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• The order of thermotolerance of four rhododendrons was determined. • ROS and secondary metabolites played important roles in rhododendrons heat response. • TFs related to heat response of rhododendrons was screened out. • Abundant genomic information of rhododendrons was provided. 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. [ABSTRACT FROM AUTHOR]

Cut rhododendron flowers have high ornamental value and are loved by consumers. It is important to clarify the senescence mechanism of cut rhododendron flowers to prolong their vase life. Transcriptome sequencing was used to investigate flower senescence in four stages of Rhododendron 'Hong Shanhu.' Using RNA sequencing, we generated approximately 100.17 Gb clean data and 173,502 annotated unigenes. Ethylene biosynthesis genes, ACOs , had different expression trends, and most ethylene signaling genes had low expression levels. The ethylene release rate and the activities of ACS and ACO were on the rise during cut flower development. The effects of 200 mg/L ethephon and 0.1 mM/L spermidine on vase life and fresh weight were limited. Most of the LEA and Dehydrin were upregulated with cut flower development and the fresh weight was decreased during fully flowering stage. Based on the different expression trends, differentially expressed genes were classified into 10 clusters. Cluster 2 showed significant upregulation between Z4 and Z3, suggesting that the genes in this cluster were upregulated, probably due to the senescence of cut flowers. The overall trend of Cluster 7 was positively correlated with the opening process of cut flowers. Cluster 9 had a peak at Z3, which indicated that the unigenes started the senescence process. bZIP53 , RAP2.4 , UIF1 , HAT5 , ILR3 , DEAR3 , SCL8 , TZF10 , MYB70 , ERF5 , ERF9 , and CBF1 had higher expression levels than the other transcription factors in these clusters, indicating that they are probably the key transcription factors involved in petal senescence. Our results indicated that 'Hong Shanhu' cut flowers were ethylene-insensitive, and their senescence was probably related to water stress. These results contribute to understanding the mechanism of senescence in cut rhododendron flowers. • 'Hong Shanhu' cut flowers are ethylene-insensitive. • Their senescence was probably related to water stress. • Most of LEA and dehydrin were upregulated during the aging process. • The ethylene release rate was on the rise throughout cut flower development. • The TFs involved in petal senescence were screened. [ABSTRACT FROM AUTHOR]