Enhancement of Sugar Yield by Introducing a Metabolic Sink in Sugarcane

Abstract

Increasing sugar content is a primary objective of sugarcane improvement programs worldwide, since sugar accumulation in sugarcane is well below the theoretical physiological limits. Despite decades of conventional breeding and molecular manipulation, sugar content has remained stagnant in the new varieties for the past few decades. Recent experiments in genetic engineering have shown that sugar content in the transgenic sugarcanes was increased significantly (SugarBooster) by creating a metabolic sink through converting sucrose into slowly digested or inert products in the vacuoles of sugar storage cells. Technically, SugarBooster lines were created by introducing a gene for sucrose isomerase or fructosyl transferase into sugarcane to generate isomaltulose- or fructants-like products. The high-value sugar isomaltulose was accumulated in storage tissues without any decrease in stored sucrose concentration, resulting in up to doubled total sugar concentrations in harvested juice. Plant lines with enhanced sugar accumulation also showed increased photosynthesis, sucrose transport and sink strength. The transgenic sugarcane line engineered to express a vacuole-targeted sucrose isomerase, which shows SugarBooster effects, was also found to accumulate sucrose to twice the concentration of the background genotype in heterotrophic cell cultures, without adverse effects on cell growth. Isomaltulose concentrations declined over successive subcultures, but enhanced sucrose accumulation was stable. Detailed physiological characterization revealed multiple processes altered in the transgenic line in a direction consistent with enhanced sucrose accumulation. Striking differences from the control included reduced extracellular invertase activity, slower extracellular sucrose depletion, lower activities of symplastic sucrose cleavage enzymes (particularly sucrose synthase (SuSy) breakage activity) and enhanced concentrations of symplastic hexose-6-phosphate and trehalose-6-phosphate in advance of enhanced sucrose accumulation. Sucrose biosynthesis by sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase was substantially faster in assays conducted to reflect the elevation in key allosteric metabolite glucose-6-phosphate. Sucrose non-fermenting-1-related protein kinase 1 (SnRK1, which typically activates SuSy breakage activity while down-regulating SPS in plants) was significantly lower in the transgenic line during the period of fastest sucrose accumulation. The SugarBooster effects elucidated from either transgenic plants or suspension-cultured cells provide hints at a control circuitry for parallel activation of key enzymes for enhanced sucrose accumulation in sugarcane. Approaches are discussed for enhancing sugar content by manipulating multiple genes.