**Genome-Wide Analysis of the** *Rhododendron* **AP2/ERF Gene Family: Identification and Expression Profiles in Response to Cold, Salt and Drought Stress**

**Zhenhao Guo 1,2, Lisi He 1,2, Xiaobo Sun 1,2, Chang Li 1,2, Jiale Su 1,2, Huimin Zhou 1,2 and Xiaoqing Liu 1,2,\***

<sup>1</sup> Institute of Leisure Agriculture, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China

<sup>2</sup> Jiangsu Key Laboratory for Horticultural Crop Genetic Improvement, Nanjing 210014, China

**\*** Correspondence: lxqjaas@aliyun.com

**Abstract:** The AP2/ERF gene family is one of the most conserved and important transcription factor families mainly occurring in plants with various functions in regulating plant biological and physiological processes. However, little comprehensive research has been conducted on the AP2/ERF gene family in *Rhododendron* (specifically, *Rhododendron simsii*), an important ornamental plant. The existing whole-genome sequence of *Rhododendron* provided data to investigate the AP2/ERF genes in *Rhododendron* on a genome-wide scale. A total of 120 *Rhododendron* AP2/ERF genes were identified. The phylogenetic analysis showed that RsAP2 genes were classified into five main subfamilies, AP2, ERF, DREB, RAV and soloist. Cis-acting elements involving plant growth regulators, response to abiotic stress and MYB binding sites were detected in the upstream sequences of RsAP2 genes. A heatmap of RsAP2 gene expression levels showed that these genes had different expression patterns in the five developmental stages of *Rhododendron* flowers. Twenty RsAP2 genes were selected for quantitative RT-PCR experiments to clarify the expression level changes under cold, salt and drought stress treatments, and the results showed that most of the RsAP2 genes responded to these abiotic stresses. This study generated comprehensive information on the RsAP2 gene family and provides a theoretical basis for future genetic improvement.

**Keywords:** *Rhododendron*; transcription factor; genome-wide; abiotic stress; gene duplication; gene expression
