analysis suggested that knockdown of <i>GhOPR9</i> could increase the susceptibility of cotton to <i>V. dahliae</i> infection. Furthermore, <i>GhOPR9</i> also modulated the expressions of jasmonic acid (JA) pathway-regulated genes under the <i>V. dahliae</i> infection. Overall, our results provided the evolution and potential functions of the <i>OPR</i> genes in cotton. These findings suggested that <i>GhOPR9</i> might play an important role in cotton resistance to <i>V. dahliae</i>.
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