Stevia (Stevia rebaudiana Bertoni) is well known for its very sweet steviol glycosides (SGs) consisting of a common tetracyclic diterpenoid steviol backbone and a variable glycone. Steviol glycosides are 150–300 times sweeter than sucrose and are used as natural zero-calorie sweeteners. However, the most promising compounds are biosynthesized in small amounts. Based on Illumina, PacBio, and Hi-C sequencing, we constructed a chromosome-level assembly of Stevia covering 1416 Mb with a contig N50 value of 616.85 kb and a scaffold N50 value of 106.55 Mb. More than four-fifths of the Stevia genome consisted of repetitive elements. We annotated 44,143 high-confidence protein-coding genes in the high-quality genome. Genome evolution analysis suggested that Stevia and sunflower diverged ~29.4 million years ago (Mya), shortly after the whole-genome duplication (WGD) event (WGD-2, ~32.1 Mya) that occurred in their common ancestor. Comparative genomic analysis revealed that the expanded genes in Stevia were mainly enriched for biosynthesis of specialized metabolites, especially biosynthesis of terpenoid backbones, and for further oxidation and glycosylation of these compounds. We further identified all candidate genes involved in SG biosynthesis. Collectively, our current findings on the Stevia reference genome will be very helpful for dissecting the evolutionary history of Stevia and for discovering novel genes contributing to SG biosynthesis and other important agronomic traits in future breeding programs.

Aconitine is the main bioactive ingredient of Aconitum plants, which are well-known botanical herbs in China. Aconitine is also notorious for its high cardiotoxicity, as it can induce life-threatening ventricular arrhythmias. Unfortunately, there are few effective antidotes to aconitine toxicity. This study aimed to evaluate the potent protective effects of the ingredients from V. baillonii on aconitine toxicity on H9c2 cell line. Cell viability was assessed by methylthiazoltetrazolium bromide (MTT). Intracellular Ca2+ concentration alteration and reactive oxygen species (ROS) generation were observed by confocal microscopy and flow cytometry, respectively. Cellular oxidative stress was analyzed by measuring malondialdehyde (MDA) and superoxide dismutase (SOD) levels. Mitochondrial membrane potential (ΔΨ) was determined using JC-1 kit. RT-PCR and Hoechst staining techniques were conducted to determine the levels of autophagy/apoptosis. The mRNA levels of dihydropyridine receptor (DHPR), ryanodine receptors (RyR2) and sarcoplasmic reticulum Ca2+-ATPase (SERCA) were measured by RT-PCR. We screened six components from V. baillonii, among which, sweroside exhibited the strongest protective effects on aconitine-induced cardiac toxicity. Sweroside suppressed the aconitine-induced mRNA expressions of NaV1.5 (encoded by SCN5A), RyR2 and DHPR, and reversed the aconitine-induced decrease in mRNA level of SERCA, thus preventing the aconitine-induced persistent intracellular Ca2+ accumulation and avoiding intracellular Ca2+ overload. We further found that sweroside restabilized the aconitine-disrupted mitochondrial membrane potential (ΔΨ) and reversed the aconitine-induced increase in the mRNA levels of cell autophagy-related factors (Beclin-1, Caspase-3, and LC3- II) in H9c2 cells. In the whole-animal experiments, we observed that sweroside (50 mg/kg) alleviated effectively aconitine-induced arrhythmias by analysis of electrocardiogram (ECG) recording in rats. Our results demonstrate that sweroside may protect cardiomyocytes from aconitine toxicity by maintaining intracellular Ca2+ homeostasis, restabilizing mitochondrial membrane potential (ΔΨ) and avoiding cell autophagy/apoptosis.

No abstract

No abstract

Nitrogen drives plant growth to the detriment of leaf sugar and steviol glycosides metabolisms in Stevia (Stevia rebaudiana Bertoni).