Transcriptome analysis and identification of the genes associated with the heat stress response in four rhododendron species

Rhododendrons are prominent ornamental plants worldwide. The molecular mechanism of thermotolerance in the Rhododendron genus is still largely unknown. Here, R. molle, R. delavayi, R. irroratum, and R. decorum were selected to determine the molecular mechanism of the heat stress response in rhododendrons by transcriptomic studies. According to the heat injury index, the order of thermotolerance was R. molle > R. irroratum > R. decorum > R. delavayi. R. delavayi was sequenced with the reference genome, while other rhododendrons were sequenced without the reference genome. Two sequencing libraries of each rhododendron, namely one control and one heat stress-treated sample, were constructed for comparative transcriptomic analysis. In R. delavayi, 90.99% clean reads were mapped, and 86,621, 121,343, and 99,992 unigenes were identified in R. molle, R. irroratum, and R. decorum, respectively. Unigenes were annotated in the KEGG, GO, and Nr databases. According to the Nr annotation results, rhododendrons had a high gene similarity with Actinidia chinensis and Camellia sinensis. Between two sequencing libraries of R. molle, R. delavayi, R. irroratum, and R. decorum, 9017, 6916, 5243, and 11751 transcripts, respectively, were further identified as differentially expressed genes (DEGs). Transcription factor (TF) families enriched in DEGs mainly include ERF, bHLH, MYB, WRKY, and MYB_related, and several TFs were commonly regulated in all plants, such as BBR/BPC7, bZIP1, bZIP12, COL15, BBX22, ERF118-2, DREB2C, HsfC1, HSF8, LBD41, PHERES, RADIALIS-like 4, SPL14, SPL9 bHLH041, Dof5.4, ERF061, and PTI6. The data obtained in this study can provide abundant genomic information for future research and help to clarify the molecular mechanisms of rhododendrons’ heat stress response.