Comparative Transcriptome Analysis Provides Insight into the Effect of 6-BA on Flower Development and Flowering in Bougainvillea

<i>Bougainvillea</i> spp. is a well-known ornamental plant that is widely applied in urban landscaping construction. The colorful bracts of <i>Bougainvillea</i> in full bloom become important for urban landscape during special festivals. Although flowering regulation measures of <i>Bougainvillea</i> attracted much attention, the underlying mechanism of flower bud differentiation and development remains poorly understood. Here, we induced flowering of <i>Bougainvillea glabra</i> 'Sao Paulo' under 6-BA treatment and conducted RNA sequencing data analysis to characterize the molecular regulatory mechanism of flower development in response to 6-BA. Transcriptome analysis indicated that a series of genes and transcription factors of cytokinin metabolism, flowering and floral development regulation, and photoperiod regulation were upregulated by the 6-BA treatment, including <i>COL</i>, <i>AP2</i>, <i>FT</i>, <i>SOC1</i>, <i>LFY</i>, <i>SPL4</i>, <i>SPL9</i>, and <i>SPL13</i>. Moreover, the expression of these important genes exhibited relatively high levels of thorns compared to apical buds, suggesting that flower bud differentiation probably starts with the thorns in <i>Bougainvillea</i>. This study confirms that 6-BA treatment at certain concentrations can promote flowering of <i>Bougainvillea</i> and provides insight into the regulatory mechanism of the growth regulator acting on early flowering of <i>Bougainvillea.</i>