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.

Steel slag is an industrial solid waste produced during the steelmaking process. To explore the application of steel slag in the agricultural field, the present experiment was carried out to study the effect of substrates with different contents of steel slag on the growth of Hydrangea macrophylla cuttings. The conventional substrate (perlite: vermiculite: peat = 1:1:1) was used as the control (CK), and the treatments were designed as T₁ (steel slag: perlite: vermiculite: peat = 1:3:3:3, v/v/v/v), T₂ (steel slag: perlite: vermiculite: peat = 1:2:2:2, v/v/v/v), T₃ (steel slag: perlite: vermiculite: peat = 1:1:1:1, v/v/v/v), and T₄ (steel slag: perlite: vermiculite: peat = 1:0:0:0, v/v/v/v). The results showed that the addition of steel slag significantly increased the substrate’s bulk density, EC, and pH and improved its water retention capacity to a certain extent. There were significant differences among different treatments in morphological indicators, root growth and development, and physiological and biochemical characteristics of cutting seedlings. All traits, including plant height, fresh weight, dry weight, root length, root surface area, root volume, the number of root tips, root activity, and soluble protein content of seedlings grown in T₃ were significantly higher than those in other substrates. The results indicated that the appropriate addition of steel slag is helpful to hydrangea cuttings’ growth, and the optimal mixing ratio is steel slag: perlite: vermiculite: peat = 1:1:1:1 (v/v/v/v). This is a significant innovation in applying steel slag in agricultural production.

Rhododendron is an important woody ornamental plant and breeding varieties with different colors is vital research goal. In this study, a flower color variation cultivar ‘Yanzhi Mi’ (pink petals) and the wild-type (WT) cultivar ‘Dayuanyangjin’ (white petals with pink stripes) were used as research objects, the pigment and transcriptome of their petals during different flower development (stage S1, S2, S3, S4 and S5) were analyzed and compared. The results showed that the derivatives of cyanidin, peonidin and pelargonidin may be responsible for the pink of mutant petals and S2 stage (buds showing color at the top but with the scales still present) was the key stage of flower color formation of mutant. In total, 412,910 transcripts and 2,780 differentially expressed genes (DEGs) were identified in pairwise comparisons of WT and mutant petals. GO and KEGG enrichment analyses of the DEGs showed that the ‘DNA-binding transcription factor activity’, ‘Flavonoid biosynthesis’ and ‘Phenylpropanoid biosynthesis’ were more active in mutant petals. Early anthocyanin pathway candidate DEGs (CHS3-CHS6, CHI, F3Hs and F3’H) were strongly correlated and up-regulated expression in mutant petals than in WT petals at S2 stage. These genes may be the key structural genes for the pink coloration of mutant petals. In the petals of mutant, two R2R3-MYB unigene (TRINITY_DN59015_c3_g2 and TRINITY_DN49281_c1_g6) were identified as repressors involved in anthocyanin regulation and were significantly down-regulated at S2 stage. This study shed light on the biochemistry and genetic mechanisms underlying the flower coloration in two Rhododendron obtusum cultivars.