| Summary: | (1) Background: <i>Populus yunnanensis</i> Dode (<i>P. yunnanensis</i>) grows in the low-latitude and high-altitude areas of southwest China. In low-latitude and high-altitude areas, plants suffer from the high intensity of UV-B (ultraviolet-b) radiation, and they have a complete regulation system to adapt to the environment of the high UV-B radiation. As natural antioxidants, anthocyanins play an important role in scavenging free radicals. <i>BBX</i> (B-box) genes are involved in anthocyanins biosynthesis. (2) Methods: By exploring the gene structure and motifs of <i>PyunBBX</i> genes (genes of <i>P. yunnanensis BBX</i> family) and the evolutionary relationship between <i>PyunBBX</i> genes and other species <i>BBX</i> genes, six <i>PyunBBX</i> genes that responded to UV-B and participated in anthocyanins biosynthesis were screened. <i>BBX,</i> with the potential to regulate anthocyanins biosynthesis, was further investigated by anthocyanins content determination and RT-qPCR (real-time quantitative polymerase chain reaction); (3) Results: After 7 days of UV-B treatment, anthocyanins were significantly accumulated, and the expression of <i>PyunBBX18</i> was up-regulated for 7 days. The expression of <i>PyunBBX12</i> was inhibited by UV-B treatment. By analyzing the RNA-seq data of leaves and bark of <i>P. yunnanensis</i>, we found that <i>PyunBBX18</i> was highly expressed in leaves and young bark; (4) Conclusions: These results showed that <i>PyunBBX18</i> and <i>PyunBBX12</i> may be involved in the response process of UV-B stress, in which <i>PyunBBX18</i> may regulate the anthocyanins biosynthesis to resist UV damage.
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