Comparative transcriptome analysis revealed gamma-irradiation mediated disruption of floral integrator gene(s) leading to prolonged vegetative phase in Stevia rebaudiana Bertoni

Genome-wide transcriptional profiling and physiological investigation elucidating the molecular mechanism of multiple abiotic stress response in Stevia rebaudiana Bertoni

Considering the major source of plant-derived low/non-calorie steviol glycosides (SGs), comprehensive physiological, biochemical, and deep transcriptional investigations were conducted to explicit deeper insight into multiple abiotic stress responses in Stevia rebaudiana. The physiological indicators including photosynthesis, chlorophyll, relative water content, shoot growth, electrolyte leakage, and SG biosynthesis were negatively impacted under drought (DS), followed by salinity (SS) and waterlogging (WS). Global transcriptional analysis revealed significant upregulated expression of the genes encoding for ROS detoxification (GST, SOD, APX, glutathione peroxidase), osmotic adjustment (alpha-trehalose-phosphate and S-adenosylmethionine decarboxylase), ion transporters (CAX, NHX, CNGS, VPPase, VATPase), water channel (PIP1, TIP) and abiotic stress-responsive candidate genes (LEA, HSPs, and Dehydrins) regulating abiotic stress response in S. rebaudiana. These inferences were complemented with predicted interactome network that revealed regulation of energy metabolism by key stress-responsive genes (GST, HKT1, MAPKs, P5CSs, PIP), transcription factors (HSFA2, DREB1A, DREB2A), and abiotic stress responsive pathways (ABA, ethylene, ion stress). This is the first detailed study to comprehend the molecular regulation of stress response and their interplay under DS, SS, and WS. The key genes and regulators can be functionally validated, and will facilitate targeted gene editing for genetic improvement of crop sustainability under changing environmental conditions in S. rebaudiana.

Cold Plasma-Induced Changes in Stevia rebaudiana Morphometric and Biochemical Parameter Correlations

Stevia rebaudiana Bertoni is an economically important source of natural low-calorie sweeteners, steviol glycosides (SGs), with stevioside (Stev) and rebaudioside A (RebA) being the most abundant. Pre-sowing seed treatment with cold plasma (CP) was shown to stimulate SGs biosynthesis/accumulation up to several fold. This study aimed to evaluate the possibility to predict CP-induced biochemical changes in plants from morphometric parameters. Principle component analysis (PCA) was applied to two different sets of data: morphometric parameters versus SGs concentrations and ratio, and morphometric parameters versus other secondary metabolites (total phenolic content (TPC), total flavonoid content (TFC)) and antioxidant activity (AA). Seeds were treated for 2, 5 and 7 min with CP (CP2, CP5 and CP7 groups) before sowing. CP treatment stimulated SGs production. CP5 induced the highest increase of RebA, Stev and RebA+Stev concentrations (2.5-, 1.6-, and 1.8-fold, respectively). CP did not affect TPC, TFC or AA and had a duration-dependent tendency to decrease leaf dry mass and plant height. The correlation analysis of individual plant traits revealed that at least one morphometric parameter negatively correlates with Stev orRebA+Stev concentration after CP treatment.

Revisiting the amalgaviral landscapes in plant transcriptomes expands the host range of plant amalgaviruses

Plant amalgaviruses are monopartite, double-stranded RNA viruses, capable of vertical transmission through seeds. An attempt to revisit plant transcriptome-assembled contigs for amalgaviral sequences identified 40 putative novel amalgaviruses in 35 plant species, nearly doubling the number of plant amalgaviruses. Of the 35 plant species, 33 are reported to host amalgaviruses for the first time, including a pteridophytic and two gymnospermic species. Coding-complete genomes of all identified viruses were recovered and the putative +1 programmed ribosomal frameshift (PRF) sites were determined. Genomes of 35 identified amalgaviruses contained the conserved +1 PRF motif 'UUU_CGN', while variant versions were predicted in five genomes. Phylogenetic analysis grouped pteridophyte- and gymnosperm-infecting amalgaviruses together in divergent sub-clades while few of the related angiosperm-infecting amalgaviruses infect members of the same plant family, reiterating the co-evolution of plant amalgaviruses and their hosts. The current study paves way for further studies on understanding biological properties of identified viruses.

Plant Specialised Glycosides (PSGs): their biosynthetic enzymatic machinery, physiological functions and commercial potential

Plants, the primary producers of our planet, have evolved from simple aquatic life to very complex terrestrial habitat. This habitat transition coincides with evolution of enormous chemical diversity, collectively termed as 'Plant Specialised Metabolisms (PSMs)', to cope the environmental challenges. Plant glycosylation is an important process of metabolic diversification of PSMs to govern their in planta stability, solubility and inter/intra-cellular transport. Although, individual category of PSMs (terpenoids, phenylpropanoids, flavonoids, saponins, alkaloids, phytohormones, glucosinolates and cyanogenic glycosides) have been well studied; nevertheless, deeper insights of physiological functioning and genomic aspects of plant glycosylation/deglycosylation processes including enzymatic machinery (CYPs, GTs, and GHs) and regulatory elements are still elusive. Therefore, this review discussed the paradigm shift on genomic background of enzymatic machinery, transporters and regulatory mechanism of 'Plant Specialised Glycosides (PSGs)'. Current efforts also update the fundamental understanding about physiological, evolutionary and adaptive role of glycosylation/deglycosylation processes during the metabolic diversification of PSGs. Additionally, futuristic considerations and recommendations for employing integrated next-generation multi-omics (genomics, transcriptomics, proteomics and metabolomics), including gene/genome editing (CRISPR-Cas) approaches are also proposed to explore commercial potential of PSGs.

An interdisciplinary approach towards sustainable and higher steviol glycoside production from in vitro cultures of Stevia rebaudiana

Stevia rebaudiana is one of the vastly acclaimed commercial plant in the world and belongs to Asteraceae family. The exclusive advantage of Stevia over artificial sweeteners is impeccable and targets its potentiality to the presence of diterpene glycosides. Moreover, the flaunting sweetness of steviol glycosides with associated medicinal benefits, turns the plant to be one of the most economic assets, globally. As compared to vegetative propagation through stem-cuttings, plant tissue culture is the most suitable approach in obtaining true-to-type plants of superior quality. During last few decades, significant in vitro propagation methods have been developed and still the research is ongoing. The present review discusses the tissue culture perspectives of S. rebaudiana, primarily focusing on the mineral nutrition, growth regulators and other accessory factors, motioning the optimum growth and development of the plant. Another crucial aspect is the generation of sweeter varieties in order to reduce the bitter-off taste, which is noticed after the consumption of the leaves. The in vitro cultures pose an efficient alternative system for production of steviol glycosides, with higher rebaudioside(s) content. Moreover, the review also covers the recent approaches pertaining to scale-up studies and genome editing perspectives.

Comparative transcriptomic of Stevia rebaudiana provides insight into rebaudioside D and rebaudioside M biosynthesis

Rebaudioside D (Reb D) and rebaudioside M (Reb M) are commercially important low/no-calorie natural sweeteners. However, they are present in a minor proportion of all steviol glycosides (SGs) in Stevia rebaudiana Bertoni (S. rebaudiana). Strain-dependent deviation in Reb D and Reb M biosynthesis is one key breach for breeding of S. rebaudiana, which has not been studied at the transcriptional level. Herein, five different S. rebaudiana varieties with distinct SGs contents, one cultivar having high stevioside content (HST), one cultivar having high Reb A content (HRA) and three cultivars having high Reb D and Reb M content (HDM₁, HDM₂, HDM₃), were selected for RNA-seq analysis. In total, 131,655 de novo assembled unigenes were found in the RNA-seq data. According to Reb D and Reb M content divergence of S. rebaudiana accessions, 2186 differentially expressed genes (DEGs) were selected as potential genes related to Reb D and Reb M biosynthesis. Weighted Gene Co-expression Network Analysis (WGCNA) was used to explore the genes associated with the Reb D and Reb M biosynthesis. The unigenes from the positively associated turquoise module formed a layered co-expression network. There are 7 UDP-dependent glycosyltransferases (UGT) and 76 transcription factors (TFs) distributing at different regions which represented varying coherence of Reb D and Reb M biosynthesis. Particularly, two TFs having a strong correlation with two UGTs in the network were also discovered. The present study provided a comprehensive insight into networks for regulation of Reb D and Reb M contents in S. rebaudiana.

Transcriptomic Characterization of Nitrate-Enhanced Stevioside Glycoside Synthesis in Stevia (Stevia rebaudiana) Bertoni

Nitrogen forms (nitrate (NO₃⁻) or ammonium (NH₄⁺)) are vital to plant growth and metabolism. In stevia (Stevia rebaudiana), it is important to assess whether nitrogen forms can influence the synthesis of the high-value terpene metabolites-steviol glycosides (SGs), together with the underlying mechanisms. Field and pot experiments were performed where stevia plants were fertilized with either NO₃⁻ or NH₄⁺ nutrition to the same level of nitrogen. Physiological measurements suggested that nitrogen forms had no significant impact on biomass and the total nitrogen content of stevia leaves, but NO₃⁻-enhanced leaf SGs contents. Transcriptomic analysis identified 397 genes that were differentially expressed (DEGs) between NO₃⁻ and NH₄⁺ treatments. Assessment of the DEGs highlighted the responses in secondary metabolism, particularly in terpenoid metabolism, to nitrogen forms. Further examinations of the expression patterns of SGs synthesis-related genes and potential transcription factors suggested that GGPPS and CPS genes, as well as the WRKY and MYB transcription factors, could be driving N form-regulated SG synthesis. We concluded that NO₃⁻, rather than NH₄⁺, can promote leaf SG synthesis via the NO₃⁻-MYB/WRKY-GGPPS/CPS module. Our study suggests that insights into the molecular mechanism of how SG synthesis can be affected by nitrogen forms.