Rhododendron micranthum is an evergreen shrub species widely distributed in China that has high ornamental and medicinal value. However, there is a lack of molecular and genomic data for this plant, which severely restricts the development of its relevant research. The objective of this study was to conduct a first genomic survey of R. micranthum and determine its whole-genome sequencing scheme. Next-generation sequencing (Illumina Hi-Seq Xten) was used to measure the genome size of R. micranthum, k-mer analysis were employed to investigate its genomic profile. Finally, we conducted bioinformatics methods to performed SSR (simple sequence repeat) prediction based on the genomic data. The genome size of R. micranthum was estimated to be 554.22 Mb. The heterozygosity ratio was 0.93%, and the sequence repeat ratio was calculated to be 49.17%. The clean reads of R. micranthum were assembled into 2,281,551 scaffolds with a N50 value of 916 bp. A total of 479,724 SSR molecular markers were identified in the R. micranthum genome, and 871,656 pairs of primers designed for application. Among of them, 100 primer pairs were validated, and 71 primer pairs were successfully amplified. In summary, the R. micranthum genome is complex with high heterozygosity and low repeated sequences. In future whole-genome research in R. micranthum, higher-depth ‘2+3’ (Illumina+PacBio) sequencing may yield better assembly results.

Rhododendron henanense subsp. lingbaoense is endemic in China. The cpDNA of R. henanense subsp. lingbaoense is a typical quadripartite structure with a length of 208,015 bp, including a large single-copy region of 110,593 bp and a small single-copy region of 2606 bp separated by a pair of identical inverted repeat regions of 47,408 bp each. The chloroplast genome contains 119 genes, including 86 protein-coding genes, four ribosomal RNA genes, and 29 transfer RNA genes. The phylogenetic analysis of R. henanense subsp. lingbaoense showed a relatively close relationship with Rhododendron delavayi.

In this study, we aimed to investigate the heat tolerance mechanism in Rhododendron henanense subsp. lingbaoense (Rhl). Rhl seedlings were treated at 40℃ (RLH), 32℃ (RLM), and 24℃ (RLC), and the changes in transcriptome and metabolome were compared. Overall, 78 differentially expressed metabolites were detected, and 8450 differentially expressed genes (DEGs) were identified. KEGG analysis revealed that the DEGs in RLH vs. RLC were mainly enriched in photosynthesis, secondary metabolic biosynthesis, and flavonoid biosynthesis. Most genes encoding glutathione-S-transferase were upregulated, whereas genes related to heat shock proteins were significantly downregulated. 31 genes related to photosynthesis were significantly upregulated (P-value < 0.001). It was speculated that these DEGs are related to the response of Rhl to high temperature stress (HTS). Overall, 9 TF families might be the key regulators of Heat stress response pathways in Rhl. Mining of DEGs revealed that the expression of some genes related to heat stress function increased highly significantly, e.g., the Rhe008987 related to Glutathione-S-transferase, Rhe016769 encoding peroxidase, and Rhe001827 encoding chalcone and stilbene synthases. Metabolome and transcriptome correlation analysis revealed that three comparison groups (RLH vs. RLC, RLH vs. RLM, and RLM vs. RLC) shared 12 metabolic pathways in which the DEMs were enriched. HTS inhibited or induced expression of genes in flavonoid biosynthesis pathway and led to decreace in kaempferol content and quercetin accumulation. HT induced expression of genes in ABC pathway, which may be one of the reasons for the significant accumulation of L-isoleucine, L-leucine, and L-proline. In this study, DEGs mining found that the expression of some genes related to heat stress function increased highly significantly. And two omics correlation analysis revealed that 12 metabolic pathways were enriched in three comparison groups. These results helped in elucidating the molecular mechanisms of response of Rhl to HTS.