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Rhododendron, large vascular plant genera with tens of thousands of cultivars, possesses valuable horticultural, medicinal, and ecological properties. However, the genetic studies have been hindered due to lack of effective genetic markers. In this study, microsatellite markers have been developed in R. obtusum Hort. ex Wats. variety 'Yanzhimi' based on RNA-seq. Di-nucleotide repeats (23,849, 63.131%) were the main type, followed by mono-nucleotide (8.544, 22.6169%) and tri-nucleotide motifs (4,078, 10.7949%). In particular, A/T (8,237, 21.804%), AG/CT (20,546, 54.388%), AAG/CTT (1,182, 3.129%), AAAG/CTTT (50, 0.132%) were dominant for mono-, di-, tri-, tetra-nucleotide repeat types, respectively. Totally, 30 EST-SSR markers gave out 196 polymorphic bands with an average of 6.53 bands per locus. NA and NE ranged from 3 to 17 and 1.28 to 6.75, with average values of 6.53 and 2.98, respectively. Rhododendron cultivars possessed relatively high genetic diversity at species level (with the average PIC value of 0.647), and frequent gene flow occurred with the mean value of 3.89. These 94 accessions could be clustered into 4 pops, and cultivars of the same origin and those bred by related parents tended to be clustered together. The clustering and fingerprint profiles effectively supported the authentication of tested cultivars. These EST-SSR markers could be used for further cultivar identification, marker-assisted selection, as well as identification of gene-trait relationship in Rhododendron species.

Rhododendron spp. is an important ornamental species that is widely cultivated for landscape worldwide. Heat stress is a major obstacle for its cultivation in south China. Previous studies on rhododendron principally focused on its physiological and biochemical processes, which are involved in a series of stress tolerance. However, molecular or genetic properties of rhododendron's response to heat stress are still poorly understood. The phenotype and chlorophyll fluorescence kinetics parameters of four rhododendron cultivars were compared under normal or heat stress conditions, and a cultivar with highest heat tolerance, "Yanzhimi" (R. obtusum) was selected for transcriptome sequencing. A total of 325,429,240 high quality reads were obtained and assembled into 395,561 transcripts and 92,463 unigenes. Functional annotation showed that 38,724 unigenes had sequence similarity to known genes in at least one of the proteins or nucleotide databases used in this study. These 38,724 unigenes were categorized into 51 functional groups based on Gene Ontology classification and were blasted to 24 known cluster of orthologous groups. A total of 973 identified unigenes belonged to 57 transcription factor families, including the stress-related HSF, DREB, ZNF, and NAC genes. Photosynthesis was significantly enriched in the Kyoto Encyclopedia of Genes and Genomes pathway, and the changed expression pattern was illustrated. The key pathways and signaling components that contribute to heat tolerance in rhododendron were revealed. These results provide a potentially valuable resource that can be used for heat-tolerance breeding.