New Insights into the Roles of Osmanthus Fragrans Heat-Shock Transcription Factors in Cold and Other Stress Responses

Sweet osmanthus (<i>Osmanthus fragrans</i>) is an evergreen woody plant that emits a floral aroma and is widely used in the landscape and fragrance industries. However, its application and cultivation regions are limited by cold stress. Heat-shock transcription factor (HSF) family members are widely present in plants and participate in, and regulate, the defense processes of plants under various abiotic stress conditions, but now, the role of this family in the responses of <i>O. fragrans</i> to cold stress is still not clear. Here, 46 <i>OfHSF</i> members were identified in the <i>O. fragrans</i> genome and divided into three subfamilies on the basis of a phylogenetic analysis. The promoter regions of most <i>OfHSF</i>s contained many <i>cis</i>-acting elements involved in multiple hormonal and abiotic stresses. RNA-seq data revealed that most of <i>OfHSF</i> genes were differentially expressed in various tissues, and some <i>OfHSF</i> members were induced by cold stress. The qRT-PCR analysis identified four <i>OfHSF</i>s that were induced by both cold and heat stresses, in which <i>OfHSF11</i> and <i>OfHSF43</i> had contrary expression trends under cold stress conditions and their expression patterns both showed recovery tendencies after the cold stress. <i>OfHSF11</i> and <i>OfHSF43</i> localized to the nuclei and their expression patterns were also induced under multiple abiotic stresses and hormonal treatments, indicating that they play critical roles in responses to multiple stresses. Furthermore, after a cold treatment, transient expression revealed that the malondialdehyde (MDA) content of <i>OfHSF11</i>-transformed tobacco significantly increased, and the expression levels of cold-response regulatory gene <i>NbDREB3</i>, cold response gene <i>NbLEA5</i> and ROS detoxification gene <i>NbCAT</i> were significantly inhibited, implying that <i>OfHSF11</i> is a negative regulator of cold responses in <i>O. fragrans</i>. Our study contributes to the further functional characterization of <i>OfHSF</i>s and will be useful in developing improved cold-tolerant cultivars of <i>O. fragrans</i>.