Under natural conditions, the plants are, usually, surrounded by a series of potential enemies. They had created strategies of defense against pathogens and herbivores' attacks, allowing its perpetuation throughout evolution. These defense mechanisms are closely associated with the synthesis of secondary metabolites, which are also worldwide used in several areas of industry. This bibliographic review was conducted aiming to better understand how plants synthesize these substances in response to biotic or abiotic stimuli. The results discussed here revealed that synthesis of plant secondary metabolites is dependent on changes in several metabolic pathways, being often directly associated with the primary metabolism. Injury by herbivores or pathogens, temperature, CO2 levels, solar radiation and drought, are the factors related to the environment that express the most significant signs of inhibition or synthesis of plant secondary metabolites. Global climatic changes recently observed can affect the metabolic pathways network and, consequently, secondary metabolites synthesis. Nowadays, genomic tools have been useful alternatives that are leading to a new revolution of plant breeding, allowing the overexpression or inhibition of these substances. Some limitations and challenges to be achieved upon the dynamics of secondary metabolite synthesis in plants are presented.

	Key words: Jasmonic acid, transcription factors, inhibition, synthesis.

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Plant secondary metabolites and its dynamical systems of induction in response to environmental factors: A review

Stevia glycosides, extracted from the leaves of the plant Stevia rebaudiana Bertoni, display an amazing high degree of sweetness. As processed plant products, they are considered as excellent bio-alternatives for sucrose and artificial sweeteners. Being noncaloric and having beneficial properties for human health, they are the subject of an increasing number of studies for applications in food and pharmacy. However, one of the main obstacles for the successful commercialization of Stevia sweeteners, especially in food, is their slight bitter aftertaste and astringency. These undesirable properties may be reduced or eliminated by modifying the carbohydrate moieties of the steviol glycosides. A promising procedure is to subject steviol glycosides to enzymatic glycosylation, thereby introducing additional monosaccharide residues into the molecules. Depending on the number and positions of the monosaccharide units, the taste quality and sweetness potency of the compounds will vary. Many studies have been performed already, and this review summarizes the structures of native steviol glycosides and the recent data of modifications of the carbohydrate moieties that have been published to provide an overview of the current progress.

Stevia Glycosides: Chemical and Enzymatic Modifications of Their Carbohydrate Moieties to Improve the Sweet-Tasting Quality

Enhanced production of steviol glycosides in mycorrhizal plants: a concerted effect of arbuscular mycorrhizal symbiosis on transcription of biosynthetic genes.

Stevia rebaudiana produces sweet steviol glycosides that are 300 times sweeter than sugar and have the beneficial effects on human health including anti-hyperglycaemic. Tissue culture is the best method with high efficacy to propagate stevia. Abiotic stress has an impact on steviol glycoside contents in stevia. Therefore, we investigated the effect of mannitol on the expression of four genes involved in the biosynthesis of stevia including UGT74G1, UGT76G1, kaurene oxidase and kaurene synthase genes and steviol glycosides accumulation in stevia under in vitro conditions. The highest expression of UGT76G1 gene occurred in the plants grown under 20 g/l mannitol. While for the kaurene synthase gene, the highest amount of gene expression was observed at 40 g/l mannitol. The results were different about kaurene oxidase gene. As the highest and lowest gene expression were seen in 50 and 30 g/l mannitol conditions respectively. There were the same results for UGT74G1 that means the most appropriate and also the most inopportune treatment for the gene expression were same as the condition for the kaurene oxidase gene. Compared with control, adding mannitol to media in all concentrations increases the expression of UGT76G1 gene. Estimation of steviol glycosides contents under different treatments of mannitol carried out by HPLC. According to the results, the highest amount of stevioside was produced under 20 g/l mannitol treatment. However, rebaudioside A was accumulated in its maximum amounts under 30 g/l mannitol. It can be concluded that adding mannitol to media in the certain concentration increases steviol glycoside contents in the stevia.

Study of gene expression and steviol glycosides accumulation in Stevia rebaudiana Bertoni under various mannitol concentrations.

Agrobacterium rhizogenes mediated “hairy root cultures” of the renowned bio-sweetener producing plant—Stevia rebaudiana, were generated to explore their yet undetected biosynthetic potentials concerning the low calorie diterpene glycoside—stevioside. Four stable rhizoclones were studied under light and dark conditions showing better growth under dark. Two of these rhizoclones revealed substantially higher photosynthetic pigment accumulation under light. Evidently, capitalization on the available inter-clonal variability first time showed the stevioside synthesizing exclusivity in the SRA4 rhizoclone under light condition, while the rest failed. The dualities of the glycoside synthesizing power amongst the two photosynthetically active rhizoclones were resolved through quantitative RT-PCR analysis of UGT85C2 gene showing positive expression in the stevioside producing rhizoclone. These findings elucidated the decisive role of UGT85C2 in combination with photosynthetic proficiency of the studied hairy root cultures in regulating the biosynthetic pathway of S. rebaudiana.

Zhongwei Wang

Papers

Environmental cues induce changes of steviol glycosides contents and transcription of corresponding biosynthetic genes in Stevia rebaudiana.