Cholesterol accumulation by suppression of SMT1 leads to dwarfism and improved drought tolerance in herbaceous plants

Plant synthetic biology as a tool to help eliminate hidden hunger

Agricultural systems are under increasing pressure from declining environmental conditions, a growing population, and changes in consumer preferences, resulting in widespread malnutrition-related illnesses. Improving plant nutritional content through biotechnology techniques such as synthetic biology is a promising strategy to help combat hidden hunger caused by the lack of affordable and healthy foods in human diets. Production of compounds usually found in animal-rich diets, such as vitamin D or omega-3 fatty acids, has been recently demonstrated in planta. Here, we review recent biotechnological approaches to biofortifying plants with vitamins, minerals, and other metabolites, and summarise synthetic biology advances that offer the opportunity to build on these early biofortification efforts.

Diversity of Phytosterols in Leaves of Wild Brassicaceae Species as Compared to Brassica napus Cultivars: Potential Traits for Insect Resistance and Abiotic Stress Tolerance

Phytosterols are natural compounds found in all higher plants that have a wide variety of roles in plant growth regulation and stress tolerance. The phytosterol composition can also influence the development and reproductive rate of strict herbivorous insects and other important agronomic traits such as temperature and drought tolerance in plants. In this study, we analysed the phytosterol composition in 18 Brassica napus (Rapeseed/canola) cultivars and 20 accessions belonging to 10 related wild Brassicaceae species to explore diverse and novel phytosterol profiles. Plants were grown in a controlled phytotron environment and their phytosterols were analysed using a saponification extraction method followed by GC-MS from the leaf samples. The B. napus cultivars showed slight diversity in eight phytosterols (>0.02%) due to the genotypic effect, whereas the wild accessions showed significant variability in their phytosterol profiles. Of interest, a number of wild accessions were found with high levels of campesterol (HIN20, HIN23, HUN27, HIN30, SARS2, and UPM6563), stigmasterol (UPM6813, UPM6563, ALBA17, and ALBA2), and isofucosterol (SARS12, SAR6, and DMU2). These changes in individual phytosterols, or ratios of phytosterols, can have a significant implication in plant tolerance to abiotic stress and plant insect resistance properties, which can be used in breeding for crop improvement.

Elucidation of sterol biosynthesis pathway and its co-regulation with fatty acid biosynthesis in the oleaginous marine protist Schizochytrium sp

Sterols constitute vital structural and regulatory components of eukaryotic cells. In the oleaginous microorganism Schizochytrium sp. S31, the sterol biosynthetic pathway primarily produces cholesterol, stigmasterol, lanosterol, and cycloartenol. However, the sterol biosynthesis pathway and its functional roles in Schizochytrium remain unidentified. Through Schizochytrium genomic data mining and a chemical biology approach, we first in silico elucidated the mevalonate and sterol biosynthesis pathways of Schizochytrium. The results showed that owing to the lack of plastids in Schizochytrium, it is likely to use the mevalonate pathway as the terpenoid backbone pathway to supply isopentenyl diphosphate for the synthesis of sterols, similar to that in fungi and animals. In addition, our analysis revealed a chimeric organization of the Schizochytrium sterol biosynthesis pathway, which possesses features of both algae and animal pathways. Temporal tracking of sterol profiles reveals that sterols play important roles in Schizochytrium growth, carotenoid synthesis, and fatty acid synthesis. Furthermore, the dynamics of fatty acid and transcription levels of genes involved in fatty acid upon chemical inhibitor-induced sterol inhibition reveal possible co-regulation of sterol synthesis and fatty acid synthesis, as the inhibition of sterol synthesis could promote the accumulation of fatty acid in Schizochytrium. Sterol and carotenoid metabolisms are also found possibly co-regulated, as the inhibition of sterols led to decreased carotenoid synthesis through down-regulating the gene HMGR and crtIBY in Schizochytrium. Together, elucidation of the Schizochytrium sterol biosynthesis pathway and its co-regulation with fatty acid synthesis lay the essential foundation for engineering Schizochytrium for the sustainable production of lipids and high-value chemicals.

Improvement of cold tolerance in maize (Zea mays L.) using Agrobacterium-mediated transformation of ZmSAMDC gene

Maize (Zea mays L.) is a food crop sensitive to low temperatures. As one of the abiotic stress hazards, low temperatures seriously affect the yield of maize. However, the genetic basis of low-temperature adaptation in maize is still poorly understood. In this study, maize S-adenosylmethionine decarboxylase (SAMDC) was localized to the nucleus. We used Agrobacterium-mediated transformation technology to introduce the SAMDC gene into an excellent maize inbred line variety GSH9901 and produced a cold-tolerant transgenic maize line. After three years of single-field experiments, the contents of polyamines (PAs), proline (Pro), malondialdehyde (MDA), antioxidant enzymes and ascorbate peroxidases (APXs) in the leaves of the transgenic maize plants overexpressing the SAMDC gene significantly increased, and the expression of elevated CBF and cold-responsive genes effectively increased. The agronomic traits of the maize overexpressing the SAMDC gene changed, and the yield traits significantly improved. However, no significant changes were found in plant height, ear length, and shaft thickness. Therefore, SAMDC enzymes can effectively improve the cold tolerance of maize.

Biosynthesis and the Roles of Plant Sterols in Development and Stress Responses

Plant sterols are important components of the cell membrane and lipid rafts, which play a crucial role in various physiological and biochemical processes during development and stress resistance in plants. In recent years, many studies in higher plants have been reported in the biosynthesis pathway of plant sterols, whereas the knowledge about the regulation and accumulation of sterols is not well understood. In this review, we summarize and discuss the recent findings in the field of plant sterols, including their biosynthesis, regulation, functions, as well as the mechanism involved in abiotic stress responses. These studies provide better knowledge on the synthesis and regulation of sterols, and the review also aimed to provide new insights for the global role of sterols, which is liable to benefit future research on the development and abiotic stress tolerance in plant.

A Comparative Transcriptomic Meta-Analysis Revealed Conserved Key Genes and Regulatory Networks Involved in Drought Tolerance in Cereal Crops

Drought affects plant growth and development, causing severe yield losses, especially in cereal crops. The identification of genes involved in drought tolerance is crucial for the development of drought-tolerant crops. The aim of this study was to identify genes that are conserved key players for conferring drought tolerance in cereals. By comparing the transcriptomic changes between tolerant and susceptible genotypes in four Gramineae species, we identified 69 conserved drought tolerant-related (CDT) genes that are potentially involved in the drought tolerance of all of the analysed species. The CDT genes are principally involved in stress response, photosynthesis, chlorophyll biogenesis, secondary metabolism, jasmonic acid signalling, and cellular transport. Twenty CDT genes are not yet characterized and can be novel candidates for drought tolerance. The k-means clustering analysis of expression data highlighted the prominent roles of photosynthesis and leaf senescence-related mechanisms in differentiating the drought response between tolerant and sensitive genotypes. In addition, we identified specific transcription factors that could regulate the expression of photosynthesis and leaf senescence-related genes. Our analysis suggests that the balance between the induction of leaf senescence and maintenance of photosynthesis during drought plays a major role in tolerance. Fine-tuning of CDT gene expression modulation by specific transcription factors can be the key to improving drought tolerance in cereals.

Altering Sterol Composition Implied That Cholesterol Is Not Physiologically Associated With Diosgenin Biosynthesis in Trigonella foenum-graecum

Diosgenin serves as an important precursor of most steroidal drugs in market. Cholesterol was previously deemed as a sterol origin leading to diosgenin biosynthesis. This study reports that cholesterol is not in parallel with diosgenin biosynthesis in Trigonella foenum-graecum. We first perturbed its sterol composition using inhibitors specific for the upstream isoprenoid pathway enzymes, HMGR (3-hydroxy-3-methylgutaryl-CoA reductase) on the mevalonate (MVA) and DXR (1-deoxy-D-xylulose-5-phosphate reductoisomerase) on the 2-C-methyl-D-erythritol-4-phophate (MEP) pathways, and have revealed that diosgenin and cholesterol reversely or differently accumulated in either the MVA or the MEP pathway-suppressed plants, challenging the previously proposed role of cholesterol in diosgenin biosynthesis. To further investigate this, we altered the sterol composition by suppressing and overexpressing the 24-sterol methyltransferase type 1 (SMT1) gene in T. foenum-graecum, as SMT1 acts in the first committed step of diverting the carbon flux of cholesterol toward biosynthesis of 24-alkyl sterols. Knockdown of TfSMT1 expression led to increased cholesterol level but caused a large reduction of diosgenin. Diosgenin was increased upon the TfSMT1-overexpressing, which, however, did not significantly affect cholesterol biosynthesis. These data consistently supported that diosgenin biosynthesis in T. foenum-graecum is not associated with cholesterol. Rather, campesterol, a 24-alkyl sterol, was indicative of being correlative to diosgenin biosynthesis in T. foenum-graecum.

Advances in steroidal saponins biosynthesis

This work reviews recent advances in the pathways and key enzymes of steroidal saponins biosynthesis and sets the foundation for the biotechnological production of these useful compounds through transformation of microorganisms. Steroidal saponins, due to their specific chemical structures and active effects, have long been important natural products and that are irreplaceable in hormone production and other pharmaceutical industries. This article comprehensively reviewed the previous and current research progress and summarized the biosynthesis pathways and key biosynthetic enzymes of steroidal saponins that have been discovered in plants and microoganisms. On the basis of the general biosynthetic pathway in plants, it was found that the starting components, intermediates and catalysing enzymes were diverse between plants and microorganisms; however, the functions of their related enzymes tended to be similar. The biosynthesis pathways of steroidal saponins in microorganisms and marine organisms have not been revealed as clearly as those in plants and need further investigation. The elucidation of biosynthetic pathways and key enzymes is essential for understanding the synthetic mechanisms of these compounds and provides researchers with important information to further develop and implement the massive production of steroidal saponins by biotechnological approaches and methodologies.

Dissecting cholesterol and phytosterol biosynthesis via mutants and inhibitors

Plants stand out among eukaryotes due to the large variety of sterols and sterol derivatives that they can produce. These metabolites not only serve as critical determinants of membrane structures, but also act as signaling molecules, as growth-regulating hormones, or as modulators of enzyme activities. Therefore, it is critical to understand the wiring of the biosynthetic pathways by which plants generate these distinct sterols, to allow their manipulation and to dissect their precise physiological roles. Here, we review the complexity and variation of the biosynthetic routes of the most abundant phytosterols and cholesterol in the green lineage and how different enzymes in these pathways are conserved and diverged from humans, yeast, and even bacteria. Many enzymatic steps show a deep evolutionary conservation, while others are executed by completely different enzymes. This has important implications for the use and specificity of available human and yeast sterol biosynthesis inhibitors in plants, and argues for the development of plant-tailored inhibitors of sterol biosynthesis.

Highlights to phytosterols accumulation and equilibrium in plants: Biosynthetic pathway and feedback regulation

Phytosterols are a group of sterols exclusive to plants and fungi, but are indispensable to humans because of their medicinal and nutritional values. However, current raw materials used for phytosterols extraction add to the cost and waste in the process. For higher sterols production, major attention is drawn to plant materials abundant in phytosterols and genetic modification. To provide an insight into phytosterols metabolism, the research progress on key enzymes involved in phytosterols biosynthesis and conversions were summarized. CAS, SSR2, SMT, DWF1 and CYP710A, the enzymes participating in the biosynthetic pathway, and PSAT, ASAT and SGT, the enzymes involved in the conversion of free sterols to conjugated ones, were reviewed. Specifically, SMT and CYP710A were emphasized for their function on modulating the percentage composition of different kinds of phytosterols. The thresholds of sterol equilibrium and the resultant phytosterols accumulation, which vary in plant species and contribute to plasma membrane remodeling under stresses, were also discussed. By retrospective analysis of the previous researches, we proposed a feedback mechanism regulating sterol equilibrium underlying sterols metabolism. From a strategic perspective, we regard salt tolerant plant as an alternative to present raw materials, which will attain higher phytosterols production in combination with gene-modification.