The chromosome-scale genome assembly, annotation and evolution of Rhododendron henanense subsp. lingbaoense.

Rhododendron henanense subsp. lingbaoense (hereafter referred to as R. henanense) is an endemic species naturally distributed in the Henan province, China, with high horticultural, ornamental and medicinal value. Herein, we report a de novo genome assembly for R. henanense using a combination of PacBio long read and Illumina short read sequencing technologies. In total, we assembled 634.07 Mb with a contig N50 of 2.5 Mb, representing ~96.93% of the estimated genome size. By applying Hi-C data, 13 pseudochromosomes of R. henanense genome were assembled, covering ~98.21% of the genome assembly. The genome was composed of ~65.76% repetitive sequences and 31,098 protein-coding genes, 88.77% of which could be functionally annotated. Rhododendron henanense displayed a high level of synteny with other Rhododendron species from the Hymenanthes subgenus. Our data also suggests that R. henanense genes related to stress responses have undergone expansion, which may underly the unique abiotic and biotic stress resistance of the species. This alpine Rhododendron chromosome-scale genome assembly provides fundamental molecular resources for germplasm conservation, breeding efforts, evolutionary studies, and elucidating the unique biological characteristics of R. henanense.

Genome survey sequencing and identification of genomic SSR markers for Rhododendron micranthum.

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 the present 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 2281551 scaffolds with a N50 value of 916 bp. A total of 479724 SSR molecular markers were identified in the R. micranthum genome, and 871656 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.

Thymol confers tolerance to salt stress by activating anti-oxidative defense and modulating Na+ homeostasis in rice root.

Modulation of plant salt tolerance has been drawing great attention. Thymol is a kind of natural chemical that has been developed as anti-microbial reagent and medicine. To date, we still have limited knowledge about thymol-modulated plant physiology. In this work, physiological, histochemical, and biochemical methods were adopted to study thymol-conferred salt resistance in the root of rice (Oryza sativa). Thymol significantly rescued root growth under salt stress. Thymol ameliorated cell membrane damage, oxidative stress, ROS accumulation, and cell death in roots under salt stress. Thymol-attenuated oxidative stress may be resulted from the activation of anti-oxidative capacity, including both enzymatic and non-enzymatic system. Thymol treatment significantly decreased Na+ content in root cells upon salt stress, which might be ascribed to the upregulation of OsSOS1 (salt overly sensitive 1) facilitating Na+ exclusion. In addition, thymol stimulated the expression of genes encoding tonoplast OsNHX (Na+/H+antiporter), which may help root cells to compartmentalize Na+ in vacuole. The results of these works evidenced that thymol was capable of inducing salt tolerance by reestablishing ROS homeostasis and modulating cellular Na+ flux in rice roots. These findings may be applicable to improve crop growth in salinity area.

Characterization and transferability of microsatellites for Gentiana lawrencei var. farreri (Gentianaceae).

PREMISE OF THE STUDY: Microsatellite markers were developed for a medicinal herb, Gentiana lawrencei var. farreri (Gentianaceae), for the future assessment of population genetic structure and potential hybridization events with related taxa. METHODS AND RESULTS: Using the 454 FLX+ sequencing platform, we obtained 81,717 clean reads with an average length of 291 bp. A total of 3031 primer pairs were designed, and 96 were selected for validation. A set of 20 fluorescently labeled primer pairs was further selected and screened for polymorphisms in three G. lawrencei var. farreri populations and one G. veitchiorum population. Among the four populations, the average number of alleles per locus was 15.2. Finally, a set of 17 unlinked loci were determined to be in Hardy-Weinberg equilibrium after two linked loci were removed. CONCLUSIONS: The identified simple sequence repeat markers will be useful for genetic diversity and evolution studies in G. lawrencei var. farreri and related taxa.

The Complete Plastome Sequences of Seven Species in Gentiana sect. Kudoa (Gentianaceae): Insights Into Plastid Gene Loss and Molecular Evolution.

The chloroplast (cp) genome is useful in the study of phylogenomics, molecular dating, and molecular evolution. Gentiana sect. Kudoa is a predominantly alpine flowering plant that is valued for its contributions to medicine, ecology, and horticulture. Previous evolutionary studies showed that the plastid gene loss pattern and intra-sectional phylogenetics in sect. Kudoa are still unclear. In this study, we compared 11 Gentiana plastomes, including 7 newly sequenced plastomes from sect. Kudoa, to represent its three serious: ser. Ornatae, ser. Verticillatae, and ser. Monanthae. The cp genome sizes of the seven species ranged from 137,278 to 147,156 bp. The plastome size variation mainly occurred in the small single-copy and long single-copy regions rather than the inverted repeat regions. Compared with sect. Cruciata, the plastomes in ser. Ornatae and ser. Verticillatae had lost approximately 11 kb of sequences containing 11 ndh genes. Conversely, far fewer losses were observed in ser. Monanthae. The phylogenetic tree revealed that sect. Kudoa was not monophyletic and that ser. Monanthae was more closely related to other sections rather than sect. Kudoa. The molecular dating analysis indicated that ser. Monanthae and sect. Kudoa diverged around 8.23 Ma. In ser. Ornatae and ser. Verticillatae, the divergence occurred at around 0.07-1.78 Ma. The nucleotide diversity analysis indicated that the intergenic regions trnH-psbA, trnK-trnQ, ycf3-trnS and rpl32-trnL constituted divergence hotspots in both sect. Kudoa and Gentiana, and would be useful for future phylogenetic and population genetic studies.