Research and Developments on Stevia
Now, the profitability of the Stevia sector is mostly governed by the economic efficiency of its farming. To remain competitive in the Stevia market, we need to derive the maximum yield from our farming against reasonable crop management expenses.
We now need more and more quantity of leaves from unit area of our Stevia farms and we need maximum amount of sweet glycosides in the leaves. Moreover, to reduce our input costs in farming, we need robust plants with resistance to diseases and environmental stresses.
It’s a real ropewalk now !
We need better Stevia plants now, which will produce more leaves per plant, more glycosides in the leaves and should be tough enough to fight all the diseases and environmental stresses.So, genetic improvement of the Stevia plants and development of high yielding varieties is the topmost priority.
Polyploidy Induction in Stevia
Polyploidy, the condition of possessing more than two complete genomes in a cell, has intrigued biologists for almost a century. Polyploidy is found in many plants and some animal species and today we know that polyploidy has had a role in the evolution of all angiosperms. In stevia, induction of polyploidy is often an important tool in its genetic improvement. In some cases, the polyploid plants have higher growth vigour, larger and thicker leaves and greater biomass production potential.
Enzymatic Transglycosilation of Steviol Glycosides
Steviol glycosides taste somewhat bitter, and show aftertaste. In order to improve of sweetness, modifications of sugar moieties were conducted by enzymatic transglycosylation. Steviol glycosides, when treated with Cyclomaltodextrin-glucanotransferase (CGTase), yield a complex mixture of products which are mono-, di-, tri and more glycosylated. Significant improvement in quality of taste was observed for most of the glycosylated products,
Steviol Glycoside Biosynthesis
Stevia plants synthesize steviol glycosides as a bio-chemical defense against some species of insects. Steviol glycosides are synthesized from products of photosynthesis and glucose breakdown in a complex biosynthetic pathway. Steviol – the backbone of steviol glycoside molecules belongs to diterpene group of compounds – which plays a multitude of biological functions in plants.
Production of all diterpene group of compounds shares a common initial biosynthetic pathway. The diterpene compounds have a basic backbone of “poly-isoprene”. Isoprene is a simple organic compound with the following chemical structure -
Genetic Modification of Yeasts for Steviol Glycoside Production
From basic building blocks, generated from photosynthesis and glucose breakdown, the synthesis of Steviol Glycosides requires 15 specialized enzymes. The organisms, which have all these 15 enzymes in functioning condition within their cells can only synthesize Steviol Glycosides. All these enzymes are proteins and the necessary software code for producing these proteins should reside in the DNA of the organism capable of producing Steviol Glycosides.
How Yeasts are Genetically Engineered for Steviol Glycoside Production
The foreign protein or enzyme, which we want to express in our production organism, is first extracted from the original organism, purified and its amino acid sequence is determined. Amino acid sequence of all the enzymes for steviol glycoside biosynthesis pathway is available in database like UniProt.
Next we can back calculate the possible nucleotide sequence in the DNA of the region which codes for the enzyme. There are several bio-informatics tools available for this job. In plant cells finding a gene in this process is not very useful, since the coding region may contain non-sense fragments into it (introns). So it is better to look for the messenger RNA for the particular protein. Once we can figure out the mRNA sequence, and its length, which in turn can give an idea about its molecular weight, we may proceed to the next stage.
In December 2015, Swiss headquartered biotech company Evolva announces that it has achieved a technical milestone in its partnership with Cargill, Inc. to commercialise EverSweet™, the next-generation fermentation based, zero-calorie stevia sweetener. Reaching this milestone triggers the payment of USD 0.5 million by Cargill to Evolva. EverSweet™ is made with the best-tasting sweetness components found in the stevia leaf, Reb M and Reb D, which deliver a great taste with better sweetness intensity, faster sweetness onset and improved sweetness quality.