<i>De novo</i> RNA sequencing transcriptome of <i>Rhododendron obtusum</i> identified the early heat response genes involved in the transcriptional regulation of photosynthesis

<div><p><i>Rhododendron spp</i>. 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” (<i>R</i>. <i>obtusum</i>) 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.</p></div

The number of unigenes identified as transcription factors in rhododendron leaves.

<p>24 h vs ck, Changed log2 ratio of Rhododendron exposed to 40°C for 4 h. B. Changed log2 ratio of Rhododendron exposed to 40°C for 24 h.</p

Survival rates of leaves and status of the ChlF parameters.

<p>A. <i>F</i>o; B. <i>F</i>v/<i>F</i>m; C. <i>F</i>v’/<i>F</i>m’; D. Survival rates of leaves. YZM, <i>R</i>. <i>obtusum “Yanzhimi”</i>; XJP, <i>R</i>.<i>hybridum</i> “Xiajinpao”; JP, <i>R</i>.<i>hybridum</i> “Jinpao”; YC, <i>R</i>.<i>hybridum</i> “Yanchun”.</p

qRT–PCR analysis of TFs in response to different heat stresses.

<p>Three bio-replicates and tech-replicates were performed. Data are presented as means±SD.</p

Enrichment of photosynthetic passway and relative unigene cluster heatmap.

<p>A. KEGG-illustrated photosynthetic passway was significantly enriched in 24-h heat treatment compared with the control. Red color indicates that the unigenes were up-regulated significantly, green color indicates that the unigenes were down-regulated significantly, and gray color indicates that the unigenes were unchanged. B. Heat map cluster of photosynthesis passway unigenes in rhododendron.</p

Transcript gene family classification of <i>R</i>. <i>obtusum</i> transcriptome.

<p>Transcript gene family classification of <i>R</i>. <i>obtusum</i> transcriptome.</p

The size distribution of transcripts and unigenes of rhododendron transcriptome.

<p>Size distribution of Illumina sequencing transcripts(left). (B) Size distribution of unigenes (right).</p

Functional annotation of rhododendron unigenes by sequence similarity search to Swiss-Prot, Pfam, GO, COG, KEGG and EGGnog database.

<p>Functional annotation of rhododendron unigenes by sequence similarity search to Swiss-Prot, Pfam, GO, COG, KEGG and EGGnog database.</p

Length distribution and GC content of the assembled transcripts and unigenes.

<p>Length distribution and GC content of the assembled transcripts and unigenes.</p

Summary of <i>R</i>. <i>obtusum</i> ‘Yanzhimi’assembled transcripts and unigenes properties.

<p>Summary of <i>R</i>. <i>obtusum</i> ‘Yanzhimi’assembled transcripts and unigenes properties.</p