Genetic Improvement of Stevia (Stevia rebaudiana Bertoni): A Potential Calorie Free Natural Sweetener

The increasing global demand for Stevia rebaudiana Bertoni, as a natural non-calorie sweetener has driven concentrated breeding efforts towards its genetic improvement for large-scale commercialization. Steviol glycosides responsible for sweetness in stevia leaves include stevioside, rebaudiosides and dulcoside. Inadequate genetic resources, characterization and the lack of high yielding, superior varieties pose major challenge in stevia production. Developing stevia varieties for high dry leaf yield, high steviol glycosides content particularly rebaudioside-A, optimum leaf-to-stem ratio, enhanced growth and photosynthetic activity, combating biotic and abiotic stresses is being targeted in genetic improvement programmes. Stevia exhibits considerable genetic diversity and in order to elucidate the genetics of steviol glycosides; the pattern of diversity has been assessed for achieving desired improvement. Recent advancements in biotechnological tools such as marker-assisted selection, genomics-assisted breeding, transcriptomics, metabolomics and genetic engineering have further accelerated the breeding progress. Marker-assisted and genomics-assisted breeding facilitate targeted improvements by identifying key genes linked to steviol glycosides biosynthesis. Genome sequencing of stevia revealed a 1416 Mb genome with over 80 per cent repetitive elements and transcriptomic analyses have identified critical glycosyltransferase genes involved in glycosylation, a critical process in steviol glycosides biosynthesis. Functional genomics and Agrobacterium-mediated transformation techniques have demonstrated the potential of metabolic engineering for enhancing steviol glycosides yield. This review highlights the progress made in stevia genetic improvement, emphasizing the integration of conventional breeding with advanced genomic tools to develop superior varieties and meet the rising demand for this natural sweetener.